Cuproptosis, How much do you know?

Copper is an indispensable transition metal that has redox properties. It serves as a cofactor for many enzymes involved in mitochondrial respiration, antioxidant defense, and the synthesis of biomolecules by providing or accepting electrons^[1].

Copper is also an essential micronutrient, and people mainly obtain it from food to maintain whole-body copper homeostasis (organ meats and shellfish are often the richest food sources, with adults typically consuming 0.8-2.4 mg of copper per day). The human body primarily absorbs copper through the small intestine, then transports it via the bloodstream to the liver, where excess copper is excreted into bile and eliminated from the body^[2][3][4].

However, once copper concentrations exceed the threshold for maintaining homeostasis, it can lead to a series of disruptions in cellular metabolic functions, ultimately resulting in cell death, a process known as "cuproptosis"^[5].

Copper-induced cell death has been known since the 1980s, but the exact mechanism of how copper ions cause cell death has been controversial^[6]. It wasn't until March 2022 that Tsvetkov et al. elucidated the mechanism of copper-induced cell death, naming it "cuproptosis". This involves the excessive accumulation of copper ions leading to abnormal aggregation of thiol-containing proteins, which disrupts iron-sulfur cluster proteins related to mitochondrial respiration, triggering a proteotoxic stress response and ultimately causing cell death^[7].

Furthermore, inhibitors of apoptosis, necroptosis, ROS-induced cell death, and ferroptosis cannot rescue copper-induced cell death. Therefore, cell death induced by copper-carrier compounds is a distinct form of cell death driven by excessive copper accumulation, unrelated to other known cell death modalities (ferroptosis, pyroptosis, etc.) (Figure 2).

Copper (Cu) is a key signaling factor in cells and is closely related to cancer. Compared to normal patients, tumor tissues and/or serum from patients with various types of cancers such as breast, lung, gastrointestinal, oral, gallbladder, and pancreatic cancers show elevated copper concentrations. Copper accumulation can promote cancer cell migration, proliferation, and tumor growth^[3].

Copper is also involved in multiple signaling pathways in tumor cells, influencing cancer development by binding and activating key molecules in these pathways (Figure 3). This has accelerated the development of Cu-coordinating compounds for anti-cancer therapy.

Two main strategies have been proposed: 1) Using Cu chelators to reduce the bioavailability of Cu, and 2) Using Cu ionophores to deliver Cu into cells and increase intracellular Cu levels.

Cu ionophores

Cu ionophores can induce copper-dependent cell death, or "cuproptosis". Examples include Elesclomol , Cu(II) bis(thiosemicarbazone) complexes [CuII(atsm) and CuII(gtsm) ], and Disulfiram (DSF). These have shown anti-cancer activities and have been evaluated in clinical trials (Table 1).

Elesclomol can transport Cu to the mitochondria, where excess Cu promotes the aggregation of lipoylated proteins and the destabilization of Fe-S cluster proteins, leading to proteotoxic stress and cell death^[7]. Elesclomol has been shown to enhance the efficacy of paclitaxel in patients with refractory solid tumors and stage IV metastatic melanoma.

In a phase III clinical trial with late-stage melanoma patients, Elesclomol showed stronger anti-tumor activity in patients with lower lactate dehydrogenase (LDH) levels compared to those with high LDH levels. Lower LDH levels indicate a higher mitochondrial metabolic state. This further validates that Elesclomol is more sensitive to cells with high levels of lipidated TCA enzymes and hyperactive mitochondrial respiration^[3].

Disulfiram (DSF) can also deliver Cu into cells and induce cuproptosis. DSF can cross the blood-brain barrier and has shown anti-cancer and/or chemosensitizing effects in clinical trials for glioblastoma patients. DSF and Cu can also downregulate the tumor suppressor gene PTEN and activate the AKT signaling pathway in human breast cancer.

Furthermore, DSF can inhibit aldehyde dehydrogenase (ALDH) and selectively target and kill ALDH+ cancer stem cells, reducing the risk of tumor recurrence. Cu ionophores can be combined with targeted therapies like TKIs and PI3K inhibitors, and are particularly effective against tumors with high mitochondrial metabolic states^[3].

For the bis(thiosemicarbazone) class of compounds, Cu II (atsm) has shown anti-cancer activity in hamsters bearing colon cancer tumors. Cu II (gtsm) exhibited tumor-killing effects on prostate cancer cells in vitro and significantly reduced the prostate cancer burden in an in situ mouse model^[3].

Cu chelators

In addition to copper ionophores, using chelating agents to deplete copper has also been shown to delay cancer metastasis by inhibiting pathological angiogenesis in various animal models.

Research has shown that many copper chelators such as Tetrathiomolybdate (TTM), Trientine, and D-penicillamine have demonstrated good anti-tumor activity in animal models and clinical trials (Table 2).

Among them, TTM has been the most extensively studied. Lowering copper levels using TTM affects the activity of MEK1/2 kinase and BRAF-driven tumorigenesis, thereby reducing the growth of BRAFV600E tumor xenografts. TTM can also inhibit copper chaperone proteins, blocking the delivery of copper to enzymes like LOX. Furthermore, in a phase II trial for malignant mesothelioma, TTM exhibited anti-angiogenic effects, delaying disease progression in stage I or II mesothelioma patients^[3].

For Trientine, it can inhibit the expression of IL-8 and exhibit anti-tumor effects in hepatocellular carcinoma. Trientine can also reduce CD31 expression and inhibit endothelial cell proliferation. D-penicillamine, on the other hand, can suppress the activity of LOX, leading to impaired collagen crosslinking, reduced VEGF expression, and delayed progression of intracranial glioblastoma^[3].

Furthermore, genes related to copper-induced cell death have been found to be associated with the tumor microenvironment, and copper can also regulate the expression of PD-L1, providing new insights into the connection between copper-induced cell death and cancer immunotherapy.