Imbalance of mitochondria and abnormalities in lipid metabolism in Diabetic Tubulopathy

Abstract

Diabetic kidney disease (DKD) is a common and serious microvascular complication in patients with diabetes. In recent years, diabetic tubulopathy has emerged as a major research focus, as its prognosis is closely associated with tubular atrophy and interstitial fibrosis. Multiple studies indicate that diabetes directly damages renal tubules, leading to mitochondrial dysfunction—characterized by impaired mitochondrial bioenergetics, excessive mitochondrial reactive oxygen species (mtROS) production, defective autophagy, and lipid metabolism disorders resulting from abnormal lipid accumulation. Consequently, this cascade triggers a series of metabolic abnormalities. However, the precise mechanisms underlying renal tubular mitochondrial dysfunction and the regulatory pathways of lipid metabolism disorders remain incompletely elucidated. A deeper understanding of the pathobiology of the tubulointerstitium will facilitate the discovery of novel biomarkers for DKD. Based on current literature, this review proposes that mitochondrial dysfunction and abnormal lipid metabolism may accelerate early-stage diabetic tubulopathy, thereby potentially improving patient prognosis.

1. Introduction

Diabetic kidney disease (DKD) is a chronic complication of diabetes and a leading cause of end-stage renal disease. The primary clinical features of DKD include a progressive increase in urinary protein and serum creatinine levels. Currently, the available therapeutic options remain limited. Although studies have primarily focused on glomerular lesions, a growing body of evidence suggests that the renal tubules play a significant role in the onset and progression of DKD. Diabetic tubulopathy (DT) has emerged as a new focus of research, and its pathogenesis is associated with various factors, including hyperglycemia, lipid accumulation, oxidative stress, hypoxia, renin-angiotensin-aldosterone system, endoplasmic reticulum stress (ERS), inflammation, Epithelial-Mesenchymal Transition(EMT), and programmed cell death. This review highlights the close relationship between DT and mitochondria as well as lipid metabolism, along with potential therapeutic strategies.

2. Tubular injury promotes DKD progression

Over the past few decades, the global prevalence of diabetes mellitus (DM) has increased substantially, largely driven by the rise in type 2 DM (T2DM) associated with obesity and metabolic syndrome [1]. This has led to an increasing burden of diabetes-related microvascular and macrovascular complications [2]. Renal microvascular damage often results in chronic kidney disease (CKD), a condition known as DKD [3]. According to current guidelines, CKD is generally defined as an abnormality in kidney structure or function persisting for >3 months, typically confirmed by a reduced glomerular filtration rate (GFR) or the presence of proteinuria, often indicated by albuminuria [4]. In both clinical practice and research, the diagnosis of diabetic kidney disease is primarily established on the following criteria: estimated glomerular filtration rate (eGFR) decline, with eGFR < 60 mL/min/1.73 m², and proteinuria, defined as a urine albumin-to-creatinine ratio ≥ 30 mg/g [5].

Beyond traditional clinical definitions based on glomerular filtration rate and proteinuria, substantial advances have been achieved in recent years in elucidating the mechanisms underlying diabetic kidney injury. The traditional concept of “diabetic nephropathy,” which primarily underscored on glomerular lesions, has gradually been replaced by the more comprehensive notion of “DKD” [6]. Concurrently, there is increasing recognition that renal tubular injury plays a key initiating and driving role in the development of DKD. In 2011, Tang et al. proposed the concept of “diabetic tubulopathy (DT),” specifically referring to the damage torenal tubular epithelial cells (RTECs) that occurs in the early stages of hyperglycemia, along with the resultant metabolic and hemodynamic disturbances [7]. Tubular injury refers to structural and functional abnormalities in RTECs caused by ischemia, toxins, inflammation, or metabolic disorders, manifesting as impaired reabsorption, secretion, and concentrating functions [8]. In DKD, renal tubular injury presents with numerous pathological features, including thickening of the tubular basement membrane, intratubular inflammatory lesions, tubular atrophy, increased apoptotic activity, interstitial fibrosis, and peritubular capillary rarefaction.

Scholars have demonstrated that DKD is primarily characterized by glomerular lesions and has a high incidence rate [9]. However, recent research has revealed that in the early stages of DKD, tubular injury may precede glomerular damage, particularly in proximal tubules. Studies have demonstrated that tubular injury is more pronounced in the early stages than in glomerular lesions [10]. While glomerulosclerosis is a hallmark feature of diabetic kidney disease, it is the extent of tubulointerstitial injury that ultimately determines the rate of renal function decline [11]. Persistent hyperglycemia has deleterious effects on the renal tubules, particularly severe effects in the proximal tubules. When blood glucose levels exceed the renal threshold for glucose, glucose appears in crude urine, thereby altering the survival microenvironment of RTECs. This condition also activates fibroblasts and pericytes, resulting in the secretion of cytokines such as transforming growth factor-β (TGF-β) and Connective Tissue Growth Factor (CTGF) [12]. These factors promote the formation of myofibroblasts, resulting in the production of substances such as collagen I, collagen III, collagen IV, fibronectin, and laminin [12], which further contribute to the fibrosis of RTECs and exacerbate inflammation progression [13]. Moreover, fibrosis of RTECs induces EMT via TGF-β, Wnt/β-catenin, and Notch pathways. This leads to irreversible damage to the tubular interstitium, which accelerates DKD progression [14–18]

Diabetic renal tubular injury involves multiple factors, such as glycated albumin, insulin resistance, and other factors related to hyperglycemia. In addition to their direct toxic effects on renal tubules, reducing sugars may undergo non-enzymatic reactions with amino groups in proteins or lipids. These reactions involve a series of complex biochemical events, including oxidative and non-oxidative molecular rearrangements known as the Maillard reaction, ultimately forming stable covalent adducts called advanced glycation end products (AGEs) [19]. In humans, irreversible advanced glycation of proteins is part of normal aging; however, this process is markedly enhanced and accelerated owing to hyperglycemia. AGEs upregulate the expression of renal tubular IL-8 and intercellular adhesion molecule-1 through Nuclear Factor kappa-light-chain-enhancer of Activated B Cells(NF-κB), Extracellular Signal-Regulated Kinase 1/2(ERK1/2), and Signal Transducer and Activator of Transcription 1(STAT-1)signaling pathways [20].

Cellular crosstalk is a phenomenon in which different or the same types of cells communicate through signaling pathways, thereby affecting their functions [21]. In DKD, cellular crosstalk exists within the glomeruli and across the renal unit. This complex and often bidirectional interaction can promote disease progression. Hasegawa et al. demonstrated the presence of retrograde transport between the proximal tubules and glomeruli, where substances secreted by the tubules are transported to the glomeruli, inducing the fusion of podocyte foot processes and subsequently triggering proteinuria. Research has demonstrated that proximal tubular cells can release nicotinamide mononucleotide (NMN), which reaches glomeruli through retrograde transport and improves their filtration capacity. However, silent information regulator 1(SIRT1) expression in the proximal tubules decreases under hyperglycemic conditions, resulting in reduced NMN secretion. This decreased NMN levels at the glomerular site and increased the expression of Claudin-1 in podocytes, causing podocyte injury and the disappearance of foot processes, thereby leading to proteinuria (Figure 1). This indicates that during DKD pathogenesis and progression, RTECs can promote disease progression through fibrosis and EMT, as well as exacerbate glomerular injury through retrograde transport, collectively facilitating DKD progression [22,23].

Figure 1.

Proposed mechanism by which high glucose induces RTEC injury. activates fibroblasts and pericytes, and promotes fibrosis. Reduced NMN delivery to glomeruli upregulates Claudin-1 in podocytes, leading to foot process effacement and proteinuria.

Moreover, tubular epithelial cells (TECs) exhibit crosstalk with glomerular endothelial cells through the angiopoietin/Tie signaling system [24]. Angiopoietin 1 (Ang 1) is predominantly expressed in RTECs and maintains endothelial integrity and function. Conversely, Ang 2 and the Tie receptor (which binds Ang 1 and Ang 2) are expressed in glomerular endothelial cells, exerting adverse effects on these cells [25]. Immunohistochemical studies in streptozotocin (STZ)-induced diabetic rats have demonstrated that Ang 1 expression initially increases with DKD progression, but subsequently declines steadily, whereas Ang 2 expression increases. Increasing the Ang 2/Ang 1 ratio promotes progressive endothelial injury accompanied by the progression of DKD.

In conclusion, cellular crosstalk between glomerular and tubular cells further illustrates that tubular injury exacerbates DKD progression. This does not negate the significance of glomerular injury in the disease process, as both play a crucial role. This review primarily addresses the implications of tubular injuries for disease progression.

3. Mitochondrial dysfunction as a core factor in the onset and progression of DT

Mitochondria serve as the central sites of cellular energy metabolism [26], providing a stable and continuous supply of energy necessary for various physiological processes. RTECs are particularly rich in mitochondria [27,28], which ensures that the high-energy demands of the renal tubules are adequately met. Impairment of mitochondrial function can lead to significant damage to tubular function and contribute to the onset and progression of the disease. Under normal physiological conditions, mitochondria maintain a balanced state across various functions. However, disturbances in mitochondrial quality control, oxidative stress, dysregulation of dynamics, and imbalances in energy metabolism can disrupt this equilibrium, leading to mitochondrial dysfunction and further details are provided in Table 1.

Table 1.

Mitochondrial injury in diabetic tubulopath.

Category of Abnormality	Key Alterations and Mechanisms	Direct Consequences	Key Molecules/Pathways Involved	References
1. Imbalance in Dynamics and Autophagy	Excessive Fission, Insufficient Fusion: Increased expression of Drp1, p-Drp1, Fis1; Decreased expression of Mfn1/2, OPA1.	Fragmented Mitochondrial Network, accumulation of dysfunctional mitochondria	Regulatory Factors: AMPK, PGAM5, STAT3, HIF-1α, Nrf2, PGC-1α.	[43–53]
2. Oxidative Stress Burst	Excessive ROS Production: Hyperglycemia-induced overload and dysfunction of the ETC, electron leakage due to CoQH₂ accumulation. Weakened Antioxidant Defense: Impaired function of antioxidant enzyme systems.	Oxidative Damage (lipids, proteins, mtDNA), activation of inflammatory (e.g. NLRP3) and fibrotic signaling pathways, leading directly to cellular injury.	ROS Sources: ETC complexes, reduced coenzyme Q (CoQH₂). Antioxidant Systems: SOD, GPx, etc. Regulatory Factors: HIF-1	[34–37,39–42]
3. Energy and Metabolic Crisis	Metabolic Reprogramming and Substrate Abnormalities: Aberrantly enhanced fatty acid oxidation (FAO) leading to lipotoxicity; altered TCA cycle intermediates. Bioenergetic Failure: Impaired ETC integrity, reduced ATP synthesis; mtDNA damage	Insufficient Cellular Energy (ATP) Supply, unable to meet high-energy demands under hyperglycemia; accumulation of toxic metabolic byproducts.	Key Metabolites: TCA cycle intermediates, lipids, amino acids. Energy-Related: Electron transport chain complexes, mtDNA, ATP synthase. Core Regulator: PGC-1α (regulates mitochondrial biogenesis and metabolism).	[54–57]
4. Hypoxia and Maladaptive Response	Renal Tubular Hypoxia: Caused by hyperfiltration and increased oxygen consumption. Dysregulated Hypoxic Response:Activation of HIF-1α signaling pathway, potentially compensatory early but disease-promoting in the long term	Exacerbates ETC dysfunction and ROS production, and interacts with dynamic imbalance to form a vicious cycle.	Key Mediator: Hypoxia-inducible factor-1α (HIF-1α). Interacting Pathways: HO-1, mitochondrial dynamics-related proteins.	[39–41]

3.1. Dysregulation of mitochondrial quality control

The maintenance of mitochondrial quality control is crucial and involves various levels, including organelles, proteins, and molecules. At the organelle level, mitochondrial dynamics such as fusion and fission [29], biogenesis [30], and mitophagy [31] are critical components. At the protein and molecular levels, mitochondrial protein quality control mechanisms, which encompass the regulation of mitochondrial proteases and molecular chaperones, play pivotal roles [32,33]. Collectively, these mechanisms affect the quantity, structure, morphology, function, and distribution of mitochondria.

Recent studies have demonstrated that the proximal tubules of patients with DKD exhibit significant alterations in mitochondrial morphology. This is characterized by fragmentation into short, rod-like, or spherical shapes, accompanied by the dissolution of cristae [32]. These findings indicate that abnormal mitochondrial quality control is crucial in DKD pathogenesis and progression. Furthermore, the administration of empagliflozin has been demonstrated to mitigate mitochondrial fission through the AMP-activated protein kinase Adenosine 5′-Monophosphate-Activated Protein Kinase/Specific Protein 1/Phosphoglycerate Mutase 5 (AMPK/SP1/PGAM5) pathway, resulting in an improvement in DT [33]. This finding further substantiates the close association between mitochondrial quality control and DT. Moreover, mitochondrial health depends on the integrity and homeostasis of the mitochondrial proteome. Disruption of mitochondrial protein homeostasis can lead to mitochondrial dysfunction, leading to cell death. Morphological changes in mitochondria are closely related to protein quality control.

In conclusion, hyperglycemic conditions induce dysregulation of mitochondrial quality control (Figure 2), which subsequently contributes to elevated levels of reactive oxygen species (ROS). The relationship between imbalance in mitochondrial quality control and ROS production is evident. The subsequent sections investigate the association between mitochondrial oxidative stress and DT.

Figure 2.

Mitochondrial dysfunction as a core factor in the onset and progression of DT.Persistent hyperglycemia triggers excessive mitochondrial ROS production in renal tubular epithelial cells, leading to oxidative stress, mtDNA mutations, and protein misfolding, which induces mitochondrial fragmentation. Concurrently, hyperglycemia increases glucose/sodium reabsorption in the proximal tubule, elevating ATP demand. To meet this demand, enhanced oxidative phosphorylation exacerbates electron transport chain (ETC) dysfunction and ROS generation, creating a self-amplifying cycle. Ultimately, reduced ATP production and accumulated mitochondrial damage cause tubular cell dysfunction, driving disease progression. Hyperglycemia further disrupts mitochondrial dynamics and autophagic flux via PGC-1α downregulation.

3.2. Mitochondrial hypoxia and imbalance of oxidative stress

ROS, such as hydrogen peroxide and superoxide anions, are chemically reactive molecules that are produced intracellularly. ROS are produced under normal physiological conditions and primarily regulate various signaling pathways. As a common by-product of oxidative phosphorylation, ROS can lead to oxidative damage in cells. However, these cells have various mechanisms for eliminating ROS. For instance, superoxide dismutase (SOD) within the mitochondria can convert superoxide anions into oxygen and hydrogen peroxide, thereby reducing the accumulation of ROS. Under physiological conditions, a dynamic balance exists between ROS production and the antioxidant system. When this balance is disrupted under pathological conditions, excessive ROS are produced along with a decline in antioxidant capacity [34], resulting in cellular oxidative stress. Excessive ROS production during oxidative stress can damage cellular proteins, lipids, and DNA, leading to lethal cellular injuries and contributing to various pathological processes.

In DT, a sustained hyperglycemic environment induces the excessive reabsorption of glucose and sodium in the proximal tubules. To maintain sodium concentrations inside and outside the cell, Na⁺/K⁺ ATPase uses a higher amount of ATP, necessitating continuous ATP production by mitochondria. This results in excessive ROS production, abnormal activity of the electron transport chain (ETC), and enhanced oxidative phosphorylation stress, thereby contributing to mitochondrial dysfunction in the RTECs (Figure 2) [35]. In the characteristic hyperglycemic microenvironment of DT, superoxide production is promoted, leading to hyperpolarization and reduced ATP production by the mitochondria. Partial inactivation of complex III in the respiratory chain results in loss of tubular cell viability [36]. The inactivation of complex III leads to the accumulation of electrons in the coenzyme Q (CoQ) pool, maintaining its highly reduced state, particularly at an elevated level of Ubiquinol(CoQH₂), which readily donates electrons to molecular oxygen (O₂) to produce superoxide anions (O₂^-), a primary source of ROS [37]. Furthermore, studies have demonstrated that under hypoxic conditions, renal tubules produce hypoxia-inducible factor-1 (HIF-1), which can reduce Mitochondrial Reactive Oxygen Species(mtROS) production and partially maintain the redox balance during early oxidative stress in the mitochondria [38]. However, this relationship is disrupted as the disease progresses, and the specific mechanisms involved have not been elucidated.

As described previously, hyperglycemia increases the reabsorption of glucose and sodium by RTECs through sodium–glucose cotransporters (SGLTs) located in the proximal tubules. This triggers hyperfiltration in the healthy glomeruli, leading to significant oxygen consumption and subsequent hypoxia. Renal hypoxia is a fundamental pathogenic mechanism underlying the initiation and progression of DKD. Hypoxia alters the function of the cytochrome chain involved in mitochondrial oxidative phosphorylation, resulting in reduced ATP production and elevated levels of ROS [39], accompanied by a weakened cellular antioxidant defense system [40]. Under typical conditions, hypoxia-inducible factor HIF-1 mitigates ROS production via multiple pathways. HIF-1 regulates energy metabolism in hypoxic environments via metabolic reprogramming, thereby decreasing ROS generation in the mitochondrial electron transport chain [41]. In a hypoxic state, the accumulation of damaged and fragmented mitochondria further exacerbates excessive ROS production and the loss of mitochondrial membrane potential, thereby leading to mitochondrial dysfunction [42].

3.3. Dysregulation of mitochondrial dynamics and autophagy

Mitochondria continuously undergo fission and fusion to adapt to changes in the cellular microenvironment, thereby satisfying energy demands under varying conditions [43]. During DKD progression, mitochondrial autophagy and fusion serve as self-protective mechanisms in the cell [44], whereas mitochondrial fission can exacerbate renal damage [45]. Research has shown that hyperglycemia inhibits mitochondrial autophagy and fusion of proximal TECs (HK-2), while concurrently causing cells to enter a state of persistent fission [46]. Moreover, a study showed that in the kidneys of mice with T2DM, a significant proportion of mitochondria in damaged renal tubules were fragmented and distributed in a disassembled form within cells. Both in vitro and in vivo experiments have shown increased mitochondrial fragmentation, reduced mitochondrial autophagy, and altered fission and fusion dynamics in renal tubular cells under hyperglycemic conditions [47].

Current research has confirmed that multiple mechanisms are involved in regulating dysregulated mitochondrial dynamics and autophagy, which influence DKD progression. Liu et al. demonstrated that the AMP-activated protein kinase(AMPK) signaling pathway and Phosphoglycerate Mutase 5(PGAM5) are involved in mitochondrial fission pathways [33]. Other studies have confirmed that stromal-derived factor-1 alpha (SDF-1α) inhibits the mitochondrial translocation of Signal Transducer and Activator of Transcription 3(STAT3) by suppressing Stromal Cell-Derived Factor −1 alpha/C-X-C Motif Chemokine Receptor 4(SDF-1α/CXCR4) signaling and the downstream phosphorylation of STAT3 at Ser727, leading to increased mitochondrial fragmentation and disruption of the mitochondrial fusion protein Optic Atrophy Protein 1(OPA1) [48].

Furthermore, researchers have developed a renal tubular cell-specific HIF-1α knockout mouse and a DKD model by injecting STZ. The results indicated that the expression levels of mitochondrial fission proteins, such as Dynamin Related Protein 1(DRP1), Phosphorylated-Dynamin Related Protein 1(p-DRP1), and Mitochondrial Fission Protein 1(Fis1), were elevated in HIF-1α^–/– mice [49], suggesting a potential regulatory relationship with the Heme Oxygenase − 1(HO-1) pathway. Lee et al. reported that under high glucose conditions, relevant fission and fusion proteins in the mitochondria of STZ-induced male C57BL/6J diabetic mice exhibited abnormal expression, such as overexpression of DRP1 and decreased levels of mitochondrial fusion protein 2(Mfn2) [50]. These findings indicate that signaling pathways involving AMPK, PGAM5, HO-1, and CXCR4 can promote DKD progression by affecting mitochondrial fusion and enhancing mitochondrial fission.

Mitochondrial autophagy is also a crucial pathway for mitochondrial quality control, with the kidneys exhibiting a higher rate of mitochondrial autophagy than other organs [51]. The regulatory mechanisms underlying this process play a significant role in DKD progression. Some researchers have found that Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1 Alpha(PGC-1α) is important for regulating the expression of the core regulatory molecule DRP1, which is involved in mitochondrial fission in the renal tubular cells of mice. Other studies have demonstrated that reducing PGC-1α expression in RTECs promotes mitochondrial fission by modulating DRP1 expression. LC3 is used as a marker for autophagy, and research has indicated that the fluorescence intensity of LC3 is reduced when mouse renal tubular cells are exposed to a high glucose environment. This suggests that mitochondrial autophagy is suppressed under hyperglycemic conditions [52]. The mechanism by which PGC-1α regulates mitochondrial autophagy may be associated with its modulation of DRP1, as studies have confirmed that downregulation of PGC-1α expression under high glucose conditions leads to increased DRP1 expression while inhibiting Mfn1 and autophagy marker expression (Figure 2) [52]. Consequently, PGC-1α may serve as a link between autophagy and mitochondrial quality control.

A study using RNA sequencing data for Kyoto Encyclopedia of Genes and Genomes(KEGG) pathway analysis indicated that RTEC-specific knockout of Tumor necrosis factor α-induced protein 8-like-1(TIPE1) (T1KO) resulted in a significant enrichment of mitogen-activated protein (MAP) genes in renal tissues compared to wild-type (WT) mice. Among these, genes associated with mitochondrial autophagy, including PTEN-induced putative kinase 1(PINK1), Parkin, Atg12, and Mfn1/2, were upregulated in the renal tissues of T1KO mice [53]. These findings confirm the role of PINK and Parkin in DT progression. Furthermore, Li et al. reported that the expression of phosphofurin acidic cluster sorting protein 2 (PACS-2) is decreased in the kidneys of patients with DKD. This reduction is associated with tubular atrophy, interstitial fibrosis, and deterioration of renal function. Mechanistic studies suggest that PACS-2 interacts with the autophagy gene beclin 1 (BECN1) and mediates the relocalization of BECN1 to the mitochondrial-associated ER membrane, thereby promoting mitochondrial autophagy, ameliorating tubular damage, and delaying DT progression [45].

Currently, there is no consensus regarding whether increased autophagy is beneficial or detrimental. However, autophagy and mitochondrial dynamics mutually regulate each other and play significant roles in DT onset and progression. This interplay implies that the intrinsic homeostasis of mitochondria may be a potential therapeutic target for DT.

3.4. Mitochondrial energy and metabolic imbalance

Mitochondrial bioenergetics represents a central cellular energy reserve, with its capacity determined by factors such as substrate availability, cellular energy demand, and functional integrity of the ETC. Studies have indicated that the energy metabolism of proximal tubular cells in mice is reprogrammed under diabetic conditions. This highlighted the enhanced role of mitochondrial fatty acid oxidation (FAO) and its contribution to lipotoxicity-induced cell damage [54]. Furthermore, metabolomic analyses of the serum and urine from db/db mice revealed differences in metabolites related to mitochondrial energy metabolism in the renal tubules, including those associated with the tricarboxylic acid cycle, lipid metabolism, glycolysis, and amino acid products [55].

Bioenergetic efficiency is also a significant research area in DT concerning energy metabolism. Mitochondrial bioenergetic reserve capacity is a key concept in this field. This reserve capacity is dependent on numerous factors, including substrate availability, energy demand, and integrity of the ETC. Current research indicates that mitochondrial metabolic dysfunction may result from the direct action of glucose-modifying mitochondria-localized proteins [56]. Alternatively, it may result from the disrupted synthesis of subunits essential for mitochondrial complex assembly caused by mutations in mitochondrial DNA (mtDNA) (Figure 2). Other studies have demonstrated that hyperglycemia can also damage mtDNA, which may progressively impair mitochondrial function over time [57].

4. Disruption of lipid metabolism in DT

4.1. Aberrant expression of key enzymes in fatty-acid oxidation (FAO)

Under physiological conditions, RTECs preferentially produce ATP via FAO [58]. However, when disease-related metabolic alterations suppress fatty acid use, glucose oxidation remains a secondary pathway. Comparative studies have demonstrated that patients with tubulointerstitial fibrosis and their corresponding murine models exhibit significant downregulation of key FAO enzymes and their regulatory factors within RTECs compared with healthy controls [59]. Messenger RNA has recently emerged as a focal point in the investigation of numerous pathologies. Notably, microRNA-21 exacerbates tubular injury and fibrosis in mice by suppressing peroxisome proliferator-activated receptor-alpha (PPAR-α) [60]. FAO is a key step in lipid metabolism, suggesting that DT progression is strongly correlated with lipid metabolism. This is elaborated further in the subsequent sections.

4.2. Lipid metabolism and lipotoxicity

Accumulating evidence indicates a robust connection between DT and lipotoxicity. In 1982, Moorhead et al. initially proposed the “lipid nephrotoxicity” hypothesis, positing that dysregulated lipid metabolism may precipitate chronic progressive kidney disease, thereby accelerating renal function decline [61]. In 1994, Young et al. at the University of Texas Southwestern Medical Center introduced the term “lipotoxicity” to describe the pathogenic beta (β)-cell alterations associated with obesity both before and during the onset of T2DM [62]. This seminal study was the first to link elevated plasma free fatty acids (FFAs) to insulin resistance and β-cell non-responsiveness under hyperglycemic conditions. Further investigations revealed that fatty acid-binding proteins (FABPs) are key drivers of disease progression. FABP1 is present in the cytoplasm of both healthy and damaged proximal tubule cells in the kidney [63]. Using multiomics approaches to identify biomarkers in the blood of patients with T2DM and DKD, researchers have found that FABP1 plays a significant role in disease progression [64].

As a frontier field in contemporary research, the gut microbiota also influences the development of DKD. Growing evidence highlights the significant role of gut dysbiosis and microbially derived metabolites in CKD, particularly in DKD [65–67]. Current evidence indicates that inflammation-driven dysbiosis disrupts cholesterol homeostasis, thereby exacerbating lipid deposition in the renal tubules of DKD patients and accelerating tubular injury [68].

In addition, inflammation is closely linked to lipid metabolism disorders. It has been demonstrated that inflammation-induced activation of the CXCL16 pathway promotes lipid accumulation in the renal tubulointerstitium, thereby contributing to tubulointerstitial injury and the progression of DKD [69].

Lipid-induced nephrotoxicity is not an exclusive cause of kidney disease [70], it can also occur because of renal dysfunction [71]. Lipotoxicity is the principal mechanism of TEC injury and is closely associated with progressive loss of renal function [72]. The pathogenic sequence of DT can be summarized as follows: chronic hyperglycemia results in a sustained surplus of energy that surpasses cellular metabolic demands, disrupts the local microenvironment, and ultimately reprograms the lipid metabolic pathways. This shift favors harmful non-oxidative routes, leading to an accumulation of toxic lipid intermediates that promote inflammation, apoptosis, oxidative stress, and fibrosis, collectively driving the initiation and progression of DT.

4.3. Lipid metabolism and oxidative stress

Lipid overload can precipitate oxidative stress, thereby contributing to DT [73]. Advanced AGEs aggravate renal inflammation and fibrosis by activating pro-inflammatory and oxidative pathways and binding to their receptor , which in turn stimulates sterol regulatory element-binding proteins (SREBPs). The latter disrupts renal lipid homeostasis by increasing fatty acid and cholesterol synthesis (Figure 3) [74]. Excessive ROS production, a customary by-product of mitochondrial oxidative phosphorylation, is closely associated with tubulointerstitial fibrosis. Lipotoxicity-induced mitochondrial injury significantly increases ROS production and upregulates profibrotic mediators, such as TGF-β and plasminogen activator inhibitor-1 (PAI-1), thereby promoting tubular fibrosis. Notably, ROS derived from lipotoxicity may also impair renal hemodynamics and systemic blood pressure, inflicting oxidative damage on tubular cells, intensifying hypoxic stress, and perpetuating a vicious cycle of injury (Figure 3) [75].

Figure 3.

Hyperglycemia promotes renal tubular injury through multiple pathways.Advanced glycation end products (AGEs), generated in the diabetic milieu, promote renal injury through two primary mechanisms: (1) Direct activation of inflammation and oxidative stress, establishing an inflammation-hypoxia vicious cycle; (2) Binding to the receptor for AGEs (RAGE), activating sterol regulatory element-binding proteins (SREBPs), leading to renal lipid metabolism disorders and endoplasmic reticulum stress (ERS). Collectively, these pathways promote tubular injury, inflammation, and fibrosis.

Thomas et al. demonstrated that rats administered albumin (ALB) complexed with FFAs (ALB + FFA) exhibited more pronounced macrophage infiltration and apoptosis than animals administered ALB alone [76]. ALB-bound FFAs enhanced mitochondrial superoxide production in rat TECs, suppressed the expression of β-oxidation enzymes in mitochondria and peroxisomes, and exacerbated intracellular lipid accumulation. Further studies revealed that the saturated fatty acid palmitate downregulates Sirtuin 3(SIRT3) in mouse proximal tubular cells. SIRT3-knockout mice exhibit compromised mitochondrial oxidative capacity and reduced SOD expression [77]. Palmitate also induces ROS formation in human HK-2 tubular cells [78] and in interstitial and podocyte populations [79].

Renal lipid accumulation induces oxidative stress. Spontaneously hypertensive rats and Wistar–Kyoto rats fed a high-fat diet (HFD) displayed elevated serum saturated fatty acid levels and concurrent renal impairment. In db/db mice, fenofibrate therapy restores the B - cell lymphoma 2/B - cell lymphoma 2 - associated X protein(BCL-2/BAX) ratio and SOD levels, alleviating oxidative stress and apoptosis and reinforcing this pathogenic link [80]. The AMPK pathway is a critical regulator of cellular energy metabolism. In diabetic mice, renal levels of phosphorylated AMPK, PINK1, Parkin, LC3-II, and Atg5 were significantly reduced. Administration of the canonical AMPK activator metformin reversed mitochondrial derangement, suppressed Nucleotide - binding Oligomerization Domain - like Receptor Family Pyrin Domain Containing 3(NLRP3) expression, and improved renal function, proteinuria, and fibrosis [81].

4.4. Lipid metabolism and KIM-1 regulation

Kidney injury molecule-1 (KIM-1), also known as T-cell immunoglobulin and mucin domain-containing protein-1 (TIM-1), is a member of the immunoglobulin superfamily [82]. It is significantly upregulated in the proximal tubules of injured or diseased kidneys [83]. KIM-1 induces renal damage via several mechanisms. It mediates the uptake of FFAs by PTECs, promotes mitochondrial fragmentation, triggers DNA-damage responses, and drives tubulointerstitial inflammation and fibrosis. Notably, KIM-1 expression is an early event in kidney pathology [82]. Evidence indicates that KIM-1 is involved in lipid internalization. KIM-1 has been identified as a scavenger receptor on PTECs because it enables them to ingest oxidized low-density lipoprotein (oxLDL) [82]. Recent studies using primary human RTECs further confirmed that KIM-1 plays a critical role in the endocytosis of palmitic acid bound to bovine serum ALB (PA-BSA). Silencing of KIM-1 significantly reduced PA-BSA uptake, whereas the uptake of fatty acid-free BSA remained unchanged. Conversely, KIM-1 overexpression increased the PA-BSA internalization. These findings demonstrate the direct role of KIM-1 in the PTEC uptake of FFAs. The study also demonstrated that KIM-1-mediated PA-BSA uptake provoked PTEC death and diverse stress responses in mice [84]. Therefore, KIM-1 has emerged as a potential therapeutic target for cancer. High-throughput screening of 14,430 small molecules identified TW37 as a KIM-1 inhibitor. TW-37 is a bcl-2 inhibitor whose structure has been determined [85]. Compared to the controls, TW37 reduced PA-BSA uptake, cell death, and inflammatory responses in PTECs. In vivo, TW37 lowered macrophage and fibroblast infiltration in kidney tissue, attenuated KIM-1-driven inflammation and fibrosis, and delayed DKD progression [84]. Collectively, KIM-1 exacerbated DKD by enhancing FFA uptake, mediating apoptosis, and facilitating PTEC endocytosis of palmitate-bound ALB, which induces mitochondrial fission and mitochondrial DNA damage. Moreover, KIM-1 activates inflammatory cells, intensifies inflammation, and promotes tubulointerstitial fibrosis [84].

4.5. Organelle crosstalk in lipid metabolism

ERS and mitochondrial dysfunction are well-recognized drivers of oxidative stress [86,87]. Notably, both are key mechanisms of lipid-induced cytotoxicity. ER is central to biosynthesis and metabolism, orchestrating protein and lipid synthesis, calcium (Ca²⁺) homeostasis, and inter-organelle communication. Disruption of ER homeostasis perturbs the intracellular protein, lipid, and Ca²⁺ balance, culminating in cellular injury or death. ERS denotes the loss of normal ER function and equilibrium, triggered by the accumulation of unfolded or misfolded proteins that overwhelm the folding capacity of the organelle. Moderate ERS is an adaptive response that restores cellular stability, whereas excessive ERS impairs ER function and induces apoptosis. In DKD, ERS was primarily caused by hyperglycemia, proteinuria, AGEs, and FFAs (Figure 3) [88]. These stimuli induce pathogenic ERS and disrupt canonical unfolded-protein-response (UPR) signaling. A recent study revealed that ERS elevates the levels of unsaturated triacylglycerol precursors while diminishing lipid-droplet formation, thereby lowering tubular resistance to lipotoxicity and promoting tubular injury and fibrosis in proximal tubular cell lines and STZ-induced diabetic mice [89].

Enhanced HIF-1α expression has been detected in renal biopsies from patients with diabetic nephropathy and in type 1 and 2 diabetic animal models [90–92]. Genetic ablation of prolyl hydroxylase domain proteins (PHDs), which curtail HIF-1α ubiquitination and degradation, enlarges lipid droplets in murine TECs. Similarly, pharmacological PHD inhibition aggravates lipid deposition in primary human renal tubular cells [93]. Several mechanisms may explain these observations. HIF-1α suppression increases the expression of PPAR-α and carnitine palmitoyltransferase-1A, thereby reducing lipid synthesis and accumulation and mitigating tubular fibrosis [94]. In hepatic models, inhibition of HIF signaling reverses the expression of the cholesterol-export gene ABCA1 and the fatty acid uptake gene ADRP, effectively correcting hypoxia-induced lipid dysregulation in C57BL/6 mice [95]. However, further research is required to determine whether similar pathways operate in DT.

Lipotoxicity can also activate the protein-kinase-R-like ER kinase (PERK) and transcription factor-6 (ATF6) branches of the UPR, inducing stromal-cell apoptosis, a process that is attenuated by PRMT1 deletion. This pattern has also emerged in HFD mouse models [96]. Obesity-induced mice are a robust platform for studying lipotoxicity. Mesenchymal stem cells that amplify hepatocyte growth factor/c-Met signaling mitigate palmitate-induced tubular damage and HFD-associated renal injury [97].

In summary, diseased kidneys exhibit a coordinated organelle response. Under hyperglycemic conditions, mitochondrial oxidative stress and ERS jointly increase ROS production and reduce lipid droplet biogenesis, whereas tubular hypoxia-induced HIFs partially counteract oxidative stress while simultaneously fostering lipid deposition. Understanding organelle crosstalk provides valuable guidance for therapeutic interventions.

5. Drug therapy

Encouraging advances have been made in the treatment of diabetic kidney disease, ranging from renoprotective agents targeting renal tubules and lipid metabolism-related drugs to mitochondria-targeted therapies. A detailed summary of this research is provided in Table 2.

Table 2.

Relevant therapeutic drugs for diabetic tubular disease.

Drug classification	Pharmacological agent	Mechanism	Citation
Renal tubular protective agents	Empagliflozin	Inhibits SGLT2, improving the survival microenvironment of renal tubular epithelial cells	[103]
Renal tubular protective agents	SIK2	Overexpression reduces endoplasmic reticulum stress (ERS), attenuating apoptosis of renal tubular epithelial cells	[104]
Renal tubular protective agents	Fraxin	Binds EGFR and inhibits its phosphorylation, reducing epithelial-mesenchymal transition (EMT)	[105]
Renal tubular protective agents	Wogonin	Modulates autophagy and inflammation mediated by the PI3K/Akt/NF-κB signaling pathway, attenuating	[107]
Renal tubular protective agents	CYA10603	Inhibits NLRP3 inflammasome activation in renal tubular cells and macrophages	[112]
Renal tubular protective agents	Pan-Src kinase inhibitor	Attenuates endoplasmic reticulum (ER) stress signaling and improves inflammation and oxidative stress	[113]
Anti-tubular cell apoptosis agents	Calcium Dobesilate	Downregulates aberrant overexpression of Bim; likely due to reduced PAI-1 levels	[114]
Anti-lipotoxicity agents	Statins	Alleviate renal function impairment in DKD patients, reduce serum inflammatory cytokine levels, and improve hemorheology	[116]
Anti-lipotoxicity agents	Fibrates	Reduce acute tubular injury induced by FFA overload via PPARα-dependent mechanisms	[120]
Anti-lipotoxicity agents	PPARα/δ Dual Agonist H11	Demonstrates coordinated metabolic benefits in renal tubular cells involving fatty acid metabolism, BCAA degradation, and glycolysis	[121]
Mitochondrial-targeting agents	Asiatic acid	Prevents renal tubular injury and mitochondrial damage by modulating the Nrf-2 pathway and mitochondrial dynamics	[122]
Mitochondrial-targeting agents	Angiotensin II type 2 receptor	Prevent reduced mitochondrial bioenergetic efficiency, increased mitochondrial ROS production, metabolic shift, and enhanced cell proliferation	[124]

5.1. Renal tubular protective drugs

5.1.1. Empagliflozin

Sodium-glucose cotransporter 2 (SGLT2) inhibitors such as empagliflozin are a new type of drug for treating T2DM [98]. Their efficacy has been verified in numerous large randomized controlled trials [99–101]. These studies have demonstrated that SGLT2 inhibitors can significantly reduce the risk of renal endpoints and exert renoprotective effects. Empagliflozin has been extensively used in clinical practice to treat T2DM, particularly in patients requiring simultaneous glycemic control and cardiovascular risk reduction [102]. Empagliflozin therapy significantly reduced the levels of renal tubular injury biomarkers, including KIM-1 and Neutrophil Gelatinase - Associated Lipocalin(NGAL), in patients with T2DM and normal albuminuria. These findings highlight the potential to protect renal tubules at early stages. SGLT2 inhibitors provide a new therapeutic approach for DKD [103].

5.1.2. Salt-inducible kinase 2(SIK2)

Salt-inducible kinase 2 (SIK2) is a salt-inducible kinase that regulates key biological processes such as energy metabolism, cell cycle progression, and apoptosis. Functional studies demonstrated that SIK2 deficiency or inactivation in diabetic mice exacerbates tubular injury and interstitial fibrosis. Research indicates that SIK2 downregulation using the specific inhibitor ARN3236 accelerates mitochondrial dysfunction, ERS, and cellular apoptosis in the renal tubules of STZ mice [104]. Based on transcriptome sequencing and molecular mechanism exploration, SIK2 overexpression can activate HSF1/Hsp70 by inhibiting the histone acetyltransferase activity of p300, which can reduce the ERS-mediated apoptosis of RTECs [104]. These findings indicate that SIK2 is a promising therapeutic target for DT.

5.1.3. Fraxin (Fr)

Fr, the primary active glycoside derived from Fraxinus rhynchophylla Hance, exhibits numerous potential pharmacological activities. It has been confirmed that Fr can directly bind to Epidermal growth factor receptor(EGFR) and inhibit its phosphorylation, thereby inhibiting the Cellular Src/nuclear transcription factor-κB(c-Src/NF-κB) signaling pathway. This decreases the incidence of renal tubulointerstitial EMT and improves diabetic recurrent implantation failure(RIF) [105]. Studies have also confirmed that Fr enhances the activation of the Connexin 43-Akt Serine/Threonine Kinase-Nuclear Factor Erythroid 2 - Related Factor 2/Antioxidant Response Element(Cx43-AKT-Nrf2/ARE) pathway to delay diabetic renal fibrosis, and Cx43 upregulation may represent a novel mechanism [106].

5.1.4. Wogonin

Treatment with woginin alleviated urinary ALB and histopathological damage in the renal tubular interstitium of diabetic mice. Studies have demonstrated that wogonin downregulates pro-inflammatory cytokine expression and autophagy dysfunction, both in vivo and in vitro. Molecular docking and cellular thermal shift analyses revealed that the mechanistic phosphatidylinositol 3-kinase (PI3K) is a target of wogonin. PI3K inhibition eliminated the protective effects of wogonin. Wogonin regulates autophagy and inflammation by targeting PI3K (an important link in the PI3K/Akt/NF-κB signaling pathway). Experiments have demonstrated that wogonin can alleviate renal tubulointerstitial fibrosis and tubular cell injury by modulating PI3K/Akt/NF-κB signaling pathway-mediated autophagy and inflammation [107]. Accordingly, wogonin may be a potential therapeutic agent for RTECs of DKD damage by targeting PI3K. Furthermore, studies have demonstrated that wogonin ameliorates renal inflammation and fibrosis in diabetic nephropathy by inhibiting the TGF-β1/Mothers Against Decapentaplegic Homolog 3(Smad3) signaling pathway [108]. Moreover, research has confirmed that wogonin alleviates kidney inflammation and fibrosis by upregulating Suppressor of Cytokine Signaling 3(SOCS3), thereby inhibiting the Toll-like receptor 4(TLR4) and Janus Kinase/Signal Transducers and Activators of Transcription(JAK/STAT) pathways [109].

5.1.5. Cya10603

CAY10603 is a specific inhibitor of histone deacetylase 6 (HDAC6) [110]. Experiments have confirmed that pharmacological inhibition of HDAC6 by various selective inhibitors, including Tubastatin A, Tubacin, ACY-738, and ACY-1215, effectively limits the progression of multiple kidney diseases [111]. Studies have observed that in the late stage of DKDDN, the expression of HDAC6 around the distal tubules is significantly upregulated, indicating that tubulointerstitial HDAC6 is associated with tubular injury [112]. Furthermore, it has been confirmed that CAY10603 exhibits therapeutic potential for Diabetic Nephropathy by inhibiting the activation of the NLRP3 inflammasome in renal tubular cells and macrophages [112].

5.1.6. Pan-Src kinase inhibitor

Fyn kinase, a member of the Src family kinase(SFK) family, is involved in ERS activation, leading to proximal tubular damage in the diabetic environment. Pancreatic Stromal Kinase Inhibitor(Pan-SKI) treatment alleviated renal injury in diabetic rats. This study demonstrated that diabetic kidney injury induced the expression of Fyn and Lyn kinases. Knockdown of Fyn, but not Lyn, inhibits P70S6K and c-Jun N-terminal Kinase/C/EBP Homologous Protein(JNK/CHOP) signaling, thereby inhibiting proximal tubular cell injury in a diabetic setting. Treatment with Pan-SKI alleviated ERS signaling and improved the structural and functional evidence of inflammation, oxidative stress, and progressive DKD, with trends similar to those observed in chlorosartan-treated rats [113]. These data emphasize that Fyn kinase is a viable target for developing therapeutic agents for DKD.

5.2. Anti-tubular apoptotic drugs

5.2.1. Calcium dobesilate

The effect of calcium dobesilate on HK2 cells under high glucose conditions was explored using Annexin V-FITC/PI double staining to detect apoptosis in cells treated with high glucose and high glucose + calcium dobesilate at different time points. Calcium dobesilate has a protective effect against high glucose-induced apoptosis of PTECs in the kidneys. Bim, an important factor in apoptosis, is exclusively expressed in PTECs in the kidneys [114]. In addition to its antioxidant effects, the potential mechanism underlying the protective effects of calcium dobesilate is the downregulation of Bim expression. A study involving 121 patients with type 2 diabetic nephropathy treated with calcium dobesilate (500 mg, thrice daily) for three months revealed that its therapeutic effects may be attributable to reduced PAI-1 levels [115].

5.3. Antilipotoxic drugs

Statins are 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors. They significantly reduced serum low-density lipoprotein cholesterol (LDL-C) levels, slightly reduced serum triglyceride (TG) levels, and increased high-density lipoprotein cholesterol (HDL-C) levels. Li et al. reported that atorvastatin could alleviate renal function impairment, reduce serum inflammatory factor levels, and improve hemorheology in patients with DKD [116]. A systematic review and meta-analysis involving 543 patients with DKD using statins suggested that statins may have beneficial effects by reducing albuminuria in this population [117]. However, these drugs have certain side effects. Studies have confirmed that long-term statin administration can exacerbate DKD through ectopic fat deposition in diabetic mice [118]. Moreover, statins may impair renal tubular function, leading to interstitial fibrosis, thereby accelerating DKD progression [119]. However, current research is insufficient. Large-scale, long-term clinical applications are required to objectively evaluate the impact of statin therapy on DKD prognosis.

Fibrates are PPAR-α agonists. Their primary effect is to reduce serum TG levels and increase HDL-C levels. Pretreatment with low-dose clofibrate reduces acute tubular injury induced by FFA overload through a PPAR-α-dependent mechanism. Prevention of renal tubular injury appears to be associated with a reduction in FFA inflow, maintenance of fatty acid catabolism, reduction in FFA intracellular accumulation, and inhibition of oxidative stress, apoptosis, and NF-κB activation [120].

H11 is a potent and balanced PPARα/δ dual agonist with a high selectivity for PPARγ. It is protective against diabetic kidney injury. H11 has demonstrated metabolic benefits by coordinating fatty acid metabolism, Branched-ChainAminoAcids (BCAA) degradation, and glycolysis in renal tubular cells [121]. These findings suggest that H11 may be a promising drug candidate for DKD treatment.

5.4. Mitochondria-targeting drugs

Studies have demonstrated that asiatic acid significantly reduces ALB levels, KIM-1 levels in urine, and serum creatinine(Scr) and Blood Urea Nitrogen(BUN) levels. Asiatic acid can prevent tubular injury and mitochondrial damage by regulating the Nrf-2 pathway and mitochondrial dynamics. Furthermore, treatment with ML385 (an Nrf2 inhibitor) eliminated the protective effects of asiatic acid on mitochondrial dynamics and renal tubules [122]. Follow-up studies are required for the development of new drugs. Additional studies have confirmed that asiatic acid can reduce TGF-β1 secretion and suppress renal tubulointerstitial fibrosis by directly inhibiting TGF-β receptor I (TGF-βR1) and activating the autophagy-lysosome system [123].

A key finding in related studies is that in the early stages of diabetes, the angiotensin II type 2 receptor (AT2R) in renal tubules can prevent the decline in mitochondrial bioenergetic efficiency, increase mitochondrial ROS production, metabolic shifts, and cell proliferation. Furthermore, we provide evidence for the presence of high-affinity mtAT2Rs and mtAT1Rs, whose densities are differentially regulated during aging and DT progression. This finding supports the idea that AT2R may partially alter mitochondrial function through direct action [124]. Accordingly, AT2R is a novel therapeutic target in DT.

6. Conclusions/expectations

Glomerular and tubulointerstitial lesions are equally crucial in DKD, and they are intricately interconnected. Numerous biological processes drive the progression of disease. Mitochondria are important pathogenic organelles central to the vicious cycle of DT. Imbalances in mitochondrial quality control, redox homeostasis, dynamics, and energy metabolism contribute to this vicious cycle. Lipid metabolism disorders are currently gaining increasing attention. Mitochondria, as key processors of lipid metabolism, can also contribute to these disorders. Lipid accumulation and lipotoxicity resulting from dysregulated lipid metabolism exacerbate disease progression.

Furthermore, inter-organelle communication between mitochondria and ER is a pathogenic factor. However, from a subcellular perspective, the precise molecular mechanisms and regulatory signaling pathways underlying this disease remain unclear. This is partially attributed to the lack of fully standardized assessment methods. Alterations in the mechanisms driving the progression of lipid metabolism disorders may affect other pathological pathways. Another significant challenge is that these subcellular changes evolve with age, disease state, and individual variation. Moreover, therapeutic strategies targeting lipid metabolism disorders remain primarily limited to experimental models and require validation in clinical trials. We believe that the intricate interplay between lipid metabolism disorders, mitochondrial dysfunction, and DT will be completely elucidated in the future, providing a robust evidence base for future therapeutic interventions for DKD.

Funding Statement

This study was financially supported by National Natural Science Foundation of China (Youth Program)(Grant No:82505536). Jiangsu Training Program of Innovation and Entrepreneurship for Undergraduates (Grant Number: 202410313010Z). Jiangsu Province Traditional Chinese Medicine Technology Development Plan (QN202427, QN202331) Natural Science Foundation of Jiangsu Province (BK20241042). The authors thank Adobe Illustrator for their assistance and support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

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