Apoptosis is a PCD process mediated by the caspase family, characterized by cytoplasmic shrinkage, chromatin condensation, nuclear fragmentation, cell membrane blebbing, and the formation of apoptotic bodies, as well as certain biochemical changes such as the exposure of phosphatidylserine to the outer plasma membrane (8–10). Apoptosis eventually leads to nuclear membrane disruption, the cleavage of many intracellular proteins (e.g., polymerase and layer membrane protein), membrane foaming, and the breakdown of genomic DNA into nucleosomal structures. Two main pathways exist in apoptosis: the intrinsic pathway and the extrinsic pathway ( Figure 1 ) (11). Besides, Studies have found that many pathways can directly or indirectly cause apoptosis, such as TGF-β/Smad signaling pathway (12).

Initiated by non-receptor-mediated and characterized by mitochondrial regulation, the intrinsic pathway occurs mainly in the cell (13). Cellular stress can trigger an imbalance in the B-cell lymphoma 2 (Bcl-2) family members, resulting in the loss of mitochondrial membrane potential and an increase in permeability, leading to the formation of the mitochondrial permeability transition pore. This allows pro-apoptotic factors to be released into the cytoplasmic matrix from the mitochondria (14). Cytochrome C released from the mitochondria binds to apoptotic protease-activating factor 1 (Apaf-1) in the presence of deoxyadenosine triphosphate or adenosine triphosphate (ATP), forming the apoptosome complex (15, 16). Apoptosome complex recruit and activate Caspase-9, leading to the formation of apoptotic bodies and further activation of Caspase-3 and Caspase-7 (17, 18). In addition, the caspase activator from mitochondria is also released into the cytoplasmic matrix, binds to the apoptosis-inhibiting protein and inactivates it, thereby enabling apoptosis to proceed (19).In addition, there are other apoptotic factors in the Caspase-independent pathway, such as apoptosis-inducing factors (AIFs), endonuclease G and Caspase-Activated DNase (CAD), which are released from the mitochondria into the cytoplasm, leading to the occurrence of apoptosis (20).

The extrinsic pathway involves the activation of apoptosis through the binding of extracellular ligands to the transmembrane domains of death receptors (9). The extracellular ligands encompass tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). Receptors include type 1 TNF receptor (TNFR1), Fas, DR3, TRAIL-R1 (DR4), TRAIL-R2 (DR5), etc. These ligands selectively interact with distinct receptors, with TNF binding to TNFR1 and TRAIL binding to its specific TRAIL receptor (21, 22). After that, it recruits and binds the cytoplasmic adaptor protein (Fas-associated protein with death domain, FADD) and procaspase-8/-10 to form a death-inducing signaling complex (DISC). In DISC, procaspase-8/-10 autocatalytically cleaves into active caspase-8, followed by caspase-3 activation, resulting in the cleavage of various proteins within cells (20).

The TGF-β/Smad signaling pathway primarily consists of TGF-β and its receptors, as well as various Smad proteins (23). After being recruited to the cell by its type III receptor from the ECM to the vicinity of the cell, active TGF-β further binds to its type II receptor, which then activates type I receptors and downstream effectors Smad2 and Smad 3. At this, activated Smad2 and Smad 3 form a complex with Smad 4, activated smad complex is transported to the nucleus, thus regulating target genes, leading to the occurrence of apoptosis (24, 25). In the latest study, Kobayashi H et al. found that circulating NBL1 promotes apoptosis of kidney cells, especially podocytes. They predicted that inhibition of NBL1 to release BMP proteins can enable their interaction with and inhibition of TGF-β signaling (26) ( Figure 1 ).

There are two routes of apoptosis, either intrinsic and extrinsic. On the left is the intrinsic pathway. Intracellular stress stimulates BH3-only protein and promotes the attachment of BAK or BAX to the outer mitochondrial membrane to form a mitochondrial permeability transition pore, leading to the release of apoptotic factors into the cytoplasm, where cytochrome C binds to Apaf-1 and the formed apoptosome complex recruits and activates Casp-9 to further activate Casp-3/7. Besides, EndoG and AIF in the mitochondria are released and move into the nucleus to break the DNA. TCM interventions: Astragalus polysaccharide (Astragalus membranaceus): Maintains mitochondrial function and suppresses apoptosis via AMPK/SIRT1/PGC-1α. In the middle is the extrinsic pathway. The combination of extracellular ligand and death receptors, successively recruits FADD, Casp-8/10, After self-activation of Casp-8/10, Casp-3 is further activated, leading to cell apoptosis, On the right are the pathways that can cause apoptosis in the DKD. Active TGFβ can further binds to TGFβR II recruited by the type TGFβRIII receptor, which then activates TGFβRI and the downstream effector Smad 2, Smad 3, phosphorylated Smad2 and Smad 3, and then transfer to the nucleus to regulate DNA transcription, leading to apoptosis.

Abbreviations: BH3-only: bcl-2 homology domain only proteins, Bcl-2: B-cell lymphoma-2, BAK: Bcl-2 homologous antagonist/killer, BAX: Recombinant Bcl2 Associated X Protein, Cyt C: Cytochrome Complex C, Casp: cysteine aspartate protease, Apaf-1: apoptotic protease activating factor-1, IAP: Inhibitor of apoptosis proteins, Smac: second mitochondria-derived activator of caspases, AIF:apoptosis inducing factor, EndoG: Endonuclease G, Fas: factor related Apoptosis, FADD: Fas Associating Death Domain Containing Protein, TNFR-1:Tumor Necrosis Factor Receptor 1, TRAIL: Tumor Necrosis Factor Related Apoptosis Inducing Ligand, TβRI/II/III: Recombinant Transforming Growth Factor Beta Receptor I/II/III, TGF-β: transforming growth factor β, TSP: thrombospondin.

Apoptosis plays an important driving role in the progression of DKD. In patients with DKD and animal models, TGF-β ligands, TGFBRs (TGF-β receptors, and downstream signaling molecules such as Smad2 and Smad3 are highly upregulated or activated in the glomerulus, tubules, and renal interstitium (27, 28). Schiffer et al. found that the expression of pro-apoptotic protein Smad1 was positively correlated with early glomerulosclerosis in a TGF-β7 transgenic mouse model (29). Targeting the TGF-β/Smad signaling pathway could reduce apoptosis in glomeruli, tubules and renal interstitium (30).

Kobayashi H et al. found that circulating NBL1 leads to early kidney structural lesions through apoptosis of kidney cells, especially podocytes, which in turn may contribute to progressive kidney function decline and progression to ESKD. They predicted that inhibition of NBL1 to release BMP proteins can enable their interaction with and inhibition of TGF-β signaling, therefore diminishing inflammation and fibrosis in kidney tissue and podocyte loss in DKD. This protein may be an accessible target to slow DKD progression to ESKD. It may be a more appropriate target in this pathway than TGF-β directly (26).

Under pathological conditions of DKD, an excessive production of ROS by podocytes activates the antioxidant defense system, leading to the release of ROS, damage to cellular oxidative components, and ultimately causing apoptosis. However, we can protect podocytes from apoptosis by targeting cellular oxidative stress-related pathways (31). For example, overexpression of Sestrin2 reduces high glucose-induced TSP-1 and ROS generation, where TPS-1 also promotes ROS generation. Sestrin2 can interact with TSP-1 and inhibit the induction effect of TSP-1 on TGF-β1/Smad 3. Therefore, Sestrin2 can alleviate oxidative stress and alleviate the apoptosis and damage of podocytes in DKD by coordinating the downstream TSP-1/TGF-β1/Smad 3 pathway (32). Chen et al. discovered that p53 can be activated by retinoic acid receptor responder protein 1 on podocytes, leading to podocyte apoptosis (33). Xu et al. found that SPAG5-AS1 inhibits podocyte autophagy and exacerbates podocyte apoptosis through the SPAG5/AKT/mTOR pathway (34). In addition to podocytes, tubular epithelial cells are found to be particularly susceptible to the dysregulation caused by diabetic conditions due to their high energy demands and dependence on aerobic metabolism. The occurrence of renal cell apoptosis gives rise to two hallmark features of advanced DKD: tubular atrophy and a significant reduction in the population of tubular epithelial cells (35). Tubular epithelial cells and macrophages form a feedback loop through extracellular vesicle transfer, promoting renal inflammation and renal cell apoptosis in DKD (36).

Pyroptosis is a newly discovered form of PCD that is associated with inflammatory responses. It is characterized by cell swelling and cell membrane rupture leading to the release of cellular contents, and activation of intense inflammatory responses (37). Pyroptosis, being the most immunogenic type of cell death, exhibits a faster progression and triggers the release of a multitude of pro-inflammatory cytokines when compared to apoptosis (38, 39).

The classical pathway of pyroptosis is initiated through the assembly of inflammasomes, which are complex structures formed by NLRP3, ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain, and pro-Caspase-1 ( Figure 2 ) (40). Upon stimulation by intrinsic or extrinsic stress signals, pattern recognition receptors are stimulated, and then NLRP3 binds to NEK7 to trigger its oligomerization (41), then pro-Caspase-1 is activated, which further cleaves pro-IL-18 and pro-IL-1β into mature IL-18 and IL-1β (42). Additionally, the gasdermin family of proteins, which possess pore-forming activity in immune responses, is activated by caspase-1 (42). Activated gasdermin family of proteins release the N-terminal domain that perforates the cell membrane, resulting in cell membrane rupture and the release of inflammatory factors such as IL-6 and TNF-α (43–45).

The non-classical pyroptosis pathway directly activates the caspase proteins, such as caspase-4, caspase-5 and caspase-11, through other pattern recognition receptors such as LPS, to mediate pyroptosis. Taking the non-classical communication pathway of GSDMD as an example, the activation of caspase leads to the cleavage of GSDMD, which triggers the efflux of K+. K+ efflux in turn activates the assembly of NLRP3 inflammasomes, leading to the cleavage of pro-IL-1β and pro-IL-18 (47). This triggers a cytokine storm, causing pyroptosis ( Figure 2 ).

In DKD, podocytes are the important target cells of pyroptosis and have specific manifestations. NLRP3 gain -of-function mutation in podocytes, such as Nlrp3 Gate-of-function mutant (NLRP3A350V), significantly aggravated glomerular damage (such as albuminuria and mesangial dilatation) of DKD. Podocyte specific knockout of NLRP3 or caspase-1 protects kidney function completely or partially (46).

The left of the figure represents the mechanism of pyroptosis in PTEC, podocyte and HRGEC cells (colored in purple) respectively; In the middle is the classical pathway of pyroptosis. Under danger signal stimulation, an inflammasome is formed, which activates pro-Caspase-1 and converts it into active Caspase-1. Caspase-1 further cleaves pro-IL-18 and pro-IL-1β, generating mature IL-18 and IL-1β. Gasdermin D is activated by active Caspase-1, releasing its N-terminal domain. This domain perforates the cell membrane, leading to membrane rupture and the release of a large amount of inflammatory factors (such as IL-6 and TNF-α), triggering cell pyroptosis. The mechanism shown on the far right is similar to apoptosis: TGF-β is transferred to the nucleus through serial reactions to regulate DNA transcription and finally, lead to pyroptosis. SS alleviate HG-induced pyroptosis and oxidative damage by regulating the Nrf2/NLRP3 signaling pathway. Tan IIA: Inhibits ROS-Txnip/NLRP3 axis, mitigating oxidative stress-induced pyroptosis. Carnosine: Targets Caspase-1 to block podocyte pyroptosis, alleviating albuminuria and renal damage. Zuogui-Jiangtang-Yishen Decoction: Reduces podocyte pyroptosis by suppressing intestinal-derived TMAO via mROS-NLRP3 axis. TSF: Inhibits NLRP3 activation via ROS-Txnip axis, attenuating renal injury. FPS suppresses podocyte pyroptosis mediated by the NLRP3 inflammasome by regulating the AMPK mTORC3/NLRP1 signaling axis in DKD. Abbreviations: Ang II: angiotensin II, AGEs: Advanced glycation end-products, HG: high glucose, PTEC: Proximal tubular epithelial cells, HRGEC: human renal glomerular endothelial cell, NOX: NADPH Oxidase, NLRP3: The NACHT, LRR, and PYD domains-containing protein 3, NEK7: NIMA-related kinase 7, ASC: adapter protein apoptosis associated speck-like protein containing a CARD, IL-18: interleukin-18, EMT: epithelial-mesenchymal transition, ROS: reactive oxygen species, PWARSN: Prader Willi/Angelman region RNA, SNRPN neighbor, SS: sorasone base, Tan IIA: Tanshinone IIA, ZGJTYS: Zuogui-Jiangtang-Yishen Decoction, TSF: Tangshen Formula, FPS: Fucinose gum.

In 2012, Dixon first proposed the concept of iron death, a unique form of PCD characterized by dysregulation of iron-mediated lipid peroxide metabolism, leading to imbalance of the redox system and ultimately to oxidative damage to cell membranes and proteins (63–66).

At present, the research on iron death focuses on three classical pathways: systemic Xc-/glutathione (GSH)/GPX 4 signaling pathway, iron metabolism signaling pathway, lipid metabolism pathway, and the emerging FSP 1/CoQ 10/NADPH system (65) ( Figure 3 ).

System Xc− is an amino acid antiporter across the plasma membrane, facilitates the release of cystine (Cys2, an oxidized form of Cys) into the extracellular space, while simultaneously expelling glutamate (Glu) from the cell in a 1:1 ratio (67). Intracellularly, Cys2 is reduced to cysteine (Cys), and Cys undergoes an enzymatic reaction to ultimately produce GSH, an essential cofactor for glutathione peroxidases (GPXs). These GPXs play a crucial role in breaking down hydrogen peroxide and lipid peroxides, thereby inhibiting the formation of lipid ROS and suppressing ferroptosis (68, 69). The mevalonate (MVA) pathway can affect the synthesis of selenocysteine tRNA by down-regulating isopentenyl pyrophosphate (IPP), further interfere with the activity of GPX4, and cause ferroptosis (70).

Ferroptosis is closely associated with the body’s iron metabolism homeostasis. In normal physiological conditions, the cell membrane’s transferrin receptor 1 (TFR1) facilitates the entry of Fe3+ into cells by transporting the complex that formed with the binding of transferrin (TF) and Fe3+, where it is then converted to Fe2+ by the STEAP3 protein and finally stored in the cell (71). Most of the intracellular iron is stored in ferritin form, while a small portion is transported into the cytoplasmic labile iron pool (LIP) through divalent metal transporter 1 (DMT1) (72, 73). Ferroportin (FPN), a membrane iron transporter, oxidizes excess Fe2+ to Fe3+ and mediates the export of iron from cells. In situations of excessive iron accumulation, an abundance of free Fe2+ within the LIP can generate hydroxyl radicals via Fenton reaction. These radicals actively participate in lipid peroxidation reactions and trigger ferroptosis (74). On the other hand, excess Fe2+ induces the activation of iron enzymes such as lipoxygenases (LOXs), further promotes lipid peroxidation in the cell membrane, and ultimately leads to cell death (75).

The accumulation of polyunsaturated fatty acids (PUFAs) is widely recognized as a characteristic feature of ferroptosis (76). This process involves the buildup of fatty acids, including both polyunsaturated and monounsaturated forms (MUFAs), which serve as substrates for lipid peroxidation (77). PUFAs are more susceptible to oxidation compared to MUFAs. Consequently, increasing PUFA levels while reducing MUFA content can facilitate lipid peroxidation and contribute to the progression of ferroptosis. Additionally, crucial enzymes participating in these enzymatic processes encompass long-chain acyl-CoA synthetase 4 (ACSL4), lysophosphatidylcholine acyltransferase 3 (LPCAT3), and arachidonate 15-lipoxygenase (ALOX15). ACSL4 facilitates the esterification of free PUFAs by incorporating them into membrane phospholipids with assistance from LPCAT3. Subsequently, ALOX15 plays a role in initiating the peroxidation process on membrane phospholipids (78). Therefore, it can be inferred that the ACSL4/LPCAT3/ALOX15 pathway promotes ferroptosis through lipid peroxidation.

In recent years, there has also been emerging research on the pathways involved in ferroptosis. Type II NADH ferroptosis inhibitor 1 (FSP1) has also been found. As an alternative mechanism of ferroptosis inhibition, the FSP1/CoQ10/NADPH system has nothing to do with the intracellular expression levels of GSH, GPX4 and ACSL4, and can protect cells from ferroptosis induced by GPX4 inhibition. FSP1 is mainly distributed in lipid droplets and plasma membranes, and can inhibit lipid peroxidation and ferroptosis by reducing lipid free radicals (79). ( Figure 3 )

Solute carrier family 7 member 11 (SLC7A11) is an active subunit of the cystine/glutamate reverse transporter system Xc− (80). Reduced SLC7A11 expression induces reduced cystine uptake, leading to depletion of GSH. p53 is a key tumor suppressor gene that functions as a transcription factor (80). Recent studies have shown that mitochondrial single stranded DNA binding protein 1 (SSBP1) induces phosphorylation of p53 serine 15 (S15) by DNA-dependent protein kinase (DNA-PK) and promotes nuclear accumulation of p53. Inhibition of SLC7A11 expression in high-fructose exposed podocytes drives ferroptosis (81).

Recent studies have found that ferroptosis involves the death of RTECs and podocyte in DKD (82). Due to the high sensitivity of podocytes to ROS, excessive ROS can cause irreversible changes in podocyte structure and function, leading to the occurrence of DKD. RTECs are one of the most mitochondria-rich cell types, so they are more susceptible to oxidative stress damage, which is also part of the pathogenesis of DKD, suggesting that this may be part of the cause of DKD caused by ferroptosis (83, 84).

There are four main mechanisms, from left to right are System Xc-/GSH/GPX4 signaling pathway, lipid metabolism pathway, the FSP1/CoQ10/NADPH system and iron metabolism signaling pathway. The italic characters in the blue box in the figure are abbreviations of drugs.

Abbreviations: Cys: cystine, Cys2: Cystine, Glu: glutamate, GSH: glutathione, GSSG: glutathione disulfide, GPX4: glutathione peroxidase 4, PUFAs: polyunsaturated fatty acids, SLC3A2: solute carrier family 3 member 2, SLC7A11: solute carrier family 7 member 11, GSR: glutathione-disulfide reductase Gene, MVA: the mevalonate pathway, IPP: isopentenyl pyrophosphate, Sec: selenocysteine, PLOOH: phospholipid Hydroperoxides, FSP1: ferroptosis inhibitor 1, Nrf2: nuclear factor erythroid2-related factor 2, HIF-1α: hypoxia inducible factor-1α, TF: transferrin, TFR1: transferrin receptor protein 1, STEAP3: DMT1: divalent metal transporter 1, HO-1: heme oxygenase-1, NCOA4: nuclear receptor coactivator 4, LIP: labile iron pool, ROS: reactive oxygen species, FPN: ferroportin, PD: platycodon grandiflorum D, MGM: mangiferin monosodium salt, Rosi: rosiglitazone, Fer-1: ferritin-1.

Studies have shown that ferroptosis indicators are up-regulated in DKD patients, and the expression of system Xc- and Gpx4 mRNA is also reduced in renal biopsy samples of DKD patients. In DKD animal models, there is a notable increase in iron content in renal tissue, which positively correlates with blood creatinine and urinary protein levels, as well as renal iron protein levels (85, 86). Iron overload, decreased antioxidant capacity, ROS accumulation, and id peroxidation are characteristic changes associated with iron death in the kidneys of streptozotocin-induced DBA/2J diabetic mice (87). Feng et al. observed that iron death causes damage in renal tubules through the hypoxia-inducible factor-1α (HIF-1α)/heme oxygenase-1 (HO-1) pathway (88). Treatment with the selective iron death inhibitor Ferostatin-1 (Fer-1) inhibits the expression of HIF-1α and HO-1, thereby alleviating renal tubular iron overload, inhibiting ROS formation, reducing oxidative stress and lipid peroxidation, thereby exerting protective effect against renal tubular injury and fibrosis in diabetic mice. To summarize, mitochondrial abundance determined the metabolic characteristics and baseline ROS levels of cells, and affected the sensitivity to ferroptosis. Oxidative stress both triggers ferroptosis and accelerates the process of ferroptosis by causing an imbalance in the antioxidant system.

In recent years, the key role of ferroptosis in the occurrence and development of kidney disease has been paid more and more attention, which may become an emerging target for clinical treatment and related drug development (89).

Autophagy is a highly conserved catabolic process that serves to degrade damaged proteins and organelles, ensuring the preservation of cellular homeostasis. Autophagy involves lysosomal degradation and recycling of unnecessary cytoplasmic proteins and organelles, which is considered a self-protective response to stress (90). Under normal physiological conditions, cells maintain low baseline levels of autophagy, and autophagy levels are rapidly increased under factors such as ROS, nutrient and energy deprivation, ER stress, hypoxia, DNA damage, and immune signaling. Mammalian cells exhibit three distinct types of autophagy, classified based on the substrate to be degraded and the pathway through which substrates are transported to lysosomes: autophagy, chaperone-mediated autophagy, and autophagy (73, 91, 92). The most extensively characterized form of autophagy is autophagy (hereafter simply referred to as autophagy), which involves the sequestration of substrates within double-membrane structures called autophagosomes that originate from the endoplasmic reticulum, followed by their fusion with lysosomes for content degradation (93). Autophagy exhibits a notable capacity for degrading entire organelles and large protein aggregates, making it the predominant form of autophagy (94).

Under normal physiological conditions, stress signaling kinases such as JNK-1 phosphorylate Bcl-2 to promote autophagy, which in turn affects regulatory proteins such as Beclin-1 to interact with class III phosphatidylinositol 3-kinase (PI3K) typical of Vps 34 to promote PI3P generation to extend with the phagocytotic membrane and attract Atg proteins. The onset of autophagy is influenced by multiple factors. The AMP-activated protein kinase (AMPK) regulates autophagy through a mammalian target of rapamycin (mTOR) -dependent and mTOR-independent pathways (95). In response to energy deprivation, the AMP/ATP ratio increases, promoting mTORC1 inactivation and ULK 1 complex activation, thereby increasing autophagy levels (96–98); AMPK can also directly initiate autophagy by phosphorylated ULK 1 (99), where activated mTORC1-phosphorylated ULK 1 disrupts the interaction with AMPK and suppresses autophagy. As mTOR integrates growth factors and nutrient signals that regulate cell growth and is responsible for inhibiting autophagy, mTOR is activated and phosphorylates specific sites on ULK 1 and ATG 13 to inhibit the activity of ULK 1-ULK 2 complex and inhibit the occurrence of autophagy (95, 100, 101), and is inhibited during starvation and cellular stress, leading to its dissociation from ULK 1 (102). ULK 1 phosphorylation promotes the recruitment and phosphorylation of various proteins, assembling the ULK 1 complex and translocates to the isolation membrane of the endoplasmic reticulum to initiate autophagy (100, 103). Moreover, Sirtuins (SIRTs) form a class of NAD + -dependent class III histone deacetylases actively involved in metabolic regulation. During cellular energy expenditure, NAD + levels increase, leading to the activation of downstream SIRT 1, and SIRT 1 directly deacetylates the transcription factor forkhead box proteins O1 (FOXO1) and FOXO3a to activate cellular autophagy and restore cellular energy homeostasis (104, 105). After initiation of autophagy, Atg 5-Atg 12 is processed under the participation of two ubiquitin-like coupling pathways, and LC3B-II integrated with it is embedded into the autophagosome membrane in the extension, becomes an autophagy marker and participates in membrane fusion and substrate selection. LC3B-II recognizes proteins such as p62/SQSTM1 on the target substrate, realizes autophagosome membrane closure after selective uptake, and realizes the fusion of lysosome with the help of related proteins such as SNAREs and Rab. After autolysosomes complete substrate degradation, the clathrin bud tube are formed, and the protolysosomes are formed by the breaking of the GTP enzyme DNM 2 (104) ( Figure 4 ).

In addition to autophagy, the specific pathways and mechanisms underlying autophagy and CMA remain to be further elucidated. In autophagy, cytoplasmic components are actively and directly engulfed by the lysosomes through invagination of the lysosomal membrane, subsequently leading to degradation within the lysosomes (106). Chaperone-mediated autophagy relies on the specific recognition of cytoplasmic protein components containing the pentapeptide motif KFERQ by heat shock cognate protein 70 (Hsc70, also known as HSPA8), and the multimeric complex formed by lysosome-associated membrane protein 2 and other cofactors, facilitating the delivery of substrates to the lysosomes for degradation (107).

Podocytes exhibit characteristically high basal autophagy activity, requiring sustained autophagic flux to preserve their functional homeostasis. Autophagy regulation in these cells is principally mediated by AMPK signaling, with supplementary modulation through the JAK2-STAT1-Tfeb axis. While acute mTORC1 inhibition potentiates autophagy via AMPK activation, chronic inhibition results in re-establishment of baseline autophagic activity. Notably. In proximal tubular epithelial cells, physiological autophagy activity remains comparatively quiescent. Nevertheless, constitutive autophagy proves indispensable for cellular homeostasis and mitochondrial quality control, with particularly robust mitophagy observed in cortical segments to accommodate their substantial bioenergetic demands. Glomerular endothelial cells critically depend on autophagy-mediated ROS clearance to sustain capillary integrity. Autophagy impairment in these cells precipitates oxidative damage, manifesting as pathological glomerular capillary dilatation, mesangiolysis, and ultimately glomerulosclerosis (108).

The core molecular mechanism of autophagy is most extensively studied in autophagy, which is often referred to as “autophagy”. The main mechanism pathway of autophagy is presented in detail in the diagram, including phagophore formation, substrate uptake, autolysosome formation, functional lysosome regeneration and regulation pathway with mammalian target of rapamycin (mTOR) as the core. At the same time, the upper right part of the diagram introduces the mechanism of autophagy process in RTECs (HK-2), glomerular mesangial cells (GMC), podocytes and renal cortical epithelial cells under high glucose (HG) environment. The italicized words in the blue box represent drugs.

Abbreviations: JNK-1: c-Jun N-terminal kinase, Bcl-2: B-cell lymphoma-2, Vps34: Vacuolar Protein Sorting 34, PI3K: Phosphoinositide 3-Kinase, Atg: Autophagy related gene, FOXO1: Forkhead box O1, FOXO3a: Forkhead box O3a, SIRTs: Sirtuins, NAD+: Nicotinamide adenine dinucleotide, LC3: MAP1LC3, microtubule-associated proteins light chain 3, PE: Phosphatidyl Ethanolamine, p62/SQSTM1: Sequestosome 1, PI4P: Phosphatydyinositol 4-phosphate, PIP5K1B: Phosphatidylinositol-4-phosphate 5-kinase, type I, PI (4, 5)P2: Phosphatidylinositol-4, 5-bisphosphate, AP2: Adaptor Protein2, DNM2: Dynamin2, mTOR: Mammalian target of rapamycin, ULK1: unc-51 like kinase 1 Gene, AMPK: Adenosine 5’-monophosphate (AMP)-activated protein kinase, SGLT2 inhibitors: Sodium-glucose co-transporter 2 inhibitors, GLP-1RAs: Glucagon like peptide-1 receptor agonists, PF: Paeoniflorin, TG: Tripterygium Glycosides, QDTS: Qi Di Tang Shen granules, QRXZYQF: Qingre Xiazheng Yiqi Fang, JTBF: Jiedu Tongluo Baoshen formula.

During the progression of DKD, the autophagic activity of podocytes decreases, along with the reduced expression levels of autophagy-related proteins, including Beclin-1, LC3-II, and Atg5-Atg12 complex (109–113). In a diabetic mouse model, podocytes exhibited defective autophagy, decreased LC3-II levels, and accumulation of p62, indicating that high glucose reduces podocyte autophagic activity (114). In diabetic mice, the expression of AGEs is increased, which activates mTOR and inhibits the nuclear translocation of pro-autophagy transcription factor EB (TFEB) to inhibit the formation and turnover of podocyte autophagosomes (115). However, other studies have yielded inconsistent results. Another study found that during high glucose treatment, podocytes show increased expression of autophagy-related proteins LC3-II, Beclin-1, and autophagosomes (116, 117), suggesting that high glucose can promote podocyte autophagy. Similar results have also been observed in primary podocytes and podocyte cell lines (115, 118). We analyzed the impact of high glucose on SVI cells (a stable podocyte cell line) and found that 48 hours of high glucose treatment induced autophagy in SVI cells, confirming the short-term high glucose can induce podocyte autophagy. On the other hand, when longer-term exposure (15 days) of high glucose exhibit inhibitory effect on podocyte autophagy, as demonstrated by the decreased LC3B-II expression in SVI podocytes after a 15-day exposure to high glucose (119).

Compared to podocytes, RTECs display lower basal levels of autophagy (120). Both in vitro and in vivo studies have observed mitochondrial fragmentation and depolarization accompanied by autophagy inhibition in RTECs under high glucose conditions (121). This suggests autophagy system inhibition is correlated with mitochondrial dysfunction. Conversely, significant upregulation of LC3-II, autophagosomes, and Beclin1 levels were observed in HK-2 cells and kidneys of diabetic rats, suggesting possible activation of the autophagy pathway under high glucose conditions (122). However, it is also found that autophagy increases after 6 and 12 hours of high glucose culture in PTEC, but is suppressed after 48 or 96 hours. This further supports that short-term high glucose exposure may activate autophagy in response to cellular stress induced by high glucose, but prolonged exposure inhibits autophagy (117).

The autophagic activity of mesangial cells is the most controversial among all types of renal intrinsic cells. In a streptozotocin-induced diabetes model, stronger GFP-LC3 staining and conversion of LC3B-II in glomeruli were observed at 4 weeks, indicating increased autophagic activity in early diabetic glomeruli under high glucose induction. On the other hand, at 8 weeks, LC3 staining decreased and p62/SQSTM1 accumulated in the cytoplasm of glomerular cells, confirming the impairment of autophagic activity in glomeruli under long-term high glucose conditions (119). Specific depletion of Atg5 to inhibit autophagy significantly exacerbated the cytotoxic effects of advanced glycation end products in mesangial cells, suggesting that activated autophagy has a renal protective effect against advanced glycation end products-induced mesangial cell injury (123).

The above research findings suggest, we speculate that short-term exposure to high glucose induces cellular autophagy to protect the kidneys, but long-term high glucose exposure inhibits autophagy, resulting in aggravated damage to renal intrinsic cells and the progression of DKD.

The management of DKD typically involves addressing lifestyle modification, glycemic control, blood pressure control, lipid management and reducing albuminuria. Metformin is commonly prescribed as the initial medication for regulating blood glucose in patients with T2DM. Emerging medications such as SGLT2 inhibitors, GLP1 receptor agonists (GLP-1RAs), non-steroidal mineralocorticoid receptor antagonists (nsMRAs) and renin-angiotensin system(RAS) inhibitors, which are known as the four pillars, offer additional treatment options for DKD (124, 125). Extensive research has demonstrated that these drugs possess renal protective properties and can effectively slow down the progression of DKD by modulating molecules and signaling pathways associated with PCD. However, nonspecific improvement of inflammation could be a secondary consequence of other beneficial effects, such as decreased glomerular hyperfiltration and albuminuria (126). Furthermore, it has been confirmed in retrospective cohort studies and clinical trials that DPP-4 inhibitors have no effect on renal outcomes except for albuminuria, so this kind of drugs will not be discussed here (127–130).