IntechOpen

Home > Books > Cell Death Regulation in Pathology [Working Title]

Open access peer-reviewed chapter - ONLINE FIRST

Revisiting Skeletal Muscle Atrophy: Links between Copper Overload, Cuproptosis, and Muscle Atropy

Written By

Zhen Shen, Sunfeng Pan, Fengjie Wu, Kaitao Luo and Yanbo Shi

Submitted: 07 February 2025 Reviewed: 12 February 2025 Published: 25 March 2025

DOI: 10.5772/intechopen.1009603

Cell Death Regulation in Pathology IntechOpen
Cell Death Regulation in Pathology Edited by Vincenzo Carafa

From the Edited Volume

Cell Death Regulation in Pathology [Working Title]

Dr. Vincenzo Carafa

Chapter metrics overview

13 Chapter Downloads

View Full Metrics

Abstract

Skeletal muscle senescence is a significant biological process in the aging of the body, marked by a reduction in muscle mass and function. In recent years, there has been growing interest in understanding the role of copper in skeletal muscle aging. During aging and various pathological conditions, skeletal muscle often exhibits an accumulation of excess copper. This abnormal buildup can trigger specific molecular mechanisms that lead to programmed cell death pathways such as apoptosis, pyroptosis, ferroptosis, and cuproptosis, as well as promote the aggregation of α-synuclein. These effects set off a series of signal cascades that ultimately result in metabolic imbalances within aging muscle fibers, including protein, mitochondrial, and satellite cell dysfunction, leading to degeneration and abnormalities in neuromuscular junctions. This forms a new pathophysiological mechanism for skeletal muscle aging and atrophy. Here, we provide a comprehensive analysis of the molecular and biological functions of copper in the regulatory network of skeletal muscle aging and atrophy, exploring the potential mechanisms of copper overload in aging muscles and the novel roles of various cell death signaling pathways induced by copper overload. Our goal is to offer potential molecular targets and therapeutic options for improving and treating skeletal muscle aging and atrophy through copper chelation strategies in clinical settings.

Keywords

  • copper
  • cuproptosis
  • skeletal muscle atrophy
  • copper metabolism
  • copper overload

1. Introduction

Copper, an essential trace element for the human body, is of vital importance in ensuring the normal functioning of life activities. In adults, the amount of copper generally ranges from 50 to 120 mg. About 50–70% is distributed in the muscles and bones. As a transition metal with redox properties, copper can participate in various biological processes in the form of ions. It achieves this by accepting and transferring electrons, playing a critical role in life-sustaining activities like energy metabolism, active oxygen scavenging, iron absorption, and signal transduction [1].

The homeostasis of copper is strictly regulated by liver metabolism. Both copper deficiency and copper overload can have a significant impact on the body and are closely associated with the onset and progression of various diseases. In recent years, diseases related to copper overload have attracted extensive attention. Research has indicated that under various pathological conditions, such as diabetes, obesity, heart failure, neurodegenerative diseases, and tumors, the copper levels in the body can increase significantly. Copper overload is a pathological process where the imbalance in intracellular copper metabolism leads to enhanced toxicity of copper ions. It has thus emerged as an important target for intervention in multiple diseases [2, 3].

Skeletal muscle, serving as a motor organ, is made up of amino acids and is rich in mitochondria. In the normal adult body, skeletal muscle constitutes approximately 40% of the total body mass. The stable maintenance of its mass forms the material foundation for skeletal muscle to carry out metabolic functions. Once skeletal muscle undergoes aging and atrophy, it poses a substantial threat to human health. Thus, exploring novel intervention targets for the treatment of skeletal muscle atrophy holds great significance.

Numerous studies have indicated that under various pathological conditions, copper overload and muscular atrophy occur concurrently [4, 5, 6, 7]. This implies that there might be a close intrinsic connection between abnormal copper metabolism and skeletal muscle atrophy. Nevertheless, the underlying mechanism of copper-overload-induced skeletal muscle atrophy remains in the stage of in-depth exploration. Notably, currently, a variety of pharmacological techniques involving copper chelating, which aim to reduce copper ion levels in the body, have achieved remarkable progress.

“Cuproptosis” refers to a process where an excessive accumulation of copper ions or disruptions in copper metabolism lead to cell dysfunction, eventually leading to cell death. In 2022, the concept of cuproptosis was first reported in the prestigious journal Science, revealing for the first time how copper ions play a unique role in cell death. This groundbreaking finding highlights a new mechanism of cell death that is distinct from well-known processes like apoptosis, necrosis, and autophagy.

Recent studies have revealed a significant potential link between copper overload, cuproptosis, and skeletal muscle atrophy. Here, we thoroughly explore the mechanisms behind muscle atrophy caused by excess copper and develop a comprehensive mechanism map. Our goal is to provide valuable insights that can support the clinical application of copper chelation technology for treating copper-related conditions or alleviating skeletal muscle atrophy.

2. Regulation of copper homeostasis and cuproptosis

In nature, copper primarily exists in the form of monovalent cuprous ions (Cu+, with reducing properties) and divalent cupric ions (Cu2+, with oxidizing properties). Copper in our diet mostly appears as Cu2+ and often combines with various substances to form stable complexes. Copper absorption mainly takes place in the duodenum and small intestine of the human body. During this process, Cu2+ is first reduced to Cu+ by the metal reductase Six Transmembrane Prostate Epithelial Antigen 1 (STEAP1) or Cytochrome b Reductase 1 (DCYB). Subsequently, Cu+ rapidly and specifically binds to the copper transporter 1 (CTR1) and then enters the intestinal epithelium. The absorbed Cu+ is then transported via the portal circulation system to be stored in liver cells. After that, it is transported to specific tissues and organs to perform their corresponding physiological functions. Excess Cu+ maintains the balance of copper ions in the body by being excreted into the bile, and a small amount of excess Cu+ is directly excreted with the feces in the form of unabsorbed metal ions (Figure 1) [8].

Figure 1.

Schematic diagram of human copper metabolism.

Upon entering the circulatory system, Cu+ combines with copper transporters such as ceruloplasmin, albumin, and trans-cupric. These complexes are then transported to various organs and tissues of the body [9]. Once Cu+ reaches the target organs and tissues, it binds to different copper-chaperone proteins, thereby exerting its catalytic effects. The main copper-chaperone proteins include cytochrome C oxidase copper chaperone 17 (COX17), copper chaperone for superoxide dismutase (CCS), and antioxidant 1 copper chaperone (ATOX1).

COX17 is mainly distributed in the cytoplasm and mitochondrial membrane. When it binds with Cu+, it is further transported to the secondary copper-binding proteins SCO 1/2 (synthesis of cytochrome C oxidase 1/2) and cytochrome C oxidase (CCO), playing a crucial role in regulating enzyme activity in the mitochondrial respiratory chain. CCS transfers Cu++ into superoxide dismutase 1 (SOD1), endowing SOD1 with the ability to combat oxidative stress and promote protein synthesis and secretion. ATOX1 transports Cu+ to the nucleus, where it combines with transcription factors to drive gene expression. Additionally, ATOX1 transfers Cu+ from the trans-Golgi network (TGN) to ATPase copper-transporting alpha/beta (ATP7A/B). Under physiological conditions, ATP7A/B transports Cu+ to the TGN. When the intracellular Cu+ content increases, ATP7A/B fuses with the plasma membrane, pumping Cu+ back into the blood circulation. The excess Cu+ is then transported via the portal vein and stored in the hepatocytes of the liver. If there is an excessive amount of copper in the body, the excess copper is excreted through bile and feces [10].

In conclusion, the equilibrium state of copper is maintained not only by duodenal absorption and bile excretion to achieve homeostasis but also by the synergistic action of copper-chaperone proteins and membrane transporter proteins. The main regulatory factors of copper metabolism homeostasis are summarized in Table 1, and the metabolic balance diagram is shown in Figure 2.

MoleculeFunctionReferences
STEAPMetalloreductase[11]
CTR1Transmembrane solute carrier, Cu importer[12]
CPMajor exchangeable plasma Cu carrier[13]
COX17Cytochrome c oxidase copper-chaperone protein[14]
CCSCopper chaperone for superoxide dismutase[15]
ATOX1Cytosolic Cu metallochaperone[16]
ATP7A/BCu exporter/Golgi apparatus Cu chaperone[17]
MT1/2Cu/Zn storage protein
SOD1Superoxide scavenger

Table 1.

Genes involved in copper homeostasis and cuproptosis.

STEAP: six-transmembrane epithelial antigen of the prostate; CTR1: copper transporter 1; CP: ceruloplasmin; COX17: cytochrome C oxidase copper chaperone 17; CCS: copper chaperone for superoxide dismutase; ATOX1: antioxidant 1; ATP7A/B: Cu2+ transporting, alpha polypeptide/beta polypeptide; SOD1: superoxide dismutase 1.

Figure 2.

Diagram of copper metabolic and cuproptosis mechanism.

3. Regulatory network of skeletal muscle atrophy

The pathological and physiological mechanisms of skeletal muscle atrophy are complex, primarily involving imbalances in protein and mitochondrial metabolism, a reduction in the number and function of satellite cells, the activation of cell death processes, and the degeneration of the neuromuscular junction (NMJ). Research has revealed that copper overload is a crucial biological factor that triggers oxidative stress. It promotes the accumulation of reactive oxygen species (ROS) through the Fenton reaction. This process inhibits the classical phosphatidylinositide 3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, ultimately resulting in a decreased protein synthesis capacity in skeletal muscle.

Furthermore, copper overload can also disrupt the homeostasis of proteins involved in mitochondrial dynamics regulation, such as mitofusin 1 and 2 (MFN1/2), optical atrophy protein 1 and 2 (OPA1/2), and dynamin-related protein 1 (DRP1). These changes are important factors contributing to mitochondrial abnormalities and the aging and atrophy of skeletal muscles. Further research indicates that reactive oxygen species (ROS) generated by copper overload have a negative regulatory effect on muscle satellite cells, hindering the maintenance of satellite cell numbers and the exertion of their functions. In the elderly body, muscle fibers damaged by normal physiological activities cannot be replenished in a timely manner, thus leading to the formation of muscle fiber atrophy phenotypes [9].

Cell death plays a crucial role in the loss of aging muscle fibers. Multiple forms of cell death, including apoptosis, pyroptosis, ferroptosis, and cuproptosis, are all involved. These different forms of cell death involve different intermediate proteins and have distinct signaling pathways. Current research shows that when there is copper overload, it may activate multiple cell death programs, promote the aggregation of α-synuclein, directly cause the degradation and loss of muscle cell contents, and simultaneously trigger the degeneration and abnormalities of the neuromuscular junction (NMJ), ultimately leading to the aging and atrophy of skeletal muscles.

Muscle atrophy is a common complication of various chronic diseases such as obesity, diabetes, heart failure, and drug addiction. Its pathological features are manifested as a decrease in muscle mass and strength decline, and its pathogenesis is complex, which may be closely related to factors such as protein metabolism imbalance, neuromuscular junction inactivation, and mitochondrial loss. Muscle atrophy not only affects normal gait and motor ability in humans, severely reducing the quality of life, but also is closely linked to the occurrence, progression, and prognosis of various diseases. In various chronic disease states, it is a key factor affecting mortality [18]. Current research shows that in pathological conditions, combating skeletal muscle atrophy is an indirectly effective method for treating certain diseases [19]. Therefore, it is particularly important to identify novel intervention targets for skeletal muscle atrophy (such as copper metabolism) to benefit its treatment.

Skeletal muscle is an important motor organ in the human body. It is composed of amino acids as the basic building blocks and is rich in mitochondria. In normal adults, it accounts for approximately 40% of the body mass. Maintaining the mass of skeletal muscle is the material basis for the metabolic function of skeletal muscle. When pathological stimulation signals from the outside world occur, skeletal muscle will respond and then develop an atrophic phenotype. The imbalance in the homeostasis of muscle proteins and mitochondria is the fundamental cause of skeletal muscle atrophy [20]. Under normal physiological conditions, skeletal muscle proteins are in a continuous dynamic balance of synthesis and degradation, and mitochondria also maintain their own quality control through continuous fusion and fission. However, once in a pathological condition, this metabolic balance is disrupted, leading to the degradation rate of muscle proteins exceeding the synthesis rate and mitochondrial fission occurring more frequently than fusion, ultimately triggering muscle atrophy.

The occurrence of skeletal muscle atrophy involves various metabolic processes and different regulatory mechanisms. In terms of protein metabolism, the synthesis of skeletal muscle proteins mainly depends on the pro-synthetic signals mediated by growth factors such as testosterone, growth hormone, and insulin-like growth factor. On the other hand, the degradation process mainly occurs through degradation pathways such as the ubiquitin-proteasome system, autophagy, apoptosis, and the activity of calpains. In terms of mitochondrial metabolism, the fusion and fission of mitochondria are precisely regulated by the dynamic balance of their respective upstream target proteins, namely mitofusins and fission proteins [21]. A deep understanding of these regulatory mechanisms and signaling pathways is the key theoretical basis for the development of therapeutic drugs for skeletal muscle atrophy, providing an important research direction for overcoming this medical challenge.

4. Copper overload and skeletal muscle atrophy

4.1 The roles and physiological functions of copper in skeletal muscle

Copper serves as a cofactor for various essential enzymes in the body and participates in various biochemical reactions within cells in an ionic form, playing a crucial role in maintaining the normal metabolism of organs. The metabolic homeostasis of copper ions within cells is jointly regulated by copper transporters and chaperone proteins. Among them, mutations in the transporter genes copper-transporting ATPase 7A (ATP7A) and copper-transporting ATPase 7B (ATP7B) can lead to copper deficiency or copper overload, which in turn trigger Menkes disease and Wilson’s disease (hepatolenticular degeneration). Exogenous copper is the primary source of copper in the body and is mainly obtained through food intake. Most of the copper in food exists in the form of organic copper bound to proteins, and this type of dietary organic copper can be absorbed by gastrointestinal cells and enter the body (Figure 1).

Copper metabolism mainly occurs in two stages: In the first stage, copper from food is absorbed in the human small intestine and enters the bloodstream, where it binds to small molecules in the serum, such as histidine, α2-macroglobulin, albumin, etc., thus forming a copper storage pool. In the second stage, most of the copper ions in the copper storage pool enter the liver via the portal vein. After being processed in the liver, they enter the bloodstream in the form of ceruloplasmin and are transported and distributed to various organs throughout the body that rely on copper, such as the brain and skeletal muscles [22]. The remaining part of the copper continues to exist in the serum in the form of binding to small molecules. The degradation of ceruloplasmin usually occurs in the liver as well. Hepatic endothelial cells can capture ceruloplasmin in the serum through transcytosis and completely remove the sialic acid residues at the N-terminus of its oligosaccharides. The desialylated ceruloplasmin, under the action of the asialoglycoprotein receptor, is then excreted into the space of Disse and subsequently endocytosed by hepatocytes. Under the degradation of hepatocyte lysosomes, ceruloplasmin is decomposed and releases copper ions. In order to maintain the metabolic balance of copper ions, excess copper ions are excreted from the liver into the bile and solidified by bile salts before being excreted from the body. This is the main pathway for copper excretion [23, 24]. However, during the electron transfer process, oxidative stress is generated. The key copper-containing enzyme in muscle cells that resists oxidative stress is Cu,Zn-superoxide dismutase (Cu,Zn-SOD) located in the cytoplasm. This enzyme can utilize a copper ion to catalyze the disproportionation reaction of superoxide and effectively inhibit the damage caused by oxidative stress to cells (Figure 1).

Copper ions are essential for myoblast proliferation and differentiation, as well as for maintaining normal metabolic homeostasis in differentiated myocytes [25]. As a key static cofactor, copper ions can participate in the construction and formation of various copper enzymes (such as oxidoreductases, oxygenases, hydroxylases, and transferases) by virtue of their own redox properties. These enzymes have flexible active sites that can maximize the transfer and transmission of electrons. Among them, the most classic copper enzyme is cytochrome C oxidase (CCO). As the terminal enzyme of the electron transport chain, it can couple electron transfer through the oxidation of cytochrome C and the reduction of oxygen, thereby enhancing the shuttle activity of the proton pump across the mitochondrial membrane and promoting the generation of ATP [26]. However, oxidative stress is generated during the electron transfer process. The key copper enzyme in myocytes to resist oxidative stress is SOD1 located in the cytoplasm. This enzyme can use a copper ion to catalyze the disproportionation reaction of superoxide and effectively inhibit the damage caused by oxidative stress to cells.

Another important site of copper ions in cells is the pathways related to the copper secretion pathway, including the trans-Golgi network (TGN), endolysosomes, secretory granules, and copper storage vesicles. The copper secretion pathway can prompt different types of cells to activate copper-dependent kinases to perform their respective functions. Moreover, in certain copper-dependent cells, copper ions can also regulate the activities of kinases such as Unc-51-like protein kinase 1/2 (ULK1/2), mitogen-activated protein kinase kinase 1 (MEK1), and phosphodiesterase 3B (PDE3B), thereby influencing the expression of autophagy-related genes (ATGs), the formation of autophagosomes, as well as cell proliferation and metabolic activities [23]. It is worth noting that some of these kinases may be associated with the signaling pathways related to skeletal muscle cell atrophy induced by copper overload [27].

4.2 Regulation of copper metabolism in skeletal muscle

Skeletal muscle is not only an organ that highly depends on copper ions and has a high metabolic rate but also a major organ where copper is distributed [28]. Copper-dependent organs (including skeletal muscle) follow common physiological principles for the uptake of copper ions. This process is regulated by intracellular copper transporters (CTRs) and copper-chaperone proteins. Specifically, copper transporters (CTR1, CTR2, and CTR3) are responsible for the uptake of copper ions. They transport copper ions across the membrane into the cell. The other two copper transporters, ATP7A and ATP7B, show tissue-specific expression. Except in hepatocytes, ATP7A is more abundantly expressed than ATP7B in most cells [28]. These two mainly regulate the distribution, storage, and efflux of intracellular copper ions. Copper-chaperone proteins can bind to intracellular copper ions, neutralize their toxicity, and transfer them to specific target proteins. The three most important copper-chaperone proteins include cytochrome C oxidase 17 (COX17), antioxidant 1 copper chaperone (ATOX1), and copper chaperone for superoxide dismutase (CCS). Different copper-chaperone proteins mediate different copper ion transfer pathways within the cell. The metabolism of copper ions in skeletal muscle is regulated by copper transporters and copper-chaperone proteins. Among them, CTR1, ATP7A, and ATP7B have been proven to be widely expressed in skeletal muscle cells [29]. CTR1 is the most crucial copper uptake transporter protein. The expression of CTR2 is cell-specific and may be more involved in intracellular copper ion transport. However, its expression in muscle cells and the related mechanisms regulating copper ion uptake are still unclear. Human CTR1 consists of 190 amino acids and fragments containing specific metal-binding sequences. The methionine 43 and 45 regions are the key to mediating copper ion uptake [30]. CTR1 can regulate the expression and abundance of CTR1 on the membrane through endocytosis and extra-membrane position migration, thereby controlling the cell’s copper ion uptake. When the intracellular copper ion content increases, the cell will enhance clathrin-and dynamin-dependent endocytosis, reduce the abundance of CTR1 on the cell membrane, and simultaneously migrate away from the membrane edge. These two aspects work together to inhibit the cell’s capture and uptake of copper ions [28], avoiding damage to the cell caused by copper overload. This automatic sensing mechanism may be mediated by specificity protein 1 (Sp1) as an intermediate [31].

After copper ions enter the cell through CTR1, they dissociate from it and then rely on different copper-chaperone proteins to mediate various copper ion transport pathways. Among these pathways, COX17 and the non-protein ligand CuL play a crucial linking role in the process of copper ion transport targeted to mitochondria. COX17 can bind copper ions by reducing its own cysteine residues and then transfer the copper ions to the mitochondrial intermembrane space, interacting with two cytochrome C oxidase synthesis proteins (SCO1 and SCO2) and participating in the CCO metabolic pathway [32]. At the same time, after CuL in the cytoplasm binds to copper ions, it can cross the outer mitochondrial membrane and enter the intermembrane space, participating in the assembly of CCO, the maturation of SOD1, and the further uptake and storage of mitochondria [32]. COX17 and CuL jointly regulate the copper ion metabolic balance in mitochondria. In addition, ATOX1 can transfer copper ions to the N-terminal regions of ATP7A and ATP7B at the membrane structures of the secretory pathway, enhancing the transport activity of the copper ion secretory pathway and thus regulating the distribution of copper ions within the cell [33]. CCS, on the other hand, transfers copper ions to SOD1 by forming a highly specific complex, and completes metal substitution and disulfide bond formation at specific sites, thereby antagonizing the damage of superoxide to cells [34]. The distribution of copper ions within the cell mainly depends on the transmembrane transport of ATP7A and ATP7B between different structures within the cell such as the trans-Golgi network (TGN), endosomes, and melanosomes. This process is crucial for the activation of copper enzymes, the storage of copper ions, and the excretion of excess copper ions and is a key link in the regulation of intracellular copper homeostasis [35].

4.3 Possible mechanisms of copper overload

Intracellular copper uptake and efflux are maintained in a precise state of homeostasis, regulated by proteins like Ctr1, ATP7A, and ATP7B. Once the uptake of copper ions surpasses the efflux, copper overload ensues. Copper overload is a biological phenomenon characterized by copper toxicity resulting from the malfunction of the copper ion metabolism mechanism. It is closely associated with neurodegenerative diseases, cardiovascular diseases, cancer, and numerous other human ailments. Currently, the exact regulatory mechanism of copper overload remains incompletely understood. This may involve multiple distinct signaling pathways and molecular mechanisms, and it is one of the key problems to be resolved in the field of copper metabolic physiology.

Aging, serving as a pathological model associated with the occurrence of copper overload, offers valuable clues for delving into the mechanism of intracellular copper overload under pathological conditions [36]. Under normal physiological circumstances, extracellular copper overload can activate the intracellular copper homeostasis regulation response mechanism. Firstly, claudin-dependent endocytosis is enhanced, which leads to a reduction in the abundance of the copper transporter CTR1 on the cell membrane. Additionally, CTR1 migrates away from the membrane edge. These combined effects inhibit the cells’ capture and uptake of copper ions, thus reducing the influx of copper ions into the cells. Secondly, the copper transporters ATP7A and ATP7B can enhance the excretion and metabolism of copper ions. Working in tandem, they jointly maintain intracellular copper homeostasis and prevent copper overload [28].

In various chronic disease conditions, the level of circulating ceruloplasmin increases significantly, leading to extracellular copper overload in skeletal muscle cells. Pathological states can disrupt the aforementioned copper homeostasis mechanism, causing metabolic disorders in copper ion uptake and efflux, and ultimately triggering intracellular copper overload. REDOX imbalance and autophagy dysfunction may also be involved in this process [36, 33]. Evidently, copper overload is not the outcome of single-factor regulation but the product of multiple signal cascade events. Further exploring and elucidating the relevant molecular mechanisms will be a crucial research direction in the field of copper metabolism physiology.

4.4 Signal involved in copper overload and muscle atrophy

Copper ions not only contribute to the formation of copper proteins but also play a profound role in regulating sugar and lipid metabolism within the body. Research has indicated that copper deficiency in the body can trigger insulin secretion inhibition and impaired glucose tolerance, which clearly demonstrates the crucial role of copper in maintaining glucose homeostasis [37]. As an endogenous regulator of lipolysis, copper ions can directly bind to the cysteine site of PDE3B, thus inhibiting its activity. Through this mechanism, copper ions further promote lipolysis via the cyclic adenylate-dependent pathway, exerting a significant impact on fat metabolism [38]. Moreover, copper ions are also vital in the mutual regulation of sugar and fat metabolism. In a high-sugar environment, ceruloplasmin can significantly enhance the oxidation of low-density lipoprotein [39]. Skeletal muscle is a key organ for carbohydrate and fat metabolism, and its health is of utmost importance for energy metabolism. The onset of skeletal muscle atrophy is often accompanied by energy metabolism disorders [40], and the regulatory function of copper ions in this process warrants further investigation and attention.

4.4.1 Copper overload inhibits skeletal muscle protein synthesis via the PI3K/PKB/Akt/mTOR signaling pathway

The PI3K/Akt/mTOR signaling pathway regulates muscle protein synthesis, and its signal transduction process is as follows: Upon stimulation by growth factors, PI3K can phosphorylate diphosphoinositol on the muscle membrane, converting it into phosphatidylinositol triphosphate. Subsequently, it phosphorylates and activates the downstream target Akt. Once Akt is activated, it promotes muscle protein synthesis through two distinct signaling pathways. Firstly, in an mTOR-independent manner, glycogen synthase kinase 3β (GSK3β) is directly inhibited, and at the same time, eukaryotic initiation factor 2B (eIF2B) is activated to promote muscle protein synthesis. Secondly, Akt phosphorylates and activates the downstream target mTOR, which mainly consists of mTOR complex 1 (mTORC1) and mTORC2.

Among them, mTORC1 is particularly sensitive to rapamycin. Rapamycin can phosphorylate and activate p70 S6 kinase, which in turn promotes the high phosphorylation of ribosomal S6 protein. This ultimately facilitates mRNA transcription and translation, leading to increased muscle protein synthesis. Additionally, activated mTORC1 can phosphorylate eIF4E binding protein 1, inhibiting its activity and further promoting mRNA translation, thus enhancing muscle protein synthesis.

It is worth noting that copper overload significantly inhibits the PI3K/Akt/mTOR signaling pathway, thereby weakening the protein synthesis capacity of skeletal muscle. This effect may be mediated by reactive oxygen species (ROS), markers of oxidative stress. Copper overload is a potent inducer of ROS, which can directly activate NF-κB and promote the overexpression of myostatin. Myostatin inhibits the expression of miR-486 through an unknown mechanism, thereby activating the activities of phosphatase and tensin homolog (PTEN), and further inhibiting the recruitment and phosphorylation of Akt [41]. Therefore, ROS can form miR-486/PTEN/PI3K/Akt signaling cascade-mediated by myostatin and inhibit myosin synthesis, which may be one of the potential mechanisms of copper overload inducing skeletal muscle atrophy.

4.4.2 Copper overload disrupts mitochondrial homeostasis in skeletal muscle

Mitochondria are organelles that exhibit a unique dependence on copper. Each mitochondrion contains approximately 45,000 to 50,000 copper ions, which play an essential and irreplaceable role in maintaining the normal metabolic functions of cytochrome c oxidase (CCO) within mitochondria and copper-zinc superoxide dismutase (SOD1). The distribution of copper ions in mitochondria displays distinct regional characteristics, being primarily concentrated in the intermembrane space and the matrix.

In the intermembrane space, the acquisition and transport of copper ions mainly rely on copper-chaperone proteins COX17 and CCS, as well as the capture and transport of copper ions in the cytoplasm by the alternative ligand glutathione (GSH) and the non-protein ligand CuL. The copper ions in the matrix are exclusively derived from CuL. After binding to copper ions in the cytoplasm, CuL traverses the intermembrane space, enters the matrix, and is stored there under the mediation of a specific copper transport receptor, SLC25A3, on the inner mitochondrial membrane. When there is a deficiency of copper ions, the copper ions stored in the matrix will be transported and released to the intermembrane space and the cytoplasm to participate in the cyclic metabolism [26]. However, the copper transport receptor on the inner mitochondrial membrane that mediates the transfer of copper ions from the matrix to the intermembrane space has not yet been clearly identified.

Copper homeostasis is important in maintaining the normal fusion, fission, and metabolism of skeletal muscle mitochondria. Copper ions participate in the electron transfer process of cytochrome c oxidase (CCO) and the oxidative phosphorylation process within mitochondria, thereby facilitating the generation of adenosine triphosphate (ATP) [42]. Under pathological conditions, the total quantity, number, and function of mitochondria undergo remodeling and alterations, which indirectly impact muscle force output and mass maintenance. The regulatory role of copper overload in this process warrants significant attention.

Mitochondria rely on their continuous fusion and fission processes to respond to changes in physiological environments and stimuli in pathological environments. The dynamic balance between fusion and fission can construct a mitochondrial functional network that is most conducive to ATP production. Fusion refers to the interconnection of different mitochondria under the action of fusion proteins, which in turn promotes the mixing and redistribution of their contents, such as metabolites, proteins, and mtDNA. This process is mainly regulated by mitofusin 1/2 (MFN1/2) and optic atrophy 1/2 (OPA1/2). During fusion, the mitochondrial membranes on both sides of the reaction are the same, which belongs to homologous fusion. Among them, MFN1/2 dominates the outer membrane fusion of mitochondria, while OPA1/2 dominates the inner membrane fusion [43]. Research has found that inhibiting MFN1/2 is sufficient to cause the accumulation of mtDNA with mitochondrial deficiency and muscle atrophy [44]. Thus, mitochondrial fusion is crucial for maintaining skeletal muscle mass, and fusion defects are important factors leading to skeletal muscle atrophy. Fission refers to the process in which mitochondria, under the action of fission proteins, grid and isolate the irreversibly damaged or unnecessary parts of mitochondria, so that they can be degraded and removed through the autophagy-lysosome pathway [45]. This process is mainly completed by the combination of dynamin-related protein 1 (DRP1) in the cytoplasm with the corresponding receptors on the outer mitochondrial membrane, such as mitochondrial fission factor 1 (MFF1), fission protein 1 (FIS1), and mitochondrial dynamics proteins of 49/51 kD (MiD49/51), and triggering the relevant signal cascade. The fission state of mitochondria is closely related to skeletal muscle mass. Excessive activation of fission can lead to excessive protein decomposition, mitochondrial functional defects, and muscle atrophy phenotypes [44].

The imbalance between fusion and fission is the main source of abnormal mitochondria and an important potential factor in inducing skeletal muscle atrophy. Copper overload can interfere with mitochondrial quality control through two pathways: Firstly, under the mediation of ROS, copper overload significantly upregulates the expression levels of DRP1 mRNA and protein in skeletal muscle, while inhibiting the expression of MFN1/2 and OPA1, thus leading to abnormal mitochondrial morphology. Secondly, copper ions can activate the MEK1/extracellular signal-regulated kinase (ERK) signaling pathway. The activated ERK can inhibit mitochondrial fusion and promote fission by regulating the oligomerization of MFN1. These evidences suggest that copper overload not only weakens mitochondrial fusion and exacerbates mitochondrial fission but also causes abnormal mitochondrial structure and functional defects, which is likely the internal mechanism of muscle atrophy.

4.4.3 Copper overload leads to excessive activation of the autophagy program in skeletal muscle through ULK1/2 mediation

Cell autophagy is a process through which cells encapsulate certain components of their cytoplasm, including mitochondria and toxic proteins, to form autophagosomes. These autophagosomes are subsequently transported to lysosomes, where hydrolytic enzymes break them down into metabolic products like amino acids. Moderate levels of cell autophagy play a beneficial role in the renewal of skeletal muscle proteins and mitochondria as well as in maintaining their homeostasis. Nevertheless, excessive activation of autophagy can result in muscle atrophy and loss.

ULK1 and ULK2 are proteins featuring highly conserved serine/threonine kinase activity. The Atg1 complex, which consists of the ULK1/2 protein kinases, the 200 kD focal adhesion kinase family interacting protein (FIP200), ATG13, and ATG101, serves as the initiator of autophagy in mammalian cells. This complex is capable of promoting the formation and expansion of the double-membrane structure of autophagosomes by activating the autophagy-related protein ubiquitin-like conjugation system. Notably, the phosphorylation and dephosphorylation of the threonine at position 180 of ULK1/2 represent a crucial step in this cellular process [23].

Copper overload plays a crucial role in inducing the autophagy program, and its action pathways may involve direct and indirect mechanisms. On the one hand, copper ions are important regulatory factors for ULK1/2 kinases. ULK1/2 contains an amino acid residue sequence for covalent copper binding, which can directly connect to copper ions. Once the copper-binding site of ULK1/2 mutates, ULK1/2 will be inactivated and lose its ability to activate the downstream substrate ATG13. Meanwhile, copper chelation treatment or disruption of the interaction between copper ions and ULK1/2 will lead to the inhibition of autophagy. These results indicate that copper overload can directly activate ULK1 and exacerbate autophagic activity. On the other hand, amino acid and energy status are important upstream regulatory factors of ULK1/2, which regulate the autophagy activity mediated by ULK1/2 through mTORC1 and AMP-activated protein kinase (AMPK), respectively. Research has found that copper overload can significantly inhibit Akt/mTORC1 in skeletal muscle cells, accompanied by the phosphorylation of beclin-1, a downstream substrate of autophagy-related genes, and autophagy activation [46]. The above evidence fully confirms the key role of ULK1/2 in copper overload-induced autophagy and skeletal muscle loss. Based on this research, further experimental verification work can be carried out using muscle cells as a model.

4.4.4 Copper overload promotes cell apoptosis, contributing to muscle atrophy

Apoptosis is a programmed cell death process that is coordinately regulated by regulatory proteins, endonucleases, protease inhibitors, and caspases. During this process, downstream signal transduction is triggered, followed by the formation of apoptotic bodies, ultimately leading to non-inflammatory self-destruction of the cell [47]. When cells are subjected to death stimuli, initiator caspases (i.e., caspase-8, −9, and −12) are mobilized and activated. They further activate effector caspases (i.e., caspase-3, −6, and −7), thereby inducing cell degradation and DNA fragmentation. Depending on the source of the stimulating signal, apoptosis can be classified into two types: extrinsic and intrinsic. Extrinsic apoptosis is triggered by the interaction between cell surface death receptors (such as tumor necrosis factor receptors) and their ligands (such as tumor necrosis factor α). Intrinsic apoptosis involves the participation of mitochondria or the endoplasmic reticulum, and its main inducers include DNA damage, hypoxia, and metabolic stress. It is worth noting that mitochondria can induce apoptosis through a caspase-independent activation mode, that is, releasing apoptosis-inducing factor (AIF) and endonuclease G to cleave and fragment DNA [48].

In skeletal muscle, muscle cells are multinucleated. When the apoptotic cascade signal is activated, individual myonuclei and a portion of the sarcoplasm are cleared, and this process is defined as myonuclear apoptosis. Significantly, myonuclear apoptosis does not result in cell death; however, it can trigger muscle fiber atrophy. Moreover, several studies have indicated that apoptotic signals may also activate the ubiquitin-proteasome system, initiating the muscle protein degradation program and thereby further promoting muscle fiber atrophy [49]. In reality, apoptosis and protein degradation often occur concurrently during muscle atrophy. Another characteristic of skeletal muscle fibers is the presence of two mitochondrial subpopulations that differ in bioenergetics and structure: subsarcolemmal mitochondria and mitochondria within muscle fibers. These two subpopulations exhibit different susceptibilities to apoptotic stimuli and may therefore be involved in different pathogenetic mechanisms underlying skeletal muscle atrophy [49].

Recently, scientists have limited understanding of the mechanisms behind copper overload and its impact on apoptosis. Studies using human cell models have revealed that exposure to high concentrations of copper ions leads to an unusual distribution of nucleophosmin and fibrillarin within the nucleoplasm, which in turn disrupts ribosomal RNA (rRNA) processing. Specifically, there is an increase in abnormal 34S rRNA due to cleavage at the A2 site. At the same time, a decrease was observed in downstream precursor rRNA (pre-rRNA) and insufficient accumulation of the 60S subunit. Moreover, transcriptome analysis shows that copper overload also affects the expression of genes involved in ribosome biogenesis. This disruption ultimately leads to apoptosis by causing nucleolar stress and through a p53-independent pathway. In studies of skeletal muscle samples, we find that copper overload not only directly triggers myolysis but also increases endoplasmic reticulum stress. This happens as a result of the simultaneous activation of initiator caspase-12, effector caspase-3, and several endoplasmic reticulum stress-related proteins, leading to cell death [50].

Based on these findings, it is hypothesized that the degradation and loss of muscle fibers induced by copper overload may predominantly occur via the intrinsic apoptotic pathway. Nevertheless, whether extrinsic apoptosis, myonuclear apoptosis, and differential disorders of mitochondrial subpopulations are implicated in this pathological process still necessitates further experimental validation.

4.4.5 The potential role of cuproptosis in muscle atrophy

Cuproptosis is a novel form of cell death distinct from apoptosis, pyroptosis, necrosis, and ferroptosis [51]. It is another metal ion-related cell death pathway following ferroptosis. Hypoxia can inhibit the cell death induced by copper ionophores, and this inhibitory effect can be blocked by hypoxia-inducible factor prolyl hydroxylase inhibitors. Meanwhile, under glycolytic conditions, cells are resistant to the killing effect of copper ions. These evidences strongly suggest that cellular respiration is a key factor mediating the occurrence of cuproptosis.

After analyzing the metabolites of ABC1 cells exposed to copper ions, researchers observed disruptions in multiple metabolic pathways related to the tricarboxylic acid cycle. However, A549 cells, which are resistant to copper ions, did not exhibit such changes. This result implies a close connection between cuproptosis and mitochondrial metabolism. Through analysis using the CRISPR technology for the whole genome, it was found that ferredoxin 1 (FDX1) and protein lipoylation are key regulatory factors in copper-overload-induced cell death. Among them, FDX1 encodes a reductase that can reduce divalent copper ions to more toxic monovalent copper ions. Protein lipoylation mainly occurs on four enzymes that regulate the tricarboxylic acid cycle and is a highly conserved post-translational modification of lysine.

In addition, since the knockout of the FDX1 gene can completely abolish protein lipoylation by inhibiting the tricarboxylic acid cycle of pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, it indicates that FDX1 is an upstream regulator of protein lipoylation. Further research shows that copper ions can directly bind to and induce the oligomerization of lipoylated dihydrolipoamide transacetylase, while causing an imbalance in iron-sulfur proteins and triggering proteotoxic stress. In summary, cuproptosis mainly occurs through the direct binding of copper ions to the lipoylated components of the tricarboxylic acid cycle (TCA). This process promotes the aggregation of lipoylated proteins and the loss of iron-sulfur cluster proteins, thereby inducing proteotoxic stress and ultimately leading to cell death [52]. Currently, the mechanism of cuproptosis in skeletal muscle cells under copper overload conditions remains unclear. Based on the existing research evidence, the possible mechanism by which copper overload mediates muscle atrophy is shown in Figure 1.

5. Conclusion and prospect

Copper plays an essential role as a cofactor for many enzymes, but excessive copper levels can lead to oxidative stress. Given the unique behavior of copper in various diseases, it is likely to become a significant research focus as a potential therapeutic target. The phenomenon of copper-induced cell death will also attract considerable attention in future studies. While there are currently no direct reports linking the mechanism of copper-induced cell death with breakthroughs in skeletal muscle atrophy treatments, progress has been made in this area. Continued in-depth research will provide a clearer scientific foundation for strategies aimed at preventing and treating copper-related diseases by targeting copper-induced cell death, thereby advancing their clinical applications.

Acknowledgments

This work was supported by grants from Medicine and Health Science and Technology Plan Projects of Zhejiang Province (YS, 2023KY1227), Science and Technology Innovation Special Project of Jiaxing Science and Technology Bureau (YS, 2024AY30006), Key Research Projects of the Affiliated Hospital of Zhejiang Chinese Medical University (YS, 2022FSYYZZ22), and the Jiaxing Key Laboratory of Diabetic Angiopathy.

Conflict of interest

The authors declare no conflict of interest.

References

  1. 1. Ruiz LM, Libedinsky A, Elorza AA. Role of copper on mitochondrial function and metabolism. Frontiers in Molecular Biosciences. 2021;8:711227
  2. 2. Bjorklund G et al. The role of zinc and copper in insulin resistance and diabetes mellitus. Current Medicinal Chemistry. 2020;27(39):6643-6657
  3. 3. Hammadah M et al. Prognostic value of elevated serum ceruloplasmin levels in patients with heart failure. Journal of Cardiac Failure. 2014;20(12):946-952
  4. 4. Wannamethee SG, Atkins JL. Muscle loss and obesity: The health implications of sarcopenia and sarcopenic obesity. The Proceedings of the Nutrition Society. 2015;74(4):405-412
  5. 5. Giha HA, Alamin OAO, Sater MS. Diabetic sarcopenia: metabolic and molecular appraisal. Acta Diabetologica. 2022;59(8):989-1000
  6. 6. von Haehling S et al. Muscle wasting and cachexia in heart failure: Mechanisms and therapies. Nature Reviews. Cardiology. 2017;14(6):323-341
  7. 7. Fukawa T et al. Excessive fatty acid oxidation induces muscle atrophy in cancer cachexia. Nature Medicine. 2016;22(6):666-671
  8. 8. Chen L, Min J, Wang F. Copper homeostasis and cuproptosis in health and disease. Signal Transduction and Targeted Therapy. 2022;7(1):378
  9. 9. Ramos D et al. Mechanism of copper uptake from blood plasma ceruloplasmin by mammalian cells. PLoS One. 2016;11(3):e0149516
  10. 10. Roberts EA, Sarkar B. Liver as a key organ in the supply, storage, and excretion of copper. The American Journal of Clinical Nutrition. 2008;88(3):851S-854S
  11. 11. Knutson MD. Steap proteins: Implications for iron and copper metabolism. Nutrition Reviews. 2007;65(7):335-340
  12. 12. Zhou B, Gitschier J. hCTR1: A human gene for copper uptake identified by complementation in yeast. Proceedings of the National Academy of Sciences of the United States of America. 1997;94(14):7481-7486
  13. 13. Hellman NE, Gitlin JD. Ceruloplasmin metabolism and function. Annual Review of Nutrition. 2002;22:439-458
  14. 14. Glerum DM, Shtanko A, Tzagoloff A. Characterization of COX17, a yeast gene involved in copper metabolism and assembly of cytochrome oxidase. The Journal of Biological Chemistry. 1996;271(24):14504-14509
  15. 15. Bertinato J, L'Abbe MR. Copper modulates the degradation of copper chaperone for Cu,Zn superoxide dismutase by the 26 S proteosome. The Journal of Biological Chemistry. 2003;278(37):35071-35078
  16. 16. Yang D et al. Copper chaperone antioxidant 1: Multiple roles and a potential therapeutic target. Journal of Molecular Medicine (Berlin, Germany). 2023;101(5):527-542
  17. 17. La Fontaine S, Mercer JF. Trafficking of the copper-ATPases, ATP7A and ATP7B: Role in copper homeostasis. Archives of Biochemistry and Biophysics. 2007;463(2):149-167
  18. 18. Lopez PD et al. Low skeletal muscle mass independently predicts mortality in patients with chronic heart failure after an acute hospitalization. Cardiology. 2019;142(1):28-36
  19. 19. Zhou X et al. Reversal of cancer cachexia and muscle wasting by ActRIIB antagonism leads to prolonged survival. Cell. 2010;142(4):531-543
  20. 20. Baehr LM et al. SnapShot: Skeletal muscle atrophy. Cell. 2022;185(9):1618-1618 e1
  21. 21. Dutt V et al. Skeletal muscle atrophy: Potential therapeutic agents and their mechanisms of action. Pharmacological Research. 2015;99:86-100
  22. 22. Dascalu AM et al. Serum levels of copper and zinc in diabetic retinopathy: Potential new therapeutic targets (review). Experimental and Therapeutic Medicine. 2022;23(5):324
  23. 23. Tsang T, Davis CI, Brady DC. Copper biology. Current Biology. 2021;31(9):R421-R427
  24. 24. Giachino A et al. Synthetic biology approaches to copper remediation: Bioleaching, accumulation and recycling. FEMS Microbiology Ecology. 2021;97(2):fiaa249
  25. 25. Vest KE et al. Dynamic changes in copper homeostasis and post-transcriptional regulation of Atp7a during myogenic differentiation. Metallomics. 2018;10(2):309-322
  26. 26. Cobine PA, Moore SA, Leary SC. Getting out what you put in: Copper in mitochondria and its impacts on human disease. 1868;2021(1):118867
  27. 27. Turski ML et al. A novel role for copper in Ras/mitogen-activated protein kinase signaling. Molecular and Cellular Biology. 2012;32(7):1284-1295
  28. 28. Lutsenko S. Dynamic and cell-specific transport networks for intracellular copper ions. Journal of Cell Science. 2021;134(21):jcs240523
  29. 29. Jin X et al. Copper ions impair zebrafish skeletal myofibrillogenesis via epigenetic regulation. The FASEB Journal. 2021;35(7):e21686
  30. 30. Puig S et al. Biochemical and genetic analyses of yeast and human high affinity copper transporters suggest a conserved mechanism for copper uptake. The Journal of Biological Chemistry. 2002;277(29):26021-26030
  31. 31. Liang ZD et al. Specificity protein 1 (sp1) oscillation is involved in copper homeostasis maintenance by regulating human high-affinity copper transporter 1 expression. Molecular Pharmacology. 2012;81(3):455-464
  32. 32. Zischka H, Einer C. Mitochondrial copper homeostasis and its derailment in Wilson disease. The International Journal of Biochemistry and Cell Biology. 2018;102:71-75
  33. 33. Yu CH et al. The metal chaperone Atox1 regulates the activity of the human copper transporter ATP7B by modulating domain dynamics. The Journal of Biological Chemistry. 2017;292(44):18169-18177
  34. 34. Banci L et al. Human superoxide dismutase 1 (hSOD1) maturation through interaction with human copper chaperone for SOD1 (hCCS). Proceedings of the National Academy of Sciences of the United States of America. 2012;109(34):13555-13560
  35. 35. Southon A, Burke R, Camakaris J. What can flies tell us about copper homeostasis? Metallomics. 2013;5(10):1346-1356
  36. 36. Masaldan S et al. Copper accumulation in senescent cells: Interplay between copper transporters and impaired autophagy. Redox Biology. 2018;16:322-331
  37. 37. Hassel CA, Marchello JA, Lei KY. Impaired glucose tolerance in copper-deficient rats. The Journal of Nutrition. 1983;113(5):1081-1083
  38. 38. Krishnamoorthy L et al. Copper regulates cyclic-AMP-dependent lipolysis. Nature Chemical Biology. 2016;12(8):586-592
  39. 39. Leoni V et al. Glucose accelerates copper- and ceruloplasmin-induced oxidation of low-density lipoprotein and whole serum. Free Radical Research. 2002;36(5):521-529
  40. 40. Yang Y, Liao Z, Xiao Q. Metformin ameliorates skeletal muscle atrophy in Grx1 KO mice by regulating intramuscular lipid accumulation and glucose utilization. Biochemical and Biophysical Research Communications. 2020;533(4):1226-1232
  41. 41. Hitachi K, Nakatani M, Tsuchida K. Myostatin signaling regulates Akt activity via the regulation of miR-486 expression. The International Journal of Biochemistry and Cell Biology. 2014;47:93-103
  42. 42. Swaminathan AB, Gohil VM. The role of COA6 in the mitochondrial copper delivery pathway to cytochrome c oxidase. Biomolecules. 2022;12(1):125
  43. 43. Tilokani L et al. Mitochondrial dynamics: Overview of molecular mechanisms. Essays in Biochemistry. 2018;62(3):341-360
  44. 44. Chen H et al. Mitochondrial fusion is required for mtDNA stability in skeletal muscle and tolerance of mtDNA mutations. Cell. 2010;141(2):280-289
  45. 45. Yoo SM, Jung YK. A molecular approach to mitophagy and mitochondrial dynamics. Molecules and Cells. 2018;41(1):18-26
  46. 46. Zhang L et al. UNC-51-like kinase 1: From an autophagic initiator to multifunctional drug target. Journal of Medicinal Chemistry. 2018;61(15):6491-6500
  47. 47. Xu X, Lai Y, Hua ZC. Apoptosis and apoptotic body: Disease message and therapeutic target potentials. Bioscience Reports. 2019;39(1):BSR20180992
  48. 48. D'Arcy MS. Cell death: A review of the major forms of apoptosis, necrosis and autophagy. Cell Biology International. 2019;43(6):582-592
  49. 49. Du J et al. Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions. The Journal of Clinical Investigation. 2004;113(1):115-123
  50. 50. Guo J et al. Copper induces apoptosis through endoplasmic reticulum stress in skeletal muscle of broilers. Biological Trace Element Research. 2020;198(2):636-643
  51. 51. Kahlson MA, Dixon SJ. Copper-induced cell death. Science. 2022;375(6586):1231-1232
  52. 52. Tsvetkov P et al. Copper induces cell death by targeting lipoylated TCA cycle proteins. Science. 2022;375(6586):1254-1261

Written By

Zhen Shen, Sunfeng Pan, Fengjie Wu, Kaitao Luo and Yanbo Shi

Submitted: 07 February 2025 Reviewed: 12 February 2025 Published: 25 March 2025

© The Author(s). Licensee IntechOpen. This content is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Your cart

Discounts available on purchase of multiple copies. View rates

Subtotal £0.00

Local taxes (VAT) are calculated in later steps, if applicable.

Support: orders@intechopen.com