Progress in the study of the role of C5a-induced tubular cell senescence in the progression of diabetic kidney disease

Abstract

Background

Chronic kidney disease (CKD) is the end stage of progressive renal disorders, and effective disease modifying treatments remain elusive. Diabetic kidney disease (DKD) is its leading cause, yet mechanisms and therapies still hold large knowledge gaps. Studies have demonstrated that tubules and interstitium constitute 90 % of renal parenchyma and drive DKD progression; high-glucose milieu induces renal tubular epithelial cell (RTEC) senescence, a process central to DKD onset and worsening.

Methods

Using PubMed as the primary data source, this review first screened literature on ‘DKD pathogenesis’, revealing that the pivotal role of ‘complement activation-induced cellular senescence’ remains insufficiently characterized. A second, focused search was then conducted on ‘complement system activation’, from which studies explicitly linking complement activation to cellular senescence were distilled. The final corpus is organized around three core dimensions: latest discoveries, current research status, and mechanism-guided therapeutic strategies.

Results

Cellular senescence, defined as the irreversible growth arrest of cells in response to damaging stimuli, involves various mechanisms such as DNA methylation, oxidative stress, DNA damage response (DDR), mitochondrial dysfunction, and the continuous production of senescence-associated secretory phenotype (SASP) factors. Complement activation induces cellular senescence through the aforementioned processes, thereby promoting and exacerbating both the onset and progression of DKD.

Conclusion

In the high-glucose milieu, complement activation drives massive C5a release, which accelerates DKD progression by inducing renal tubular cell senescence. C5a-receptor antagonists have already demonstrated potent renoprotective effects, positioning C5a as a central target for future DKD drug development.

Introduction

DKD has long been recognized as a significant health burden. Studies have established that aldose reductase (AR) functions as the initial rate-limiting enzyme of the polyol pathway. Under hyperglycaemic conditions, AR activity is markedly up-regulated, initiating a cascade in which glucose is reduced to sorbitol and subsequently oxidized to fructose. Fructose metabolism generates a substantial burst of reactive oxygen species (ROS), eliciting oxidative stress that directly compromises renal parenchymal cells, while concomitant uric acid production imposes an additional nephrotoxic burden. Sustained AR activation further promotes aberrant deposition of extracellular matrix components including collagen and fibronectin thereby accelerating renal fibrogenesis [1,2]. Collectively, these events precipitate and exacerbate DKD, thereby constituting a critical driver of the relentless progression of chronic kidney disease CKD to end-stage renal disease.

Although DKD is not traditionally viewed as an immunomodulatory disease, the activation of innate immune cells, particularly the complement system, has emerged as a key mediator in its pathogenesis [3]. The complement system, a crucial component of innate immunity, is activated in hyperglycemic environments. This activation is tightly coupled to the aforementioned AR activation: excessive AR activity not only depletes NADPH, accumulates sorbitol, and triggers a burst of ROS [4,5], but also it renders oxidized lipids and protein products as targets for complement activation [6,7]. These targets drive the release of a series of inflammatory mediators that, in turn, further up regulate AR gene transcription [8], forging a positive-feedback loop that intensifies cellular senescence.

One potential mechanism for the development of DKD is cellular senescence. Cellular senescence is a largely irreversible state of proliferative arrest triggered by telomere attrition, DNA damage, oxidative stress, or inflammation [9,10]. Although proliferation is halted, these cells remain metabolically active and acquire a hyper-secretory phenotype termed the SASP, characterized by sustained release of pro-inflammatory cytokines, chemokines, growth factors, and proteases [11–13]. Interestingly, aldose reductase is activated under high glucose, and treatment with aldose-reductase inhibitors not only suppresses senescence markers such as SA-β-gal and p21 but also markedly curbs the release of SASP factors [14–16]. Ultimately, the SASP actively remodels the tissue microenvironment, fuels chronic inflammation, and drives fibrosis, thereby accelerating disease progression.

Cell senescence is closely associated with the activation of the complement system. There are three known pathways of complement activation, and the complement system is activated by a cascade of reactions, each of which results in the formation of C3 converting enzyme, which forms a multimeric complex with more C3b molecules, followed by the formation of C5 converting enzyme, which breaks down C5 to C5a and C5b [17]. C5a is a major effector molecule of the complement cascade, which exerts its effector function by binding to and activating its G-protein-coupled receptor, C5a receptor(C5aR), leading to pro-inflammatory signaling cytokine and chemokine release, and induction of vasodilatation [18]. It was found that C5a stimulation of renal tubular epithelial cells up-regulated the gene expression of senescence mediators, especially those mediated by the Wnt/β-catenin pathway. Recently, aberrant DNA methylation has been demonstrated to play a significant role in renal aging [19]. Giuseppe et al. discovered that C5a can induce aberrant methylation in regions associated with cell cycle control and Wnt signaling [18]. Recent evidence demonstrates that abundant C5a generated by complement activation on one hand markedly up regulates senescence markers such as p53 and p21, triggering DNA methylation disarray and mitochondrial dysfunction [20]; on the other, it further amplifies inflammatory responses via pathways including the senescence associated Wnt/β-catenin axis thereby accelerating senescence of renal parenchymal cells [21]. Inhibition of complement activation can significantly protect renal parenchyma, preventing tubular injury and renal fibrosis.

In addition to the use of biomarkers to improve diagnosis, intervention, and treatment before irreversible kidney damage occurs to reduce the significant morbidity associated with DKD, it is also essential to understand the main mechanisms of DKD development and to target DKD control therapies to the process of DKD progression. Interventions targeting senescent cells prior to the development of DKD may be prophylactic. This review discusses the significant role of the complement factor C5a in promoting renal tubular epithelial cell senescence and its implications for the progression of DKD, as well as the potential of C5aR inhibitors in the future prevention and treatment of DKD.

Complement system

The complement system plays a key role in protecting the host from invasive factors and is an essential component of the immune system. It promotes the clearance of harmful microorganisms and damaged cells from the body by facilitating the action of antibodies and immune cells, thereby maintaining health and internal environmental stability. The complement system is composed of various soluble proteins, receptors, and regulatory factors, all of which play a specific role in the cascade of reactions following complement activation. The complement cascade is typically activated through three different pathways: the classical pathway, which is activated by the binding of pattern recognition molecule C1q to immune complexes such as IgG and IgM; the lectin pathway, or the MEL pathway, is activated by the binding of mannose residues expressed on the surface of invading pathogens to MBL (mannose-binding lectin). and the alternative pathway, which involves the spontaneous activation of C3 molecules in the serum through surface binding or tick-over activation [22,23]. In all three pathways of complement activation, activation eventually converges at protein C3, leading to a common downstream cascade. C3 is cleaved by C3 convertase into anaphylatoxins (C3a) and an opsonin fragment (C3b), with C3b being central to complement system activation. Subsequently, C5 convertase is formed, which cleaves C5 to produce C5b-9, and then form membrane attack complex (MAC), ultimately resulting in cytotoxic effects on target cells [24,25].

Activation of the classical pathway begins with the formation of an antigen-antibody complex, which is formed by the binding of the immune complex to the C1q domain of complement C1. This complex then cleaves C2 and C4, generating C4b2b (C3 convertase), which attaches to the cell membrane surface and subsequently initiates the complement cascade [26,27]. MBL as a classic activator of the lectin pathway, after binding to mannose residues, MBL is activated. This activation allows MBL to associate with MBL-associated serine proteases (MASPs), which in turn become activated. The activated MASPs cleave C4 and C2, forming C3 convertase (C4b2b). The C3 convertase then cleaves C3 to produce C3b. Subsequently, C5 convertase (C4b2b(3b)) is formed, which cleaves C5 into C5a and C5b. Eventually, C5b binds to cell membrane surfaces and associates with C6-9 to form the C5b-9. The MAC inserts into the cell membrane, disrupting the local phospholipid bilayer, and creates a transmembrane hydrophilic channel, ultimately leading to cell lysis [27,28]. As for alternative pathways, due to the presence of a large amount of C3 molecules in serum, they can be activated in a way that does not rely on antibodies when encountering foreign substances such as viruses, bacteria, and biomaterials. Additionally, C3 can undergo low levels of spontaneous cleavage [27,29]. Once the C3b produced binds to some solid-phase surfaces (cellular membranes) it adheres to the membrane surface, and subsequently binds to serum factor B (FB), and factor D (FD) to form C3 convertase (C3bBb) of alternative pathway, which in turn cleaves C3 to produce more C3b. This results in a positive feedback loop leads to the deposition of more C3b. When C3b reaches a certain level leads to an amplified complement cascade response [26,27,30].

Activation of the complement system in a high glucose milieu

Hyperglycemia, a hallmark of diabetes, has been shown to activate the complement system, particularly through the lectin pathway. Studies have demonstrated elevated levels of complement components in the plasma and urine of diabetic patients with renal involvement, suggesting that local complement activation plays a significant role in renal damage [30–33]. The accumulation of advanced glycation end-products (AGEs) and oxidative stress in a high-glucose environment further exacerbates complement activation, leading to renal tubular injury and fibrosis.

Indeed, the clinical and pathological relevance and mechanisms of the three complement activation pathways in DM remain unclear. Current evidence indicates that serum C1q is markedly elevated in patients with type 2 diabetes and cognitive impairment, suggesting that the classical pathway is preferentially linked to neurologic complications [34]. In diabetic kidneys, its activation remains primarily based on the ‘C1q-AGEs’ binding hypothesis [35]. However, direct evidence for the in situ triggering of this complex in renal tissue is currently lacking. In the lectin pathway, serum levels of MBL and Collectin-11 are elevated in DKD [35], yet laser-capture microdissection coupled to LC-MS/MS (LMD-LC-MS/MS) barely detects LP-specific proteins such as MBL or MASPs in glomeruli. This suggests that LP operates predominantly on renal tubules rather than glomeruli [30], although its precise mechanisms remain to be elucidated. The alternative pathway can be triggered either by spontaneous C3 hydrolysis generating low-level ‘tick-over’ or by glycation-induced inactivation of complement regulatory proteins [24], which mechanism predominates in the diabetic milieu remains unresolved. Current proteomic data indicate enrichment of alternative pathway components in glomeruli, whereas evidence for their deposition within tubular cells is still lacking. Collectively, all three complement activation routes participate in diabetic pathology, yet their dominant contexts, initiating ligands, and downstream effector mechanisms remain to be defined, need to be parsed out through integrated mechanistic–clinical studies.

Li XQ and colleagues measured seven complement components (C1q, MBL, Bb, C4d, C3a, C5a, sC5b-9) in plasma and urine of 68 DM patients with renal biopsies. They found significantly higher levels of these components in patients with renal involvement compared to those without, suggesting that all three complement pathways may be activated in the circulation of DM patients. It’s interesting to note that the levels of complement components in urine also showed similar results, with elevated levels observed in patients with renal involvement compared to those without. However, there was no significant difference in the levels of C1q in the urine between the two groups of patients, suggesting that C1q levels may not necessarily indicate renal involvement, or may be regulated differently in the context of DKD [36]. Previous studies have highlighted that C1q can bind to altered self-proteins, such as AGEs and oxidized low-density lipoprotein (ox-LDL) both of which are relevant to DM [37]. Indeed, the lack of significant correlation between C1q levels and clinical pathological parameters suggests that the activation of the classical pathway may not have a direct or prominent role in the development of DM. It’s could imply that other complement pathways might be more critical in the pathogenesis and progression of DM. Jing-Min Zheng and colleagues recruited 62 DN patients confirmed by renal biopsy to investigate local complement activation mechanisms. Their study found that C1q was expressed in the glomerular area of the kidneys, but there was no significant change in C1q levels in the tubular interstitial area compared to normal controls [30], complement C3 is also deposited to some extent in the glomerulus [38]. These results suggest that C1q may be more involved in glomerular changes rather than tubular changes in diabetic nephropathy, the deposition of C1q and C3 in the glomeruli may be associated with poorer renal survival rates in patients with DKD. However, these results does not support a role for the classical pathway in tubular interstitial injury. Additionally, the team investigated the expression of alternative pathway factor B and key molecules of the lectin pathway, MBL and MASP1. The results revealed elevated expression of MBL and MASP1 in the tubulointerstitial tissue of diabetic patients. This increase in expression was closely correlated with the degree of tubulointerstitial damage. Moreover, they found that levels of MBL and MASP1 were associated with the levels of the complement activation end product MAC in the tubulointerstitial, suggesting that activation of the MBL pathway is closely linked to renal tubules injury [36]. Under hyperglycemic conditions, glucose reacts with proteins or lipids to form AGEs. The extensive accumulation of AGEs triggers the activation of the lectin pathway, leading to excessive activation of the complement system, which subsequently exacerbates the pathophysiological process of DKD. This mechanism is widely recognized as a primary factor in the progression of DKD [39]. Regarding complement factor B, studies have observed that the increased expression of factor B is primarily localized to the brush border of renal tubular cells, with no significant distribution in the tubular basement membrane (TBM) and tubulointerstitial (where MAC is mainly distributed). This indicates that there is no significant correlation between the levels of factor B and the renal tubule injury marker MAC [30].

In summary, local activation of the complement system under hyperglycemic conditions plays a crucial role in the progression of DKD, leading to renal damage, particularly tubular interstitial injury. Increased complement components in plasma and urine further confirm the importance of local complement system activation in DKD. Complement components can be locally synthesized by renal tubular epithelial cells, glomerular capillaries, and mesangial cells. However, current research primarily indicates that the activation of MBL pathway is significantly involved in tubular injury in DKD [36], while the exact roles of the classical and alternative pathways in tubular damage require further investigation (Figure 1).

Figure 1.

The three pathways of complement system activation.

Activation of the MBL pathway in a high glucose milieu

MBL is a pattern recognition molecule that typically binds to carbohydrate ligands on various pathogens, triggering complement activation. Under normal conditions, MBL does not bind to mannose residues on healthy human glycoproteins due to insufficient binding strength, and complement activation is inhibited by regulatory proteins like CD59 and DAF present in circulation and on cell surfaces. However, once MBL binds to its ligands, it forms a complex with MASPs (MBL-Associated Serine Proteases), activating MASPs and leading to the formation of C3 convertase. This results in the production of large amounts of C3a and C5a, ultimately inducing complement cascade reaction.

Current research suggests that hyperglycemia leads to glycosylation of pattern recognition molecules and complement regulatory proteins, resulting in uncontrolled activation of the lectin pathway. The uncontrolled activation of the complement system is an important mechanism that promotes the progression of DKD [39]. Clinical trials have confirmed that serum levels of MBL are significantly higher in diabetic patients with established kidney damage compared to those without kidney damage [40,41]. MASPs, as key factors in activating the lectin pathway of the complement system, are upregulated in the renal tubular cells of diabetic rats [42]. Additionally, studies have shown that diabetic mice with MBL gene knockout exhibit reduced kidney injury markers, such as less renal fibrosis and lower urinary albumin excretion. This suggests that activation of the complement system’s lectin pathway plays a significant role in the development of DKD [43].

Previous research has confirmed that long-term hyperglycemia in diabetes leads to increased protein glycosylation and subsequent impairment of protein function. This glycosylation can affect various proteins, including those involved in inflammation and kidney damage, contributing to the progression of diabetic complications [44,45]. MBL is known to recognize carbohydrate ligands, but it has also been suggested that it may recognize other glycosylated molecules, apoptotic cells, and cellular debris in the tubulointerstitial. This recognition can lead to the activation of the complement system in the tubulointerstitial, thereby exacerbating damage to renal tubular cells. Damage to the renal tubules and tubulointerstitial is a common and common feature shared by patients with DKD. Persistent cellular debris and increased glycosylated molecules in the damaged tissue can be recognized by MBL, triggering further activation of the MBL pathway. This positive feedback loop exacerbates tubular injury. Additionally, studies have shown that diabetes can lead to MBL deposition in the glomeruli and an increase in plasma levels of complement activation product C3a. This elevation might be due to the recognition of new epitopes induced by hyperglycemia [31]. In diabetic patients, complement regulatory proteins undergo glycation modifications, leading to their inactivation. Once inactivated, these regulatory proteins cannot effectively suppress complement activation, resulting in accelerated complement attack and deposition of MAC [46]. The increased deposition of the MAC in diabetic tissues may induce the release of growth factors. These growth factors, which are essential for stimulating the proliferation of cells within the vascular wall, can thereby promote the development of proliferative vascular diseases and advance the progression of diabetic nephropathy (Figure 1).

Cellular senescence

Cellular senescence refers to the irreversible arrest of cell proliferation, often triggered by DNA damage, oxidative stress, mitochondrial dysfunction, as well as protein misfolding, and inflammation [47]. Cellular senescence typically refers to the state in which normal cells exit the cell cycle due to damage. In this state, the residual metabolic activity of the cells enables them to secrete a variety of cytokines, known as the SASP. SASP can amplify the senescence signals through autocrine and paracrine mechanisms [48]. Increasing evidence suggests that the accumulation of senescent cells is associated with renal function loss during the progression of DKD, and senescent cells can be found in the proximal renal tubules and podocytes of patients [49].

When cells face damaging attacks, such as DNA damage, accumulation of damage-associated factors in chronic disease tissues, and metabolic injuries, they activate repair pathways that can partially restore cellular functional integrity. However, if the damage is irreversible, cells will undergo programmed cell death or enter senescence. If DNA damage remains unresolved, it triggers the DNA damage response involving sensor kinases, ultimately leading to cell cycle arrest through the upregulation of p21 mediated by p53.

Once the senescence program is activated, the cells can no longer proliferate [50–52]. Studies have confirmed that senescent cells accumulate at the site of the cause of many chronic diseases [21]. After their formation, senescent cells can have diverse roles; they may actively contribute to organismal health by neutralizing potential tumor cells. However, they can also negatively affect tissue function, playing a detrimental role in age-related diseases and various chronic conditions through mechanisms like inflammation and tissue damage.

Klotho was initially identified as an anti-aging molecule. In mouse experiments, Klotho-deficient mice exhibited premature aging phenotypes, while those overexpressing Klotho showed a 30% increase in average lifespan and demonstrated protective effects against various pathological phenotypes, particularly in kidney diseases [53]. As an anti-aging factor, Klotho is highly expressed in the kidneys, primarily in TECEs. It plays a crucial role in anti-aging, anti-fibrosis, and anti-inflammation in kidney-related diseases. As chronic kidney disease progresses, levels of soluble Klotho in circulation significantly decline [54,55] Soluble Klotho acts as an endocrine factor that is vital for regulating and maintaining kidney function [56]. Therefore, inhibiting the negative regulatory effect of complement component C5a on Klotho is particularly important, as it may help improve kidney health and slow disease progression. Additionally, recent studies have revealed that the complement component C1q can activate the Wnt signaling pathway, which is antagonized by the protein Klotho. The Wnt signaling pathway plays a crucial role in cellular senescence and aging-related diseases [21]. Interestingly, DNA methylation modifications have also been shown to accelerate kidney aging, and high methylation of the Klotho gene promoter can lead to reduced expression of the Klotho gene [57].

The importance of renal tubular epithelial cell damage in renal diseases

Renal tubules and the interstitial compartment constitute the majority of the renal parenchyma, making them critical targets in the pathogenesis of kidney diseases. Hyperglycemia-induced oxidative stress and AGE accumulation lead to hypertrophy and senescence of renal tubular cells, ultimately contributing to renal fibrosis. This suggests that early detection and intervention targeting tubular health may be crucial in preventing further deterioration of kidney function [58]. Future research could shift its focus from a glomerulus-centered to further exploring the role of the renal tubules in DKD. This shift may lead to a better understanding the disease mechanisms and identify potential therapeutic targets for improving kidney health.

The significant reabsorption function of proximal renal tubules is the primary reason for meeting the high oxygen demands of the kidneys. The hyperglycemic environment created by increased glucose reabsorption leads to the accumulation of AGEs, increased production of reactive oxygen species (ROS), and oxidative damage, resulting in hypertrophy of the proximal renal tubules. Ultimately, this leads to cell cycle arrest and an aging phenotype in tubular cells. An increasing body of evidence suggests that tubular lesions are not secondary to glomerular damage, but rather represent early and initial characteristic changes [41,42]. Moreover, damage to the renal tubules can lead to more widespread glomerular dysfunction. Studies have found that tubular damage resulting in albuminuria is a predictive factor for the progression of DKD. In summary, the epithelial cells, play a central role in the pathogenesis of DKD [59,60].

The hyperglycemic environment not only leads to the activation of the complement system but also directly induces cellular senescence in renal tubular cells [61,62]. Metabolic dysfunction occurring in the long-term hyperglycemic environment of diabetes leads to cellular senescence. The accumulation of senescent cells further exacerbates metabolic dysfunction, inflammation, and tissue damage, resulting in the continued progression of DKD. Ultimately, this cascade of events contributes to the premature aging of the kidneys [63,64]. Studies have shown that in age-matched comparisons, the cellular senescence markers, including cell cycle inhibitor p16INK4A and senescence-associated β-galactosidase (SA-β-gal), are significantly elevated in tissue samples from patients with DKD compared to the control group. These markers are predominantly found in renal tubular cells [49]. Future investigations into the potential mechanisms underlying renal tubular cell senescence in DKD may offer new therapeutic strategies aimed at mitigating the progression of DKD and improving patient prognosis.

C5a mediates renal tubular epithelial cells senescence in DKD

C5a is key effector molecules in the complement cascade, with the MBL pathway primarily activated in a hyperglycemic environment, leading to the generation of significant amounts of C3a and C5a. The release of C5a triggers the formation of the MAC, resulting in cell lysis as the final step of this cascade. C5a exerts its effects by binding to and activating its G protein-coupled receptor, C5aR. Importantly, complement C5a is primarily associated with renal tubules, as both C5a and C5aR are predominantly expressed in renal tubular cells, suggesting that targeting the C5a/C5aR axis may thus represent a promising therapeutic strategy for mitigating the progression of DKD [65].

Castellano and colleagues reported for the first time that C5a induces cellular senescence through epigenetic modifications in RTEC [66] Previous studies illustrate the involvement of C5a/C5aR1 in the development of cellular senescence [67]. Melinda T. Coughlan et al. detected elevated levels of the SASP marker IL-6 in the renal cortex of diabetic mice models, while the expression significantly decreased after the knockout of the C5aR1 gene. Additionally, the expression of the anti-aging gene klotho increased due to the knockout of C5aR1 [68]. The study found that in renal biopsy tissues of DKD patients, there was an increased deposition of C5a in the renal tubules. As the severity of renal tubular injury and interstitial fibrosis in DKD patients increased, the levels of C5a in the kidneys also rose correspondingly. These results indicate a strong positive correlation between C5a signaling and the progression of DKD [65].

Additionally, the C5a receptor inhibitor PMX53 can reduce the expression of the cell cycle arrest marker p21 protein, mitigating cellular senescence in diabetic patients. These results provide the first evidence that C5a can induce cellular senescence under diabetic conditions, thus playing a role in the progression of DKD [68]. The C5a/C5aR1 signaling pathway can release pro-inflammatory signals, induce vasodilation, and promote the release of cytokines and chemokines. Similarly, although senescent cells have lost their ability to divide, their high metabolic activity supports the release of pro-inflammatory cytokines, chemokines, and growth factors. In a high-glucose environment, the complement system is activated, leading to the excessive production of C5a. The C5a/C5aR signaling pathway can activate the nuclear factor kappa B (NF-κB) in blood mononuclear cells, resulting in a significant downregulation of Klotho, thereby inducing cellular senescence through an NF-κB mediated mechanism [54,67,68]. Complement activation leads to chronic inflammation, which induces cell cycle arrest in renal tubular epithelial cells, thereby exacerbating cellular senescence. In a high-glucose environment, an excessive number of glucose molecules bind to proteins or lipids without enzymatic regulation, resulting in misfolded proteins and the accumulation of pro-oxidative macromolecules known as AGEs The significant accumulation of AGEs in renal tissues not only induces oxidative stress within cells, producing high levels of ROS, but also activates the NF-κB signaling pathway. This activation further downregulates Klotho expression, intensifying cellular senescence [39,69–71]. Additionally, the generation of ROS and the accumulation of AGEs caused by high blood glucose levels can lead to DNA damage, accelerating the early senescence of glomerular and renal tubular epithelial cells. One of the key conditions for the significant production of SASP factors is the persistence of DNA damage signaling. In senescent cells, several SASP factors accumulate in large quantities and further enhance senescence signaling through a paracrine mechanism [47,48,52]. Concurrently, the majority of SASP regulators converge on the NF-κB transcription factors, which collectively regulate SASP factors and jointly induce cellular senescence in various contexts [72].

C5a induces cellular senescence by regulating Wnt/βcatenin pathway gene methylation

Wnt/β-catenin signaling pathway plays a critical role in cellular self-renewal and differentiation, significantly contributing to organ development and tissue repair. Under normal circumstances, Wnt/β-catenin activity is typically suppressed in adults [42]. However, in the context of prolonged diabetes, which activates the complement system, this pathway can be reactivated. Short-term Activation: Short-term activation of the Wnt/β-catenin pathway can facilitate kidney tissue repair and regeneration. This is crucial in response to acute injuries where cellular proliferation and differentiation are necessary to restore normal function. Dysregulation and Long-term Activation: Conversely, if Wnt/β-catenin signaling is persistently or excessively activated, it can lead to cellular senescence in renal cells. This senescence is characterized by a halt in the cell cycle and an increase in the secretion of pro-inflammatory factors, contributing to a detrimental microenvironment. Ultimately, the continuous activation of Wnt/β-catenin signaling promotes renal fibrosis, a process where excess extracellular matrix components accumulate, leading to the progressive loss of kidney function. This progression marks the transition from a reversible acute injury phase to a chronic disease state [40,73–75]. In the process of renal fibrosis, the activation of the Wnt/β-catenin pathway plays a decisive role in driving renal tubular senescence [76]. Previous studies have confirmed that exposure to complement significantly activates the Wnt/β-catenin pathway, leading to the manifestation of an aging phenotype [21]. Interestingly, the hyperglycemic environment in diabetes also induces an increase in Wnt expression in renal tubular epithelial cells, thereby activating the Wnt/β-catenin pathway [41,73]. Renal tubular epithelial cells are the primary source of WNT proteins in the kidney, and continuous activation of WNT proteins, such as WNT9a, can induce renal tubular senescence [77], characterized by increased expression of cell senescence-associated proteins like p16, p19, p53, and p21 [75]. Therefore, it is reasonable to speculate that the large amounts of C5a produced by complement activation in a hyperglycemic environment may exert its effects through the C5a/Wnt/β-catenin pathway. Previous study suggests methylation modification of DNA accelerates kidney aging [19,78].

Research indicates that Studies have established that shifts in DNA-methylation patterns are intimately linked to cellular senescence and are finely tuned by extrinsic (nutritional, hormonal) and intrinsic cues, thereby modulating the aging trajectory [79–81]. To dissect complement’s role in this process, Giuseppe’s group performed genome-wide methylome profiling of renal cells and found that C5a induces hypomethylation of three pivotal genes BCL9, CYP1B1, and CDK6 leading to their marked up-regulation. These genes sit at the hub of cell-cycle and apoptosis networks and are all enriched in the Wnt/β-catenin senescence pathway [82,83], Thus, in a high-glucose milieu, complement-derived C5a not only triggers Wnt/β-catenin signaling but also cooperates by demethylating key genes within this pathway, thereby amplifying pro-senescence gene expression and markedly elevating senescence markers such as SA-β-Gal, p53, and p21 [84], culminating in accelerated senescence of renal tubular epithelial cells. These findings further consolidate the central role of complement activation in renal fibrosis and cellular aging [21], and provide a new theoretical basis for targeted intervention in diabetic kidney disease (Figure 2).

Figure 2.

In a high-glucose environment, complement C5a activates the NF-κB and Wnt/β-catenin signaling pathways, which are involved in cellular senescence.

C5a is capable of inducing cellular senescence through the NF-κB/Klotho

Klotho is a protein closely associated with human aging, often referred to as a longevity protein. It is a single-pass transmembrane protein that exists in three isoforms: α-Klotho, β-Klotho, and γ-Klotho. The α-Klotho isoform, hereafter referred to as Klotho, is primarily produced in the kidneys and is predominantly expressed in the proximal and distal renal tubules, where it exerts protective effects on the kidneys. Klotho is a key pleiotropic protein that is involved in the regulation of multiple biological pathways associated with aging and kidney diseases. Relative reduction of plasma soluble alpha-klotho in diabetic patients predicts the onset/development of diabetic nephropathy [54,55].

In patients with DKD, a higher degree of interstitial fibrosis and tubular atrophy is closely associated with premature cellular aging in renal tissues. The progression of chronic kidney disease is a multifaceted pathophysiological process involving various cellular pathways. Among these, the Wnt/β-catenin signaling pathway plays a critical role in regulating cellular aging, making it essential in the onset and progression of diabetic kidney disease. The activation of the Wnt pathway is considered a major regulatory factor in the development of DKD [73,85], and is also a primary inducer of cellular senescence [68,77,86]. Previous studies have shown that Klotho can regulate the activation of the Wnt signaling pathway and acts as an antagonist of Wnt [87,88]. In the context of Wnt pathway activation, Klotho can bind to various Wnt proteins (such as Wnt1, Wnt4, and Wnt7a), inhibiting their activity and thereby suppressing cellular senescence induced by Wnt signaling [53,86]. Additionally, earlier research has demonstrated that modulation of the complement system can influence the functionality of Klotho. In cases of renal ischemia-reperfusion injury, inhibiting C5aRcan restore Klotho expression, thereby exerting a protective effect on renal function [54,68].

NF-κB plays a critical role in promoting the expression of numerous inflammation-related genes and is significantly involved in the progression of kidney diseases [89]. The chronic hyperglycemic environment associated with diabetes leads to the accumulation of metabolic waste, activation of the immune system, release of inflammatory mediators, and inflammatory cytokines such as tumor necrosis factor (TNF), interleukin-1 (IL-1), and interleukin-6 (IL-6) accumulate in renal tissues, triggering the activation of the NF-κB pathway. This pathway not only exacerbates inflammation but also contributes to the pathological processes that drive the progression of diabetic kidney disease [90–92]. In addition, high glucose levels induce the activation of Toll-like receptor 4 (TLR4) and p38 mitogen-activated protein kinase (MAPK) in renal tubular epithelial cells. Both TLR4 and p38 MAPK can trigger the activation of the NF-κB pathway [93,94]. Furthermore, the accumulation of AGEs produced in diabetes also exerts an activating effect on NF-κB [95]. NF-κB signaling is considered crucial in regulating Klotho production. It is composed of a heterodimer formed by various members of a gene family, including RelA (p65), c-Rel, RelB, NF-κB1 (p50/p105), and NF-κB₂ (p52/p100) [96]. Research conducted by Yuchen Chien and colleagues through proteomic analysis of aging chromatin revealed that the NF-κB subunit p65 predominantly accumulates on the chromatin of senescent cells. Their findings identified the p65 subunit of NF-κB as one of the most significantly enriched transcriptional modifiers that bind to aging chromatin.

This evidence demonstrates that the upregulation of p65 coordinates and drives changes in the expression of genes associated with cellular senescence [72]. In summary, the activation of the NF-κB pathway plays a significant role in the senescence of RTECs in DKD. Previous studies have shown that the activation function of p65/RelA is impaired when the complement system is inhibited using the C1 inhibitor C1-INH, which helps maintain the expression of Klotho. Furthermore, in in vitro experiments, exposing TECs to C5a and then using the specific NF-κB inhibitor CAPE demonstrated that NF-κB inhibition can reduce the decrease of Klotho induced by C5a. This process indicates that complement inhibition can prevent the activation of NF-κB signaling in renal tubular epithelial cells, thereby protecting Klotho production and further confirming the significant role of complement C5a in the regulation of NF-κB [54,67,68]. In conclusion, these results indicate that C5a in the context of diabetes may stimulate renal proximal tubular epithelial cells and lead to a significant reduction in Klotho through an NF-κB-dependent mechanism. This reduction weakens the inhibitory effect of Klotho on the activation of the Wnt signaling pathway, thereby promoting the senescence of renal tubular epithelial cells. Furthermore, in diabetic kidney disease, Klotho also participates in the premature senescence of renal tubular epithelial cells by regulating mechanisms such as DNA damage, oxidative stress, and telomere shortening [88]. Therefore, the regulation of Klotho by C5a plays a significant role in cellular senescence (Figure 2).

C5a induces cellular senescence through the NF-κB/SASP

SASP has been shown to play a significant role in many chronic diseases. It refers to a series of inflammatory cytokines, chemokines, growth factors, and proteases secreted by senescent cells, which are crucial for maintaining the senescent microenvironment [11,97]. Cell cycle arrest-related senescence is closely associated with the secretion of the SASP [98,99]. SASP components actively participate in the aging process by enhancing senescence-associated growth arrest in an autocrine manner. Key SASP factors include IL-6 and IL-8. Research has shown that there is a reciprocal influence between SASP secretion and cellular senescence in DKD. Specifically, the senescence of proximal tubular cells induces SASP, while the secretion of SASP further exacerbates cellular senescence [100].

With the progression of DKD, renal tubular epithelial cells undergo senescence due to various damaging stimuli. Senescent cells release a series of SASP factors, which accumulate in large amounts within these cells and can further enhance senescence signaling through autocrine mechanisms [99,101]. In a hyperglycemic environment, the accumulation of glucose molecules, without enzyme regulation, leads to misfolding and the buildup of pro-oxidative macromolecules, resulting in the accumulation of AGEs in the kidneys. This accumulation generates excessive ROS, creating a persistent DNA damage signal, which is necessary for the production of SASP factors [48,98]. Typical chemokines associated with SASP include IL-6, IL-8, MCP-1, CTGF, and PAI-1 [102], Giuseppe Castellano and colleagues found that exposing renal tubular epithelial cells to C5a significantly increased the gene expression of IL-6, MCP-1, and CTGF. Moreover, the elevated IL-6 gene expression observed in mouse renal cortex was markedly reduced upon C5aR1 knockout gene [68]. These evidences demonstrates that complement C5a regulation can exacerbate tubular senescence by inducing the SASP [21].

Additionally, the Yuchen Chien group demonstrated that the NF-κB p65 subunit is among the most prominently enriched transcriptional regulators on senescent-cell chromatin, and that p65 is intimately associated with both pro-senescence and SASP genes. Inhibiting NF-κB activity markedly down-regulates SASP genes that depend on this factor, establishing NF-κB as the core master regulator of the SASP [72]. Subsequent work revealed that NF-κB not only initiates the SASP by modulating the DNA-damage response [103]. but can also be re-activated by the SASP component IL-1α [102], creating an ‘NF-κB → SASP → NF-κB’ positive-feedback loop that perpetually amplifies inflammatory signals and accelerates cellular senescence [104]. These findings demonstrate that the C5a/C5aR axis ignites an ‘C5a/NF-κB/SASP’ cascade in renal tubular epithelial cells, thereby orchestrating their senescence. This axis elucidates how inflammatory signals and cellular senescence synergistically amplify each other and opens a novel therapeutic target for diabetic kidney disease (Figure 3).

Figure 3.

In a high-glucose environment, complement C5a promotes the massive production of ROS and AGEs, stimulates neutrophils and macrophages to transform into active cells involved in inflammatory responses, induces a decrease in Tregs, and leads to mitochondrial dysfunction, thereby causing cell cycle arrest and promoting the occurrence of cellular senescence.

C5a exposure upregulates the protein levels of p53 and p21, thereby promoting cellular senescence

In DKD, renal cell senescence serves as the mechanistic basis for kidney aging and is characterized by an essentially irreversible growth arrest accompanied by changes in cell morphology and epigenetics. Senescence can be triggered by various forms of stress, and mechanistically, the process of senescence ultimately leads to cell cycle arrest through the activation of p21 mediated by p53 [51]. Recognized biomarkers of aging include β-galactosidase (SA-β-gal), p53, p21, and p16, among others. Following DNA damage, the primary role of p53 is to induce cell cycle arrest via p21, thereby leading to cellular senescence [76,105].

As a transcription factor, p53, upon activation, is involved in the regulation of autophagy, DNA damage repair, cell cycle arrest, and senescence [106]. The DNA damage response (DDR) is a key factor in cell cycle arrest., and cellular senescence is closely related to persistent DDR. During cell proliferation, the gradual erosion of telomeres leads to telomere shortening. When double-strand breaks occur, these shortened telomeres are detected by the cell, triggering the DDR, which activates p53 to induce cell cycle arrest via p21, ultimately executing senescence [105,107].

p21 is a cyclin-dependent kinase inhibitor that primarily interacts with p53 to mediate p53-dependent cell cycle arrest [108]. Senescence predominantly affects renal tubular cells in CKD. In a diabetic environment, p21 is the most notable gene continuously induced by hyperglycemia. The levels of p21 in renal tubules and urine are correlated with the severity of DKD. Even with improved blood glucose levels, p21 can still be persistently induced, and diabetes-related complications continue to exist, a phenomenon known as hyperglycemic memory [108,109]. Specifically, p21 promotes the functional decline of renal tubular cells by mediating cell cycle arrest and regulating pathways related to cellular senescence. Under hyperglycemic conditions, the persistent induction of p21 not only exacerbates the senescence process of renal tubular cells but may also lead to cellular dysfunction and structural changes, further driving the pathological progression of DKD [108].

TP53 encodes p53, which is crucial for regulating premature aging and cell cycle arrest. Previous studies have shown that exposure to C5a elicits a marked increase in p53 and p21 protein levels, with p21 induction strictly dependent on prior p53 activation, ultimately driving a chronic senescent state and irreversible cell-cycle arrest [21]. Given that p21 levels in the renal cortex are closely associated with albuminuria, this suggests that p21 plays a significant role in C5a-induced aging in DKD. Moreover, in diabetes, the pathways related to the transcriptional regulation of cell cycle genes by TP53 are upregulated and can be reversed by PMX53 [68]. Overall, these findings indicate that C5a exposure activates p53, ultimately leading to senescence in renal tubular epithelial cells, which is crucial for the progression of DKD (Figure 3).

C5a promotes senescence of renal tubular epithelial cells by inducing mitochondrial dysfunction

Oxidative stress plays a crucial role in the development of vascular complications in diabetes, particularly in DKD. It is a fundamental factor that establishes the link between hyperglycemia and DKD. The persistent hyperglycemic environment is the primary culprit that triggers an excessive production of ROS, ultimately leading to oxidative stress.

Mitochondria are the energy sources of eukaryotic cells and play a crucial role in providing energy for cellular functions. The accumulation of damaged mitochondria in renal tubular epithelial cells may be a key factor in their premature aging [110]. Research has found that in DKD, there are abnormalities in mitochondrial lipid metabolism, particularly in the remodeling of cardiolipin, indicating structural and bioenergetic impairments in the mitochondria. This leads to a reduced energy state, characterized by decreased ATP production, which results in a decline in the AMP to ATP ratio, a key indicator of cellular energy load. This drop triggers the activation of the energy sensor AMP-activated protein kinase (AMPK). However, prolonged activation of AMPK can promote cellular senescence. In fact, the ability of AMPK to induce cell cycle arrest depends on p53 [111]. Subsequently, p53 upregulates the transcription of p21 to regulate cell cycle arrest and senescence, while p53 itself can be modulated by C5a.

Under diabetic conditions, the excessive generation of ROS triggered by hyperglycemia is dependent on mitochondrial dysfunction [112]. This dysfunction leads to DNA damage and an increase in renal oxidative stress, which in turn accelerates the aging of renal tubular epithelial cells. The accumulation of ROS not only contributes to cellular damage but also exacerbates the progression of DKD by impairing cell function and promoting cellular senescence [113]. Oxidative stress is considered one of the contributing factors to telomere shortening [114]. The gradual erosion of telomeres subsequently triggers a sustained DDR, which induces cellular senescence. In diabetic patients, telomere loss is associated with renal cell senescence, proteinuria, and the progression of DKD [115]. Research has shown that inhibiting the C5a/C5aR axis can restore lipid metabolism in renal mitochondria. Studies conducted on human primary proximal tubular cells indicate that C5aR1 signaling interferes with mitochondrial respiratory function and promotes the generation of ROS, exacerbating DNA damage [66]. Moreover, the treatment with the C5aR inhibitor PMX53 has been shown to restore the composition of cardiolipin in diabetic patients to levels resembling those of non-diabetic individuals, indicating that C5a may regulate mitochondrial homeostasis [66]. These findings underscore the significant role of the C5a/C5aR axis in the pathology of DKD, particularly through its regulation of mitochondrial function and contribution to cellular senescence. By modulating mitochondrial health, the C5a/C5aR pathway influences various cellular processes, including oxidative stress responses and energy metabolism, which are crucial in the progression of DKD. The restoration of cardiolipin composition further suggests that targeting the C5a/C5aR axis could be a promising therapeutic strategy for preventing or alleviating the effects of DKD by maintaining mitochondrial integrity and function, thereby mitigating the associated cellular aging processes. This highlights the potential of C5aR inhibition as a novel intervention in managing diabetic kidney disease and its complications (Figure 3).

C5a is involved in the modulation of inflammatory responses that induce cellular senescence

CKD is often accompanied by chronic inflammation and cellular senescence, suggesting that targeting cellular senescence and inflammation may provide new strategies for the treatment of this disease. DKD, as one of the main causes of CKD, is a common cause of renal failure. The traditional view is that it is caused by microvascular complications due to hyperglycemia and hemodynamic changes [116]. Recent studies have highlighted the pivotal role of inflammation in the pathogenesis and progression of DKD, making it a hot topic in research [117]. Evidence from in vitro experiments, pathological examinations, and epidemiological studies all underscore the significance of inflammation in DKD [118–120].

Preclinical research continually reveals that the inhibition of the complement system can alleviate inflammation and fibrosis, offering protective effects against DKD. This suggests that new treatment strategies based on complement system inhibition may play a significant role in future clinical treatments for DKD.

Renal aging is associated with various physiological changes, including chronic low-grade inflammation. In patients with DM, increased levels of pro-inflammatory cytokines in serum, urine, and renal tissue are closely related to proteinuria, which is a significant factor in the progression of DKD and directly leads to renal tubular and interstitial damage [118]. The principal regulators of inflammation include cytokines such as IL-1, IL-6, IL-8, and TNF, all of which are implicated in the development and progression of DKD [3,121,122]. Studies have indicated that the expression levels of IL-8 in renal tubular epithelial cells and in the serum of patients with diabetes are higher than those in healthy individuals, and it has been considered as a predictive marker for the development of DKD in diabetic patients [121,123–125]. Furthermore, TNF mRNA and protein levels are elevated in the glomeruli and proximal renal tubular epithelial cells of patients with diabetes, revealing a close link between TNF and the injury of renal tubular epithelial cells [126].

Hyperglycemia accelerates the progression of DKD by promoting inflammatory cell infiltration and the secretion of various cytokines and chemokines in RTEC. In-depth research has identified the molecular mechanisms related to inflammation triggered by diabetes in the kidney, including long-term alterations in glycometabolism leading to increased production of AGEs, ROS, and SASP, as well as the activation of nuclear factor kappa-B (NF-κB) [92,127], JAK/STAT, and AKT-mTOR signaling pathways [116,120,128]. AGEs not only bind to the NLRP3 inflammasome to generate an inflammatory response [129], but also increase the expression of serum amyloid A (SAA), creating a persistent inflammatory environment [128]. The SASP exacerbates inflammation and induces secondary senescence, leading to the progression of renal damage [130]. Macrophages are the primary infiltrating cells in the kidneys of patients with diabetes [131,132], with the M1 phenotype being pro-inflammatory and capable of producing TNF-α, which accelerates the inflammatory process in DN. This is associated with the activation of NF-κB and JAK/STAT pathways, collectively forming an inflammatory environment [133,134].

Complement component C5a, as an anaphylatoxin produced during the activation of the complement system, plays a significant role in inflammatory responses. In DKD, C5a can recruit various inflammatory cells to the site of injury, including neutrophils, monocytes, and T lymphocytes, among other inflammatory cells [135,136]. And by binding to C5aR1, it induces the polarization of macrophages towards the M1 phenotype, promoting the production of pro-inflammatory cytokines [137,138] C5a can also participate in basic cellular regulatory processes by modulating chloride channels and transporters, as well as inducing significant changes in the morphology and membrane-forming abilities of neutrophils [139]. These processes are related to the migration and chemotaxis of neutrophils, transforming them into migratory cells capable of invading sites of inflammation [135]. Infiltrating T cells, including Th1, Th2, Th17, and regulatory T cells (Tregs), can produce pro-inflammatory cytokines such as IFN-γ and TNF-α, leading to kidney damage [140,141]. Interestingly, complement receptors help regulate Tregs. In STZ induced diabetic mice, both C5aR1 deficiency and PMX53 treatment can prevent the reduction of FoxP3+ Tregs [66,142], which are believed to play a role in ameliorating inflammation in diabetic kidneys [143,144]. Furthermore, PMX53 treatment reduces urinary albumin and cytokines such as 8-isoprostane and IL-18 in diabetic mice, alleviating inflammation [66]. Meanwhile, the C5aR1 antagonist W-54011 decreases IL-6 levels and reduces the mRNA expression of pro-inflammatory gene markers like TLR2, MCP-1, and the macrophage marker F4/80 [145].

An increasing number of studies have indicated that inflammation is an endogenous factor of aging [146], and is involved in the development and progression of DKD. Under conditions of chronic inflammation, immune cells such as macrophages and lymphocytes release a large number of pro-inflammatory cytokines, including IL-6 and TNF-α, which can induce cellular senescence [147]. Conversely, the accumulation of senescent cells secretes the SASP, promoting chronic inflammation and inducing the senescence of normal tissue cells and immune cells, which impairs the immune system and leads to the accumulation of senescent cells and inflammatory factors, forming a vicious cycle of inflammation and aging [50,130]. Research has demonstrated that activated NF-κB is primarily detected in cortical tubular epithelial cells [148]. Once activated, NF-κB can regulate the expression of numerous genes and plays a crucial role in inflammatory responses. In cultured renal tubular epithelial cells, various stimuli can activate the classical NF-κB pathway, leading to the transcriptional regulation of multiple pro-inflammatory molecules [92,149]. At the same time, activated NF-κB negatively regulates the anti-aging gene Klotho, which collaboratively promotes cellular senescence. Additionally, chronic hyperglycemia leads to the production of ROS, resulting in oxidative stress. Oxidative stress is believed to amplify inflammatory responses, while inflammation, in turn, promotes oxidative processes through inflammatory mediators [150]. Oxidative stress can cause telomere shortening and induce DNA double-strand breaks, which have been confirmed as significant factors in cellular senescence [151,152].

In summary, research has demonstrated that complement C5a regulation can exacerbate tubular senescence by inducing the in vivo SASP. Inhibition of the complement pathway can prevent the activation of NF-κB signaling in renal tubular epithelial cells. Additionally, the C5a/C5aR axis may alleviate oxidative stress by regulating mitochondrial homeostasis, thereby modulating cellular senescence. Consequently, in the tubular epithelial cells of DKD, complement C5a plays a significant role in various pro-inflammatory mechanisms, while persistent chronic inflammation further promotes cellular senescence. The absence of C5aR and the corresponding inhibitors can reduce various inflammatory factors and mitigate the inflammatory response, suggesting that future therapies targeting the complement system, particularly the inhibition of the C5a/C5aR axis, hold great potential for the treatment of DKD (Figure 3).

Other complement components activated in hyperglycaemic settings: C3a and MAC

Studies have confirmed that Antagonists targeting either C5a or C3a receptors significantly attenuate albuminuria and renal fibrosis in STZ induced diabetic mice. In diabetic rat models, levels of C3a, C5a, and MAC are overexpressed in renal parenchyma [153]. In type 1 and type 2 diabetic patients with albuminuria, plasma levels of C5a and C3a rise in parallel, and concentrations of C3a, C5a, and soluble MAC in both plasma and urine are markedly higher in DKD patients than in diabetics without complications [36].

As another core effector of the complement system, C3a binds C3aR on podocytes and tubular epithelial cells to amplify inflammatory responses [154,155], suppress cAMP generation, trigger reactive oxygen species bursts and collapse of mitochondrial membrane potential, and subsequently activate the Wnt/β-catenin and TGF-β/Smad3 pathways, leading to mesangial matrix expansion and tubulointerstitial fibrosis. Genetic deletion of C3aR or treatment with PMX53 markedly reduces proteinuria, glomerulosclerosis, and inflammatory cell infiltration in STZ and db/db models [35,39], implying that its activation may also play a crucial role in the induction of cellular senescence.

On the other hand, the membrane attack complex (MAC, C5b-9) is physiologically restrained by the complement regulator CD59. Under hyperglycaemic conditions, CD59 undergoes glycation-induced inactivation, loses its inhibitory effect on MAC, and results in pronounced MAC deposition on podocytes, tubular epithelial cells, and glomerular endothelial cells [39,156]. MAC deposition elevates intracellular Ca²⁺, activating the NLRP3 inflammasome and promoting caspase-1 cleavage and maturation, thereby driving release of IL-1β and IL-18 and amplifying local inflammation [46,157]. Concurrently, MAC induces mitochondrial dysfunction and triggers a burst of reactive oxygen species [39]. This promotes podocyte detachment, tubular cell apoptosis or senescence, and accelerates glomerulosclerosis and tubular atrophy [156]. C5aR antagonists (avacopan, PMX53) indirectly suppress C5b-9 assembly and have been shown in animal DKD models to reduce MAC deposition and fibrosis [39].

Collectively, although complement components C3a and MAC are equally pivotal in diabetic complications, their precise mechanisms of action, pathological impact, and whether they also accelerate disease progression by inducing cellular senescence in DKD remain to be thoroughly elucidated.

Future outlook

The activation of the complement system, particularly through the lectin pathway, is a significant contributor to renal damage in DKD. Targeting complement activation, especially the C5a/C5aR axis, holds promise for developing novel therapeutic strategies to prevent or slow the progression of DKD. Future research should focus on elucidating the underlying mechanisms and exploring the potential of C5a receptor antagonists as therapeutic agents.

Although the liver is the primary site for complement synthesis, local complement activation in the renal tubules is increased under injurious stimuli, which may contribute to the progression of kidney diseases by promoting tissue and cellular damage. This local activation can lead to further inflammation and injury in the kidney, exacerbating conditions like DKD. The association between C5a levels and kidney disease progression emphasizes the potential role of the complement system in renal pathophysiology, suggesting that targeting complement activation could represent a novel therapeutic strategy to mitigate kidney damage in diabetic patients [65]. However, future research must clarify whether complement activation in high-sugar environments is predominantly localized or systemic including differences in activation intensity, duration, and effector molecule profiles while simultaneously evaluating the efficacy and safety of complement inhibitors in both renal and systemic settings. Only then can drug benefits be precisely quantified in real-world disease contexts, providing reliable evidence for clinical application.

cellular senescence, particularly the senescence of renal tubular epithelial cells, plays a significant role in the development and progression of DKD. Underscoring complement targeted inhibition as a promising therapeutic avenue to attenuate renal injury and improve outcomes in patients with diabetes. The C5a-receptor antagonists PMX53 and W-54011 reviewed here have been shown in db/db mice and STZ rats to markedly suppress inflammatory cytokine release, ameliorate micro-inflammation, and thereby delay cellular senescence. PMX53 additionally restores mitochondrial homeostasis and alleviate oxidative stress, while valproic acid via indirect down regulation of C5aR1 attenuates high-glucose–induced C5aR1 expression and reduces both senescence markers and the SASP [66,158]. While our findings suggest that targeting complement activation may offer a novel therapeutic avenue to reduce renal injury and improve prognosis in patients with diabetes, several limitations must be addressed by future research. First, the evidence base is low-level, with most conclusions derived from animal or cellular studies and lacking validation in large clinical cohorts or randomized controlled trials. Second, critical pharmacological data are missing for PMX53, W-54011, and valproic acid; comprehensive pharmacokinetic, toxicological, and dose–response studies are urgently needed to define safe therapeutic windows, optimal dosing regimens, and administration strategies. Third, current animal models inadequately recapitulate the genetic heterogeneity, metabolic profiles, and comorbidity landscape of human DKD. Future work should incorporate humanized mice, kidney organoid-on-chip platforms, and patient-derived ex vivo perfusion systems to enhance translational fidelity and success.

Conclusions

Within a hyperglycemic environment, the activation of the complement system significantly contributes to the induction of cellular senescence, particularly in proximal tubular epithelial cells, thereby playing a crucial role in the development and progression of DKD. Although all three complement pathways are activated in the circulation of patients with DKD, current research predominantly focuses on the core role of the MBL pathway, with the roles of the classical and alternative pathways in DKD requiring further exploration.

The complement system, particularly C5a, plays a crucial role in promoting renal inflammation and cellular senescence in a high-glucose environment. Cellular senescence is a stable cell-cycle arrest elicited by diverse injurious stimuli. In renal tubular epithelial cells, C5a accelerates this process through multiple pathways, thereby driving the progression of DKD. Current evidence positions C5a-mediated senescence as an ‘upstream trigger’ in DKD: hyperglycaemia promotes abundant C5a generation, which evokes oxidative stress and DNA damage, pushing tubular cells into senescence and unleashing a robust SASP. SASP factors, in turn, intensify inflammation and fibrosis while further activating the complement system, perpetuating elevated C5a levels and establishing positive feedback loop that relentlessly amplifies renal injury.

In summary, this review systematically delineates how C5a/C5aR signaling, through multiple pathways, mediates renal tubular senescence and accelerates DKD progression (Figure 4).

Figure 4.

Schematic illustration of the mechanism by which the complement C5a-driven pathway induces cellular senescence under high-glucose conditions.

Funding Statement

No funding was received.

Disclosure statement

The authors declare no competing financial interests or personal relationships.

Data availability statement

Date availability is not applicable to this article as no new data was generated during the study.