Mitochondrial Copper Homeostasis in Diabetic Kidney Disease: Emerging Links Between Metabolic Stress, Respiratory Chain Dysfunction, and Renal Injury

Abstract

Diabetic kidney disease (DKD) remains a leading cause of chronic kidney disease and kidney failure despite major therapeutic advances. Current disease-modifying therapies reduce, but do not abolish, residual risk, indicating that additional cell-intrinsic mechanisms convert diabetic metabolic stress into progressive renal injury. Mitochondrial dysfunction has emerged as a central node in this process because renal tubular cells, podocytes, endothelial cells, and mesangial cells rely on mitochondrial oxidative phosphorylation to sustain transport, filtration-barrier integrity, redox control, and adaptive repair. Copper is an essential metal cofactor for cytochrome c oxidase (COX; respiratory chain complex IV), and mitochondrial copper delivery is required for COX assembly, electron transfer, proton-pumping capacity, and ATP generation. This review integrates established evidence from DKD pathobiology, mitochondrial biology, and copper homeostasis with emerging and indirect evidence to examine mitochondrial copper dyshomeostasis as a potential mechanistic link in DKD. Established findings support mitochondrial dysfunction in DKD and the essential role of copper in COX/complex IV assembly and respiratory-chain function. We further propose that diabetic metabolic stress may disturb copper uptake, intracellular trafficking, mitochondrial copper delivery, or copper use by COX, resulting in functional mitochondrial copper depletion or misallocation. Such disturbances could contribute to COX dysfunction, oxidative phosphorylation failure, ATP depletion, mtROS amplification, impaired mitochondrial quality control, and subsequent renal injury. However, direct DKD-specific evidence demonstrating impaired mitochondrial copper trafficking or functional mitochondrial copper depletion remains limited, and this mechanism should therefore be regarded as a testable hypothesis rather than an established pathogenic pathway. We also distinguish this mechanism from cuproptosis, examine intersections with ferroptosis, and outline biomarker, multi-omics, metallomics, and therapeutic opportunities.

Introduction

Diabetic kidney disease is one of the most common and consequential microvascular complications of diabetes and is a major cause of CKD, kidney failure, cardiovascular events, and premature mortality worldwide.^1,2 Contemporary guidance emphasizes early risk stratification, RAS blockade in albuminuric disease, SGLT2 inhibitors, GLP-1 receptor agonists, nonsteroidal mineralocorticoid receptor antagonists, blood-pressure control, and metabolic management.^3,4

DKD is not a simple consequence of hyperglycemia. It reflects the interaction of hyperglycemia, AGEs, lipotoxicity, glomerular hyperfiltration, increased proximal tubular solute transport, inflammation, oxidative stress, hemodynamic stress, and microvascular hypoxia.^5,6 SGLT2-centered physiology illustrates this metabolic burden: filtered glucose and sodium increase proximal tubular transport work, and maladaptive tubular-glomerular feedback contributes to glomerular hypertension and energy mismatch.⁷ Distinguishing DKD from nondiabetic CKD is also important because DKD often combines glomerular, tubular, vascular, and interstitial lesions rather than a single compartmental injury.⁸ In this setting, proximal tubular epithelial cells are especially vulnerable because they require high ATP flux to sustain sodium, glucose, protein, and organic solute handling.

Mitochondria are therefore not passive bystanders in DKD. Human metabolomics revealed a signature consistent with mitochondrial dysfunction in DKD,⁹ and renal mitochondrial energetics are increasingly recognized as a determinant of tubular work, oxygen consumption, and susceptibility to hypoxic or toxic stress.¹⁰ Reviews and experimental studies consistently describe reduced OXPHOS capacity, altered substrate use, mtROS accumulation, mtDNA injury, impaired mitochondrial dynamics, defective mitophagy, and disturbed mitochondrial biogenesis in DKD.^11–13 These abnormalities are highly relevant to diabetic tubulopathy because transport demand increases at the same time that mitochondrial energy supply may fall.¹⁴ Both glucose-driven and non-glucose stressors can converge on mitochondrial injury, suggesting that mitochondrial failure is a common pathway rather than a late nonspecific event.^15,16

Copper biology provides a focused and under-integrated entry point into this mitochondrial problem. Copper is required for several cuproenzymes, but within mitochondria its most direct bioenergetic role is in cytochrome c oxidase (COX/complex IV), the terminal ETC complex responsible for electron transfer to molecular oxygen and a major contributor to proton-gradient formation and ATP production. This direct dependence on copper provides the principal rationale for focusing on COX/complex IV in this review. Cellular copper must be acquired, buffered, escorted by chaperones, delivered to organelles, and inserted into target enzymes with minimal free copper exposure.^17–19 CTR1/SLC31A1 mediates major copper uptake across cellular membranes,²⁰ ATP7A/ATP7B and chaperone systems support intracellular distribution and export,^19,21 and mitochondrial COX maturation depends on specialized copper-delivery and assembly machinery.²² Importantly, copper dysregulation in DKD should not be reduced to whether serum or total tissue copper is increased or decreased. Systemic copper imbalance, renal tissue copper redistribution, mitochondrial copper availability, and cuproptosis represent related but mechanistically distinct levels of copper biology. Systemic copper indices reflect whole-body or circulating copper status; tissue redistribution refers to altered renal copper abundance or localization; mitochondrial copper availability concerns whether copper can be appropriately delivered to and utilized by mitochondrial cuproenzymes such as COX; and cuproptosis represents a distinct copper-dependent regulated cell-death process.

The central knowledge gap is that, although diabetes and DKD have been associated with altered copper indices and copper-related biomarkers,²³ and mitochondrial dysfunction is well established in DKD, direct evidence that diabetic metabolic stress disrupts mitochondrial copper trafficking, copper delivery to COX, COX assembly, or complex IV activity in defined renal cell types remains limited.^24,25 Accordingly, this review examines whether mitochondrial copper dyshomeostasis, including the possibility of functional mitochondrial copper depletion, may provide a mechanistic link between diabetic metabolic stress and respiratory-chain dysfunction. We propose that impaired mitochondrial copper availability could compromise COX/complex IV function, thereby contributing to OXPHOS impairment, ATP depletion, mtROS amplification, defective mitochondrial quality control, and renal cell injury. This sequence is presented as a biologically plausible and testable hypothesis rather than an established DKD pathway. The objective of this review is therefore to integrate evidence from DKD pathobiology, mitochondrial bioenergetics, and copper biology to evaluate the mitochondrial copper–COX–respiratory-chain relationship in DKD. Its novelty lies in shifting attention from bulk systemic or tissue copper abnormalities and cuproptosis toward compartment-specific mitochondrial copper availability and COX-dependent bioenergetic dysfunction. The overall conceptual framework linking diabetic metabolic stress, mitochondrial copper dyshomeostasis, respiratory-chain dysfunction, and renal injury is summarized in Figure 1.

Figure 1.

Proposed framework linking diabetic metabolic stress, mitochondrial copper dyshomeostasis, and DKD progression. Diabetic metabolic stressors, including hyperglycemia, advanced glycation end-products (AGEs), lipotoxicity, inflammation, oxidative stress, glomerular hyperfiltration, and tubular transport overload, may disturb cellular copper trafficking and alter mitochondrial copper availability. Copper uptake through CTR1 and intracellular handling involving ATOX1 and ATP7A/B are shown together with the mitochondrial copper–COX machinery, including COX17, SCO1/2, and COX11. We propose that disturbed mitochondrial copper handling may result in functional mitochondrial copper depletion, thereby impairing COX/complex IV function and potentially contributing to OXPHOS failure, ATP depletion, mtROS amplification, and mitochondrial quality-control failure. These mitochondrial abnormalities may in turn contribute to renal cell injury, inflammation, fibrosis, and DKD progression. Solid arrows indicate established or directly supported copper-handling relationships, whereas dotted arrows denote proposed or incompletely validated relationships in DKD; bidirectional dashed arrows indicate reciprocal interactions among renal injury, inflammation, and fibrosis. Functional mitochondrial copper depletion is presented here as a testable hypothesis rather than an established mechanism in DKD.

This article is a narrative review. Relevant literature was identified through targeted searches of PubMed/MEDLINE and the Web of Science Core Collection using combinations of terms related to diabetic kidney disease, copper homeostasis, mitochondrial copper, cytochrome c oxidase/complex IV, copper chaperones, mitochondrial dysfunction, and cuproptosis. Reference lists of key original studies and reviews were also screened for additional relevant publications. Priority was given to original mechanistic studies, DKD-specific evidence, and studies directly addressing mitochondrial copper trafficking or COX biology. Searches were last updated on 1 July 2026. Because this was a narrative rather than systematic review, no PRISMA-based study-selection procedure or formal risk-of-bias assessment was performed. Throughout the review, evidence is distinguished as direct DKD-specific evidence, kidney-relevant supportive evidence, or mechanistic/hypothesis-generating evidence, and proposed relationships are identified as such where direct validation is lacking.

Copper Homeostasis and Mitochondrial Copper Trafficking

Cellular Copper Handling

Copper is essential but potentially toxic because redox-active copper can catalyze damaging reactions if it accumulates in poorly buffered pools. Mammalian cells therefore maintain extremely low free copper and depend on coordinated uptake, buffering, compartmentalization, chaperone-mediated delivery, and export.^17,18 CTR1/SLC31A1 is a key high-affinity copper importer, and mechanistic work has shown that its structure and trafficking help determine copper entry across cellular membranes.²⁰ Once inside the cell, copper is distributed through a network that includes ATP7A and ATP7B for secretory-pathway delivery and export, ATOX1 for copper transfer to P-type ATPases, CCS for metallation of Cu/Zn-superoxide dismutase, and specialized mitochondrial copper-delivery factors.^19,22

The discovery of ATX1/ATOX1 as a soluble copper chaperone established the principle that copper is not simply dissolved and diffused through cytosol but is handed to target systems through ligand-exchange reactions.²⁶ CCS similarly showed how a dedicated chaperone supports SOD1 metallation and antioxidant defense.²⁷ Structural and biochemical studies of copper chaperones further clarified that copper transfer depends on metal-binding motifs, redox-sensitive thiols, and protein-protein recognition.²⁸ These mechanisms matter for DKD because diabetic stress changes redox tone, protein glycation, inflammatory signaling, and mitochondrial membrane potential, any of which could alter copper delivery without necessarily changing total tissue copper.

Mitochondrial Copper Trafficking and COX Assembly

Mitochondrial copper is primarily discussed in relation to COX/complex IV, although mitochondrial copper pools may participate in additional processes. COX contains metal centers that require precise cofactor insertion during assembly. Early genetic studies identified human respiratory-chain assembly genes including SCO1, COX15, and COX11,²⁹ and COX17 was localized as a protein involved in mitochondrial copper transport.³⁰ Yeast studies showed that SCO1 and SCO2 could suppress mitochondrial copper-recruitment defects,³¹ while assembly-focused reviews placed these proteins within the broader process of COX biogenesis.³² Human SCO1 and SCO2 were subsequently shown to have independent but cooperative functions in copper delivery to COX.³³

Copper transfer within the COX assembly pathway is not a single-step event. Cox17 can transfer copper to Sco1 and Cox11 in yeast systems,³⁴ and human Sco1 and Sco2 function as copper-binding proteins.³⁵ Reviews of mitochondrial copper trafficking describe a relay involving COX17, SCO1, SCO2, COX11, and other assembly factors that support maturation of the CuA and CuB centers required for complex IV activity.^36,37 Functional experiments add weight to this model: COX17 knockdown impairs COX assembly and supramolecular organization,³⁸ and redox regulation can influence COX biogenesis through ROS-sensitive assembly factors.³⁹ SLC25A3, historically known as a mitochondrial phosphate carrier, has also been identified as a mitochondrial copper transporter required for COX biogenesis.⁴⁰ More recent structural and biochemical work has detailed coordination of metal-center biogenesis in human COX,⁴¹ while newer reviews emphasize redox control of COX assembly and function.⁴²

These data define the central mechanism of this review: copper trafficking enables COX assembly; COX assembly enables complex IV activity; complex IV activity enables efficient electron transfer, proton pumping, and ATP generation. When copper delivery is impaired, the immediate pathological consequence is not simply “abnormal copper”, but reduced respiratory-chain capacity. The relevance to kidney disease is underscored by evidence that COX17 can restrict renal fibrosis by preserving mitochondrial copper homeostasis and complex IV activity in renal experimental systems.²⁴ Although this study was not designed as a DKD-specific proof of functional mitochondrial copper depletion, it provides a kidney-relevant precedent for linking COX17, mitochondrial copper, complex IV, and fibrotic injury. The cellular copper-trafficking network and the specialized delivery of copper to mitochondrial COX/complex IV assembly machinery are illustrated in Figure 2.

Figure 2.

Mitochondrial copper trafficking and COX/complex IV assembly. Cellular copper enters primarily through CTR1/SLC31A1 and is subsequently distributed through specialized intracellular copper-handling pathways. In the cytosol, ATOX1 transfers copper to ATP7A/ATP7B at the Golgi/trans-golgi network to support copper distribution, secretion, and export, whereas CCS facilitates copper delivery to SOD1 and antioxidant defense. Mitochondrial copper availability involves distinct transport and delivery mechanisms, including SLC25A3 and the COX-specific copper-trafficking machinery. COX17, SCO1/SCO2, COX11, together with other COX assembly factors, participate in copper-dependent COX metallation, maturation, and complex IV assembly. Diabetic ROS, advanced glycation end-products (AGEs), loss of mitochondrial membrane potential, and inflammatory signaling may perturb components of this machinery, although direct DKD-specific evidence remains limited. Defective mitochondrial copper delivery could therefore impair COX maturation and compromise complex IV assembly and function. The DKD-specific effects shown in the upper panel should be interpreted as proposed or incompletely validated relationships rather than established causal pathways.

Functional Mitochondrial Copper Depletion

Functional mitochondrial copper depletion is proposed here to describe a state in which total cellular or tissue copper may be normal or elevated, but mitochondria cannot access, deliver, or use sufficient copper for COX assembly and respiratory-chain function. This concept differs from systemic copper deficiency, copper overload, and cuproptosis. It focuses on compartmentalized copper bioavailability and copper-dependent enzymatic function rather than on bulk elemental abundance. The biological rationale for this construct derives from established mechanisms of intracellular copper trafficking and copper-dependent COX metallation and assembly, whereas copper-excess-associated cell death represents a mechanistically distinct process. Accordingly, functional mitochondrial copper depletion should be regarded as an operational and testable hypothesis in DKD rather than an established pathophysiological entity.^{17–22,25,36–42}

This distinction is particularly important in diabetes and DKD. Serum copper, urinary copper, ceruloplasmin, and total renal copper reflect systemic copper status, renal handling, protein binding, inflammation, or tissue redistribution but cannot by themselves establish whether the mitochondrial COX assembly pathway receives sufficient usable copper. Accordingly, increased urinary copper excretion or renal copper redistribution could theoretically coexist with inadequate mitochondrial copper availability, although this specific state has not yet been directly demonstrated in DKD. Experimental and clinical literature also indicates that altering copper availability may have context-dependent effects; reducing redox-active copper may be beneficial in some settings, whereas excessive or misdirected copper removal could theoretically compromise copper-dependent respiratory enzymes.^{23,25,36–42}

The working model is therefore one of copper misallocation: diabetic metabolic stress may shift copper distribution, damage copper chaperones, alter transporter expression, or impair mitochondrial import, producing COX dysfunction despite no simple systemic copper deficiency. This model is testable. It predicts that DKD tissue or high-glucose renal cells will show altered COX17/SCO1/SCO2/COX11/SLC25A3 expression or function, lower COX assembly or activity, reduced OCR and ATP, increased mtROS, and renal injury markers; it also predicts that restoring mitochondrial copper delivery, rather than indiscriminate copper supplementation, would improve bioenergetic readouts.

The working model is therefore one of copper misallocation: diabetic metabolic stress may shift copper distribution, alter redox-sensitive copper-chaperone or COX-assembly-factor function, change transporter expression, or perturb mitochondrial copper handling, potentially compromising COX function without requiring systemic copper deficiency.^{17–22,26–28,39–42} This model generates testable predictions rather than summarizing findings already demonstrated in DKD. Specifically, DKD tissue or diabetic renal-cell models may exhibit alterations in COX17, SCO1, SCO2, COX11, or SLC25A3 expression or function together with impaired COX assembly or activity, reduced OCR and ATP production, increased mtROS, and renal injury markers. The model further predicts that selective restoration of mitochondrial copper delivery would improve bioenergetic readouts more effectively than indiscriminate copper supplementation.^24,36–42

Copper Dysregulation in Diabetes and Diabetic Kidney Disease

Copper Imbalance in Diabetes

Diabetes is associated with altered trace-element metabolism, but the literature does not support a single uniform direction for copper changes. A narrative review of copper in diabetes and DKD concluded that copper may contribute to oxidative stress, inflammation, endothelial dysfunction, and renal injury, while also noting heterogeneity across studies and biological compartments.²³ Older and more recent clinical studies have reported alterations in urinary copper excretion, serum and urinary copper/zinc status, serum ceruloplasmin, Cu/Zn ratios, and urinary trace-element profiles in diabetes and DKD, although these indices can be influenced by inflammation, albuminuria, kidney function, nutritional status, and assay context.^43–50

Urinary copper excretion was reported to be increased in type 2 diabetic patients with nephropathy,⁴³ and combined serum/urinary copper and zinc analyses in Chinese participants suggested that copper-zinc balance may differ across prediabetes, diabetes, and complication states.⁴⁴ Serum trace-element studies also associated altered copper, iron, magnesium, and zinc with diabetes-related proteinuria,⁴⁵ and a South Indian case-control study found altered copper and magnesium status in diabetic nephropathy.⁴⁶ Microalbuminuria has been correlated with urinary copper in type 2 diabetes,⁴⁷ while serum ceruloplasmin predicted diabetic nephropathy progression in Korean men with type 2 diabetes.⁴⁸ More recent work linked the Cu/Zn ratio under inflammatory conditions with DKD in the Asahi Diabetes Complications Study,⁴⁹ and a 2024 cross-sectional analysis implicated urinary trace elements in DKD.⁵⁰ These findings support copper dyshomeostasis, but not a simplistic copper-deficient or copper-overloaded phenotype.

Metabolic Stress as a Driver of Copper Redistribution

Diabetic metabolic stress could plausibly perturb copper handling at multiple levels, although direct DKD-specific evidence remains limited. Hyperglycemia, AGEs, lipotoxicity, inflammation, oxidative stress, and mitochondrial dysfunction are established components of the diabetic renal milieu.^5,6,12–16 At the same time, intracellular copper transfer depends on metal-binding motifs, redox-sensitive thiols, and protein–protein interactions, and COX biogenesis is susceptible to redox regulation through ROS-sensitive assembly machinery.^{26–28,39,42} Accordingly, diabetic redox and inflammatory stress could potentially alter copper-chaperone or COX-assembly-factor function and thereby reduce copper-transfer efficiency. Loss of mitochondrial membrane potential and broader mitochondrial injury may further perturb the intramitochondrial environment required for efficient COX biogenesis, although a copper-specific effect of this process has not been directly demonstrated in DKD.^{10–16,39–42}

Animal evidence supports the plausibility of renal copper redistribution during diabetes. Streptozotocin-induced diabetic rats showed copper accumulation in soluble and particulate fractions of renal cortex.⁵¹ Copper-selective chelation ameliorated diabetes-evoked renal fibrosis and albuminuria and suppressed pathogenic TGF-beta activation in diabetic rat kidneys.⁵² Proteomic profiling after copper chelation identified renal targets associated with reversal of diabetic nephropathy in rats,⁵³ and trientine combined with RAS blockade improved glomerular morphology in hypertensive experimental diabetic nephropathy.⁵⁴ These studies are important but should be interpreted carefully: beneficial effects of copper chelation may reflect reduced redox-active copper or altered fibrotic signaling, not necessarily correction of mitochondrial copper delivery.

Copper Dysregulation in DKD

Copper-related DKD research has recently shifted toward transcriptomic and cell-death signatures. Bioinformatic studies have proposed copper-related biomarkers and molecular mechanisms in diabetic nephropathy,⁵⁵ identified immune and cuproptosis-related genes by WGCNA and machine learning,⁵⁶ and described cuproptosis-related ceRNA networks and immune infiltration in DKD.⁵⁷ These studies provide hypothesis-generating maps, but they do not directly measure mitochondrial copper pools, COX metalation, or complex IV activity.

The key gap is therefore mechanistic resolution. Existing DKD copper data are strongest for altered systemic or urinary copper indices, renal copper redistribution, and copper-associated gene signatures. They are weaker for the specific axis of mitochondrial copper delivery, COX assembly, and respiratory-chain activity. The next stage should move from copper abundance to copper function: subcellular copper measurement, native gel analysis of respiratory complexes, COX activity assays, copper-chaperone redox-state analysis, Seahorse OCR profiling, ATP quantification, and mtROS imaging in defined renal cell types.

Mitochondrial Copper Homeostasis and Respiratory Chain Dysfunction in DKD

COX/complex IV Dysfunction as the Central Mechanistic Bridge

COX/complex IV is the most direct bioenergetic target of mitochondrial copper dyshomeostasis. DKD studies repeatedly implicate reduced mitochondrial oxidative metabolism and altered respiratory-chain activity.^9,11 Because complex IV lies at the terminal step of the ETC, impaired COX assembly or activity can increase upstream electron pressure, compromise proton-gradient formation, reduce ATP production, and promote mtROS. In a diabetic kidney, where proximal tubular transport and glomerular adaptation increase energy demand, this defect is expected to have larger consequences than in cells with lower respiratory dependence.

Several DKD observations are compatible with this model. Human metabolomic data support mitochondrial dysfunction as a DKD signature.⁹ Diabetic tubulopathy is characterized by mitochondrial dysfunction, altered substrate use, and energy stress.¹⁴ Pharmacological studies targeting mitochondrial pathways, including Nrf2/PINK1 signaling and PGC-1alpha-regulated energy homeostasis, can attenuate renal injury in diabetic models.^58,59 These studies do not prove copper-dependent COX failure, but they show that mitochondrial energy machinery is a disease-relevant and modifiable node.

ATP Depletion and Tubular Energy Mismatch

The proximal tubule provides the clearest physiological context for functional mitochondrial copper depletion. It has dense mitochondria and a high ATP requirement for sodium reabsorption, glucose reabsorption, protein uptake, acid-base handling, and organic solute transport. The diabetic state increases filtered glucose and sodium load and enhances proximal tubular reabsorptive work, creating a demand-side rise in ATP use.^7,60 If mitochondrial copper delivery is impaired, COX dysfunction would reduce ATP supply at the same time that transport demand rises.

The downstream phenotype may include lower OCR, ATP depletion, mitochondrial membrane depolarization, impaired FAO, increased KIM-1/NGAL, tubular dedifferentiation, inflammation, and fibrosis. Defective FAO in renal tubular epithelial cells is sufficient to promote kidney fibrosis,⁶¹ while DKD-specific lipotoxicity reviews and lipid-metabolism studies emphasize how lipid overload and mitochondrial stress interact in diabetic kidneys.^62,63 KIM-1 has also been linked to fatty-acid uptake by renal tubular cells and progressive DKD, connecting injury-marker expression to tubular metabolic remodeling.⁶⁴

SGLT2 inhibitors indirectly support the importance of tubular energy demand. Their renal benefits cannot be explained only by glucose lowering; they alter proximal tubular workload, tubuloglomerular feedback, oxygen consumption, and metabolic signaling.^7,60 If functional mitochondrial copper depletion contributes to COX failure, SGLT2 inhibition may reduce the energetic stress imposed on compromised mitochondria, even without directly correcting copper trafficking.

mtROS Amplification and Mitochondrial Damage

Respiratory-chain impairment involving COX may increase upstream redox pressure and favor electron leakage and mtROS generation. Increased mtROS can damage mtDNA, membrane lipids, and respiratory-chain proteins, while redox-sensitive COX-assembly and copper-trafficking machinery may also be affected.^{10–13,39,42} We therefore propose that mtROS could further compromise mitochondrial copper handling and COX competence, creating a potential self-reinforcing bioenergetic injury loop. DKD studies strongly support mitochondrial oxidative stress and mtROS as contributors to renal injury,^12,13,65,66 but the copper-dependent initiation and reinforcement of this feedback loop remain to be directly demonstrated in DKD.

DKD literature strongly supports mtROS as an injurious amplifier. Mitochondrial dysfunction and oxidative stress are repeatedly implicated in DKD progression,^12,13 and CD36 was shown to promote NLRP3 inflammasome activation through the mtROS pathway in renal tubular epithelial cells of diabetic kidneys.⁶⁵ Reviews of mitochondrial inflammation in DKD further connect mitochondrial injury, mtROS, mtDNA release, and inflammatory signaling.⁶⁶ This makes mtROS a plausible convergence point between copper-dependent respiratory-chain failure and downstream inflammatory damage. As illustrated in Figure 3, mitochondrial copper dyshomeostasis and mtROS may form a self-amplifying bioenergetic injury loop that progressively aggravates COX dysfunction and mitochondrial damage.

Figure 3.

Proposed bioenergetic injury loop associated with mitochondrial copper dyshomeostasis in DKD. Functional mitochondrial copper depletion is proposed to reduce the availability of usable mitochondrial copper for COX/complex IV assembly and function. If present in DKD, impaired COX function could compromise electron transfer and oxidative phosphorylation, leading to reduced ATP production and increased electron leakage or redox pressure. Increased electron leakage may promote mitochondrial ROS (mtROS) amplification, resulting in mtDNA damage, membrane injury, respiratory-protein damage, and dysfunction of copper-trafficking machinery. These alterations could further disrupt mitochondrial copper trafficking and impair COX function, thereby forming a potential self-reinforcing mitochondrial injury loop. The initiation and copper-dependent reinforcement of this loop in DKD remain hypothetical and require direct experimental validation.

Mitochondrial Quality-Control Failure

Respiratory-chain impairment also affects mitochondrial dynamics and mitophagy. ATP depletion, membrane-potential loss, and mtROS can alter DRP1-mediated fission, MFN/OPA1-dependent fusion, PINK1/Parkin signaling, and selective autophagic clearance. Kidney autophagy is essential for homeostasis and stress adaptation,⁶⁷ and DKD reviews describe loss of mitochondrial quality control as a major contributor to disease progression.⁶⁸ Mitochondrial dynamics reviews further emphasize that excessive fragmentation and defective fusion can perpetuate renal cell injury in DKD.⁶⁹

Experimental data support this extension. MitoQ improved tubular injury mediated by mitophagy through Nrf2/PINK1.⁵⁸ Pyruvate kinase M2 activation protected against diabetic glomerular pathology and mitochondrial dysfunction.⁷⁰ Berberine inhibited DRP1-mediated mitochondrial fission and dysfunction in podocytes⁶² and promoted PGC-1alpha-regulated mitochondrial energy homeostasis in DKD.⁵⁹ The broader role of PGC-1alpha and mitochondrial biogenesis in kidney disease supports this bioenergetic interpretation.⁷¹ AMPK activation alleviated renal tubulointerstitial fibrosis by activating mitophagy,⁷² and PACS-2 improved tubular injury by facilitating ER-mitochondria contact and mitophagy.⁷³ AKAP1 contributed to impaired mtDNA replication and podocyte mitochondrial dysfunction in DKD.⁷⁴ More recent work shows that preserving tubular mitochondrial quality control through YAP1 mitigates DKD.⁷⁵ These studies collectively position quality-control failure downstream of respiratory dysfunction and upstream of progressive renal injury.

Potential Renal Cell-Specific Consequences of Mitochondrial Copper Dyshomeostasis

Proximal Tubular Epithelial Cells

Proximal tubular epithelial cells are a biologically plausible major site for the proposed mechanism because of their high mitochondrial density, heavy transport workload, FAO dependence, and exposure to filtered glucose, albumin, lipids, and toxins. However, direct evidence demonstrating impaired mitochondrial copper trafficking in proximal tubular cells in DKD remains limited. The following model is therefore inferred from established diabetic tubulopathy, mitochondrial biology, and copper-dependent COX function.^14,15 If mitochondrial copper availability becomes insufficient, COX activity and ATP production may decline, whereas mtROS, FAO impairment, defective mitophagy, and tubular injury responses may increase.^61,64

This tubular model also explains why copper indices can appear inconsistent. Tubular injury may increase urinary copper excretion or ceruloplasmin-containing vesicle release, whereas mitochondrial copper use may still be insufficient. Urinary exosomal studies show that exosomes can better capture kidney-specific protein alterations than whole urine in DKD,⁷⁶ and urine exosomal ceruloplasmin has been proposed as an early marker of underlying kidney disease.⁷⁷ These data support the idea that urinary copper-related proteins may reflect tubular stress, transport, or vesicle biology rather than mitochondrial copper availability alone.

Podocytes

Podocytes require intact mitochondrial metabolism to maintain actin cytoskeleton organization, slit diaphragm integrity, autophagy, and adaptive stress responses. Direct evidence linking podocyte mitochondrial copper trafficking to DKD is limited, so strong causal statements should be avoided. Nevertheless, mitochondrial dysfunction is a recognized component of podocyte injury in DKD.⁷⁸ Astragaloside IV attenuated podocyte apoptosis by improving mitochondrial dysfunction through Nrf2-ARE/TFAM signaling,⁷⁹ and RIPK3 promoted albuminuria in diabetic podocytopathy through PGAM5-DRP1 signaling and mitochondrial dysfunction.⁸⁰ These studies suggest that if functional mitochondrial copper depletion impairs COX in podocytes, downstream effects could include mtROS accumulation, cytoskeletal injury, nephrin/podocin disruption, foot-process effacement, and albuminuria.

Single-cell and glomerular studies add cell-type resolution. Experimental diabetic glomeruli show dynamic single-cell RNA changes across glomerular cell populations,⁸¹ and early human diabetic nephropathy has been mapped at single-cell resolution.⁸² These approaches should be leveraged to determine whether COX assembly factors, copper transporters, and mitochondrial stress signatures are altered in podocytes at specific stages of DKD.

Glomerular Endothelial Cells and Mesangial Cells

Glomerular endothelial cells are specialized components of the glomerular filtration barrier characterized by fenestrations and a glycocalyx-rich luminal surface. In DKD, glomerular endothelial injury is associated with oxidative stress, glycocalyx disruption, altered NO bioavailability, and increased filtration-barrier permeability.⁸³ Experimental evidence further demonstrates mitochondrial oxidative stress and dysfunction in glomerular endothelial cells during DKD, supporting mitochondrial injury as a biologically relevant endothelial mechanism.⁸⁴ However, direct evidence linking mitochondrial copper trafficking or copper-dependent COX dysfunction to these endothelial phenotypes remains lacking. Accordingly, the copper–COX framework proposed here predicts that impaired mitochondrial copper availability could exacerbate endothelial bioenergetic and redox stress; this relationship should currently be regarded as hypothesis-generating rather than established in DKD.

Mesangial cells contribute to matrix deposition, glomerular expansion, and sclerosis. Mitochondrial ROS and metabolic stress can activate TGF-beta/Smad, NF-kappaB, MAPK, and extracellular matrix programs. MAGI2 was reported to ameliorate podocyte apoptosis through TGF-beta-Smad3/nephrin communication,⁸⁵ underscoring the relevance of TGF-beta-linked glomerular signaling in diabetic injury. In mesangial and stromal compartments, functional mitochondrial copper depletion would be expected to amplify ROS-sensitive profibrotic signaling rather than act as an isolated lesion. The cell-specific consequences of mitochondrial copper dyshomeostasis in proximal tubular cells, podocytes, glomerular endothelial cells, and mesangial cells are summarized in Figure 4. Thus, the endothelial and mesangial consequences proposed here remain hypothesis-generating and require direct validation of mitochondrial copper handling and COX function in these cell types.

Figure 4.

Proposed renal cell-specific consequences of mitochondrial copper dyshomeostasis in DKD. Mitochondrial copper dyshomeostasis is proposed to impair COX/complex IV function, reduce ATP production, and increase mitochondrial ROS (mtROS), although direct evidence for these copper-dependent events in specific renal cell populations remains limited. In proximal tubular cells, impaired COX-dependent bioenergetics could contribute to reduced ATP availability, transport dysfunction, tubular injury-marker induction, and tubulointerstitial injury and fibrosis. In podocytes, increased mtROS and mitochondrial dysfunction may contribute to cytoskeletal and slit-diaphragm injury, nephrin/podocin disruption, foot-process injury, and albuminuria. In glomerular endothelial cells, mitochondrial dysfunction may reduce NO bioavailability and promote permeability abnormalities, whereas in mesangial cells it may favor extracellular-matrix accumulation and glomerular remodeling. These cell-specific processes may collectively contribute to tubular injury, albuminuria, fibrosis, glomerulosclerosis, and DKD progression.

Potential Convergence on Inflammation and Fibrosis

If mitochondrial copper dyshomeostasis occurs in DKD, its downstream bioenergetic and redox consequences could potentially converge on inflammatory and fibrotic pathways. COX dysfunction and mtROS can promote mtDNA release, NLRP3 inflammasome activation, NF-kappaB signaling, and cGAS-STING responses. Reviews of cGAS-STING, inflammasomes, and pyroptosis describe extensive crosstalk among these inflammatory pathways,⁸⁶ and kidney disease reviews emphasize interactions between mitophagy and inflammasomes.⁸⁷ In DKD, defective mitophagy may allow damaged mitochondria to persist and continuously release ROS and mitochondrial danger signals.

Chronic inflammation then drives fibrotic remodeling through TGF-beta/Smad, Wnt/beta-catenin, Notch, macrophage activation, and matrix accumulation. Molecular pathway reviews of DKD place inflammation and fibrosis at the center of progression.^5,6 The copper–COX hypothesis therefore proposes a potential compartment-specific bioenergetic link between diabetic metabolic stress, mitochondrial danger signaling, and fibrotic remodeling, but this sequence remains to be directly demonstrated in DKD.

Copper Dyshomeostasis Network: Cuproptosis, Ferroptosis, Biomarkers, and Therapeutic Implications

Mitochondrial copper dyshomeostasis, cuproptosis, and ferroptosis should be viewed as intersecting but non-equivalent processes. Functional mitochondrial copper depletion, as proposed in this review, represents a potential bioenergetic state characterized by insufficient usable copper for COX function. Cuproptosis instead reflects copper-excess-associated proteotoxic cell death involving lipoylated mitochondrial proteins, whereas ferroptosis is an iron-dependent lipid-peroxidation pathway. Their potential convergence lies primarily in mitochondrial dysfunction, redox imbalance, and susceptibility to cell injury rather than in a single linear copper-driven pathway.

Mitochondrial Copper Depletion vs Cuproptosis

Mitochondrial copper depletion and cuproptosis are both copper-related but mechanistically distinct. Functional mitochondrial copper depletion, as used here, means insufficient mitochondrial copper availability for COX assembly and respiratory-chain activity. Its central outputs are reduced complex IV activity, OXPHOS failure, ATP depletion, and mtROS amplification. Cuproptosis, by contrast, is a copper-induced cell-death process linked to copper binding to lipoylated TCA-cycle proteins, protein aggregation, Fe-S cluster protein loss, and proteotoxic stress.^88,89 Iron and copper can also interact with broader regulated cell-death networks, including ferroptosis and cuproptosis.⁹⁰

This distinction matters for DKD. Bioinformatic cuproptosis signatures in diabetic nephropathy are useful for hypothesis generation,^56,57 but they do not demonstrate that renal cells are dying primarily through cuproptosis. A kidney-focused copper review similarly emphasizes that copper-related renal disease spans several mechanisms rather than a single death pathway.²⁵ The present review therefore does not argue that DKD is simply cuproptosis. Instead, it emphasizes that copper dyshomeostasis may impair respiratory-chain function before, alongside, or independently of copper-induced cell death.

Interaction with Ferroptosis and Oxidative Stress

Ferroptosis is better established than cuproptosis in kidney injury and DKD. It was originally defined as an iron-dependent nonapoptotic cell-death pathway,⁹¹ later conceptualized as a regulated cell-death nexus linking metabolism, redox biology, and disease.⁹² GPX4 is a key regulator of ferroptotic lipid peroxidation,⁹³ and Gpx4 inactivation can trigger acute renal failure in mice.⁹⁴ DKD-focused reviews and experiments link ferroptosis to tubular lipid peroxidation, mitochondrial redox imbalance, GSH/GPX4 impairment, and inflammation.⁹⁵

Copper dyshomeostasis may interact with ferroptosis in several ways. COX dysfunction and mtROS can increase lipid peroxidation pressure, while disrupted Fe-S cluster biology and mitochondrial damage may sensitize cells to iron-dependent death. NAC attenuated ferroptosis in diabetic nephropathy by maintaining mitochondrial redox homeostasis through the SIRT3-SOD2/GPX4 pathway.⁹⁶ Empagliflozin reduced renal tubular ferroptosis through AMPK/NRF2 signaling,⁹⁷ and dapagliflozin alleviated DKD through HIF-1alpha/heme oxygenase-1-mediated ferroptosis regulation.⁹⁸ These studies suggest that mitochondrial redox state, tubular metabolism, ferroptosis, and antidiabetic therapies intersect. Whether copper-dependent COX dysfunction contributes upstream to ferroptotic vulnerability in DKD remains an open question requiring direct experimental testing.

Biomarker and Multi-Omics Opportunities

Systemic copper markers may be useful but insufficient. Serum copper, urinary copper, ceruloplasmin, and Cu/Zn ratio may identify copper imbalance, inflammation, or renal handling abnormalities.^43,48–50 However, they cannot directly report mitochondrial copper availability. More mechanistic biomarkers should include COX17, SCO1, SCO2, COX11, SLC25A3, COX activity, complex IV assembly, OCR, ATP, mitochondrial membrane potential, mtROS, mtDNA damage, KIM-1, NGAL, nephrin, podocin, UACR, and eGFR.

Urinary biomarker work provides a practical bridge to translation. Reviews have summarized urinary biomarkers for DKD and early diabetic nephropathy.^99,100 Urinary exosomes may better represent kidney-specific changes than whole urine,⁷⁶ and exosomal ceruloplasmin suggests that copper-related proteins can be detected in kidney-derived urinary vesicles.⁷⁷ Renal metabolomics, proteomics, and multi-omics can capture energetic and inflammatory phenotypes not visible from single analytes.^101–103

Single-cell and spatial approaches are particularly important because mitochondrial copper dyshomeostasis may be cell-type and stage specific. Human and experimental DKD single-cell studies have mapped early glomerular and tubular responses,^81,82 treatment-responsive cellular states,¹⁰⁴ human DKD atlases integrating single-cell and spatial transcriptomics,¹⁰⁵ and mechanosensitive injury pathways in early diabetic nephropathy.¹⁰⁶ Transcriptomic comparisons of early and advanced diabetic nephropathy further show that disease stage influences molecular programs.¹⁰⁷ Future studies should overlay copper-trafficking genes, COX assembly factors, mitochondrial stress signatures, and spatial metal imaging onto these atlases.

Therapeutic Implications

Landmark RAS trials established that losartan and irbesartan reduce albuminuria progression and hard renal outcomes in type 2 diabetes with nephropathy or microalbuminuria.^108–110 More recently, canagliflozin, dapagliflozin, and empagliflozin showed kidney-protective effects across diabetic and broader CKD populations,^111–113 while finerenone reduced kidney and cardiovascular outcomes in albuminuric CKD with type 2 diabetes.^114–116 GLP-1 receptor agonist evidence has also matured, with liraglutide showing renal benefits in secondary analyses and semaglutide reducing major kidney events in the FLOW trial.^117,118 Yet many patients continue to progress, implying that current therapies do not fully interrupt the intracellular conversion of diabetic metabolic stress into structural renal injury.

Against this therapeutic background, the implication of the proposed mitochondrial copper–COX framework is not simply to “give copper” or “remove copper”. Copper is required for COX function, but excess labile copper may promote oxidative stress or copper-induced cell death. Excessive chelation could reduce pathogenic redox-active copper yet also compromise copper-dependent enzymes. If the proposed mitochondrial copper–COX mechanism is experimentally validated in DKD, a potential therapeutic objective would be to restore appropriate copper distribution and mitochondrial copper delivery while preserving COX assembly and respiratory-chain function without increasing labile copper toxicity.

Several existing therapeutic themes align with this goal. Mitochondria-targeted or mitochondria-protective strategies are being explored in DKD.¹¹⁹ SGLT2 inhibitors reduce proximal tubular workload and alter renal metabolic physiology.⁶⁰ Finerenone may ameliorate mitochondrial dysfunction through PI3K/Akt/eNOS signaling in diabetic tubulopathy.¹²⁰ Broader mitochondrial metabolic reprogramming is increasingly recognized in DKD.¹²¹ Accordingly, modulation of COX17, SCO1, SCO2, COX11, or SLC25A3 should currently be regarded as an experimental strategy for mechanistic testing rather than as an established therapeutic target in DKD. The broader copper-dyshomeostasis network, its intersections with cuproptosis and ferroptosis, and the associated biomarker and therapeutic opportunities in DKD are integrated in Figure 5. To clarify the varying levels of evidence supporting the proposed mitochondrial copper–COX framework in DKD, the major mechanistic components, supporting evidence, and key limitations are summarized in Table 1.

Figure 5.

Copper-related states, intersecting stress pathways, and potential translational directions in DKD. Proposed functional mitochondrial copper depletion, copper redistribution, copper excess with potential cuproptosis, and ferroptosis represent mechanistically distinct but potentially interacting processes. These processes may converge through mitochondrial dysfunction, oxidative stress, Fe–S cluster disturbance, lipid peroxidation, inflammation, and fibrosis rather than forming a single linear copper-driven pathway. Candidate biomarkers and mechanistic readouts include systemic copper indices, copper-trafficking proteins, COX activity, oxygen-consumption rate (OCR), ATP, mitochondrial ROS (mtROS), KIM-1, NGAL, UACR, and eGFR. Potential future experimental strategies include restoring appropriate copper distribution, improving mitochondrial copper delivery, protecting COX function, reducing mtROS, and enhancing mitophagy. These strategies remain investigational and should not be interpreted as established therapeutic approaches in DKD.

Table 1.

Evidence Framework for Mitochondrial Copper Dyshomeostasis in DKD

Mechanistic Component	Current Evidence	Evidence Level	Interpretation / Limitation
Systemic/urinary copper abnormalities	Human diabetes/DKD studies report altered copper-related indices	Direct DKD-associated evidence	Does not establish mitochondrial copper status
Renal copper redistribution	Experimental diabetic kidney studies	DKD-supportive evidence	Total tissue copper does not indicate mitochondrial availability
Mitochondrial dysfunction in DKD	Human and experimental DKD studies	Established DKD evidence	Does not establish copper dependence
COX copper trafficking	COX17/SCO1/SCO2/COX11/SLC25A3 mechanistic studies	Strong mechanistic evidence, largely non-DKD	Supports biological plausibility
COX17–renal fibrosis link	Non-DKD renal experimental model	Kidney-relevant supportive evidence	Not definitive evidence for DKD
Functional mitochondrial copper depletion	Proposed in this review	Hypothesis	Requires direct mitochondrial copper and COX validation
Cuproptosis in DKD	Mainly bioinformatic/signature studies	Hypothesis-generating	Does not prove cuproptotic cell death
Ferroptosis in DKD	Experimental DKD evidence	Relatively established	Connection to copper–COX remains uncertain
COX17/SCO1/SCO2/COX11/SLC25A3 as therapeutic targets	No established DKD therapeutic validation	Experimental possibility	Requires gain/loss-of-function and rescue studies

Challenges and Future Perspectives

Direct Evidence Remains Limited

The most important limitation is that direct DKD evidence for mitochondrial copper dyshomeostasis is still limited. Current data support altered copper indices in diabetes and DKD, copper redistribution in diabetic kidney models, and the centrality of mitochondrial dysfunction in renal injury. However, few studies directly measure mitochondrial copper content, copper-chaperone function, COX metal-center maturation, complex IV assembly, and COX activity in renal cells under diabetic conditions. The hypothesis of functional mitochondrial copper depletion should therefore be presented as a testable mechanism, not an established conclusion.

Systemic Copper Levels Do Not Equal Mitochondrial Copper Availability

Future studies must separate systemic copper status from mitochondrial copper bioavailability. Serum copper, urinary copper, tissue total copper, and ceruloplasmin are useful but insufficient. A rigorous validation pipeline should include mitochondrial isolation, subcellular copper quantification by ICP-MS or imaging methods, copper-sensitive probes when validated, COX assembly analysis by blue-native electrophoresis, COX activity assays, OCR and ATP measurement, mtROS imaging, mitochondrial membrane-potential assays, and rescue or knockdown of COX17, SCO1, SCO2, COX11, and SLC25A3.

Cell Type and Disease Stage Matter

The consequences of mitochondrial copper dyshomeostasis are likely to vary by cell type and disease stage. Proximal tubular cells may show energy mismatch and transport failure; podocytes may show cytoskeletal injury and slit-diaphragm loss; endothelial cells may show NO reduction and microvascular dysfunction; mesangial cells may show matrix accumulation. Early DKD may feature adaptive mitochondrial remodeling, whereas advanced DKD may show irreversible quality-control failure, fibrosis, and nephron loss. Cell-type-resolved omics and spatial analysis should therefore be combined with functional mitochondrial assays.

From Hypothesis to Translational Validation

A practical validation strategy would proceed in three steps. First, clinical DKD samples should be profiled for copper markers, urinary vesicle proteins, COX assembly factors, mitochondrial injury markers, UACR, eGFR, KIM-1, NGAL, nephrin, and podocin. Second, db/db mice, STZ-induced diabetes, high-fat diet models, and high-glucose/lipotoxic renal cell systems should be used to test whether diabetic stress alters mitochondrial copper delivery and COX activity. Third, gain- and loss-of-function experiments targeting COX17, SCO1, SCO2, COX11, and SLC25A3 should determine whether restoring copper delivery rescues COX activity, ATP, mtROS, mitophagy, and renal injury. If validated, mitochondrial copper homeostasis could become a mechanistic basis for DKD subtyping, risk prediction, and precision intervention.

Conclusion

DKD is a multifactorial disease in which metabolic, hemodynamic, inflammatory, and oxidative stresses progressively affect tubular, glomerular, endothelial, and interstitial compartments. Mitochondrial dysfunction is well documented in DKD and is relevant to renal bioenergetics, redox control, and cellular adaptation. Independently, copper is an essential cofactor for COX/complex IV, and appropriate mitochondrial copper delivery is required for COX assembly and respiratory-chain function. These established observations provide the biological basis for considering mitochondrial copper handling in DKD.

However, functional mitochondrial copper depletion in DKD remains a proposed and testable hypothesis rather than an established disease mechanism. We propose that impaired mitochondrial copper availability could compromise COX/complex IV function and thereby contribute to OXPHOS impairment, ATP depletion, mtROS accumulation, defective mitochondrial quality control, and renal injury. Direct DKD-specific evidence linking altered mitochondrial copper trafficking to these downstream events remains limited. Accordingly, future studies combining subcellular copper measurements, COX assembly and activity assays, mitochondrial functional phenotyping, and cell-type-resolved approaches will be required to determine whether this framework is mechanistically valid and whether it can ultimately inform biomarker development or therapeutic intervention.

Funding Statement

The authors declare that no funding was received for this study.

Abbreviations

AGEs, advanced glycation end-products; CKD, chronic kidney disease; COX, cytochrome c oxidase; DKD, diabetic kidney disease; ECM, extracellular matrix; eGFR, estimated glomerular filtration rate; ETC, electron transport chain; FAO, fatty acid oxidation; GLP-1RA, glucagon-like peptide-1 receptor agonist; KIM-1, kidney injury molecule-1; mtDNA, mitochondrial DNA; mtROS, mitochondrial reactive oxygen species; NGAL, neutrophil gelatinase-associated lipocalin; OCR, oxygen-consumption rate; OXPHOS, oxidative phosphorylation; RAS, renin-angiotensin system; SGLT2, sodium-glucose cotransporter 2; UACR, urinary albumin-to-creatinine ratio.

Data Availability

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Author Contributions

Bo Hou: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. Jiahao Liang: Investigation, Visualization, Writing – original draft, Writing – review & editing. Xingrui Li: Conceptualization, Supervision, Writing – review & editing. All authors made substantial contributions to the work reported, participated in drafting, revising, or critically reviewing the manuscript, approved the journal to which the article was submitted, reviewed and approved all versions of the manuscript, and agree to be accountable for all aspects of the work.

Disclosure

The authors report no conflicts of interest in this work.