Copper is a trace metal required for many enzymes, but it becomes toxic when it accumulates. Cells maintain this element at low levels through transport and storage systems. When these control mechanisms are overwhelmed, copper becomes toxic, but the precise way it killed cells was historically unclear.
Recent discoveries have unveiled a unique form of regulated cell death triggered by an excess of copper, a process named cuproptosis. This process is distinct from other known cell death pathways because it involves a direct attack on specific components within the cell’s energy-producing machinery. This finding has illuminated the mystery of copper toxicity and opened new avenues for understanding and treating diseases.
The Cellular Targets: TCA Cycle and Lipoylation
The tricarboxylic acid (TCA) cycle is a series of chemical reactions within the mitochondria that serves as the primary pathway for cells to convert fuel into usable energy. It operates by breaking down molecules to power the synthesis of adenosine triphosphate (ATP), the cell’s main energy currency. The efficiency of the TCA cycle is directly linked to the cell’s metabolic health.
For the TCA cycle to operate correctly, several of its enzymes require a modification known as protein lipoylation. This process involves attaching a fatty acid molecule called lipoic acid to these enzymes. This attachment acts as a swinging arm, helping to shuttle intermediates between different active sites of the enzyme complexes. Without lipoylation, these enzymes cannot perform their roles effectively, and energy production is impaired.
Two enzyme complexes that depend on lipoylation are the pyruvate dehydrogenase (PDH) complex and the α-ketoglutarate dehydrogenase (KGDH) complex. The PDH complex is responsible for converting pyruvate into acetyl-CoA, the primary entry molecule for the TCA cycle. The KGDH complex catalyzes a subsequent step within the cycle itself. The lipoylated components of these complexes are the specific molecules that copper targets.
Copper’s Direct Engagement with Lipoylated Proteins
Cells possess an intricate network of proteins that manage copper levels, preventing its accumulation to toxic concentrations. Chaperone proteins transport individual copper ions, and specialized pump proteins in cell membranes actively export any excess. This system of copper homeostasis can be overwhelmed, however, by genetic disorders or by exposure to compounds called copper ionophores, which shuttle the metal directly into the cell.
When intracellular copper levels rise beyond the cell’s management capacity, free copper ions interact with cellular components. The toxic form of copper, the cuprous ion (Cu+), binds directly to the lipoic acid attached to TCA cycle enzymes. This interaction is highly specific; other metals like iron, zinc, or nickel do not trigger the same effect.
The binding occurs specifically with the lipoylated DLAT subunit of the PDH complex, and this direct engagement is the initiating event of cuproptosis. The copper hijacks the lipoylated machinery that is part of the cell’s central metabolic pathway. This is not simply an inhibition of the enzyme’s function but the start of a chain reaction that leads to widespread cellular damage and death.
The Cascade to Cell Death
The binding of copper to lipoylated proteins triggers their immediate aggregation. The affected proteins, such as DLAT, begin to clump together, forming large, non-functional clusters or oligomers. This oligomerization is a direct consequence of the copper-lipoic acid interaction and represents a toxic gain-of-function for these proteins.
This initial aggregation sets off a broader wave of protein instability. The clumps of copper-bound proteins lead to the rapid loss of iron-sulfur (Fe-S) cluster proteins. These proteins are involved in numerous cellular processes, including DNA replication and the electron transport chain. The destabilization of Fe-S cluster proteins impairs multiple mitochondrial functions.
The combined effect of protein aggregation and the loss of Fe-S cluster proteins induces a condition of severe proteotoxic stress. The cell’s quality control systems, which are responsible for clearing aggregated proteins, become overwhelmed. The cell activates a heat shock response, producing proteins like HSP70, but it is insufficient to cope with the scale of the crisis.
This cascade of events culminates in the failure of mitochondrial function and the death of the cell. Unlike other cell death pathways, cuproptosis is a direct collapse of the cell’s metabolic core. It is driven by the specific interaction between copper and lipoylated proteins, leading to an unrecoverable state of proteotoxic stress.
Physiological and Therapeutic Relevance
The discovery of cuproptosis explains the cellular damage in diseases of copper overload, such as Wilson’s disease. This genetic disorder is caused by mutations in the ATP7B gene, which codes for a protein that excretes excess copper. When this protein is defective, copper accumulates in tissues like the liver and brain. The cuproptosis mechanism, involving the aggregation of lipoylated proteins, directly accounts for the liver cell death in this condition.
This cell death mechanism also presents therapeutic possibilities in oncology. Many cancer cells exhibit altered metabolic states and are highly dependent on the TCA cycle to fuel their rapid growth. This heightened reliance on mitochondrial respiration makes them potentially more susceptible to cuproptosis than healthy cells. Inducing this form of cell death could be a way to selectively target and eliminate tumors.
Researchers are exploring this strategy by using copper-binding molecules, or ionophores, to intentionally elevate copper levels within cancer cells. One such molecule, elesclomol, showed promise in early clinical trials, although its mechanism was not understood at the time. With the identification of cuproptosis, there is renewed interest in using elesclomol and similar compounds to treat cancers that are particularly dependent on mitochondrial metabolism.