Preclinical and clinical investigations have consistently revealed ERS marker upregulation in renal tissues of patients with DKD and relevant animal models (15–17). Pathological changes in DKD tend to start early with thickening of the basement membrane, gradually spreading to glomeruli, microvessels and tubules. By contrast, changes in glomerular hyaluronan appear in the late stages of DKD (18,19). Persistent urinary protein abnormalities stem predominantly from four sources: i) Renal filtration barrier defects, including those affecting the basement membrane, podocytes and associated cells (20); ii) damage across renal tubular segments (21); iii) renal interstitial functional impairment (22); and iv) immune cell population dysregulation (23). In conclusion, renal histopathology reveals that ERS impacts DKD in three notable manners: Glomerular filtration membrane injury, renal tubular reabsorptive dysfunction and renal interstitial remodeling with immune cell dysregulation (24).

Studies in DKD model mice have revealed that ERS contributes to glomerular injury, as demonstrated by increased GRP78/caspase-12 expression and reduced Bcl-2 levels (25–27) Notably, genetic deletion of ER protein 44 in db/db mice upregulated ATF6, XBP1 and CHOP, further indicating ERS involvement in glomerular filtration membrane injury (28). Composed of endothelial cells, basement membrane and podocytes, the glomerular filtration membrane is important for blood filtration. Damage to these components drives early DKD progression and proteinuria (27). Surface-bound GRP78 drives extracellular matrix (ECM) accumulation through PI3K/Akt activation, while the AMP-activated protein kinase (AMPK)/mTOR pathway promotes late-stage PERK/CHOP upregulation to enhance autophagy, and the IRE1/NF-κB pathway mediates ERS-induced inflammation in podocytes (29–31). Compared with individuals without DKD, renal biopsy analyses of patients with DKD have revealed increased GRP78 and caspase-12 expression in human glomerular podocytes (28,32). The elevation of ATF6 and PERK in human podocytes triggers lipid metabolism disorders and inflammation, which are suppressed by JNK-mediated insulin activity (33). Mechanistically, the high glucose-induced elevation of PERK, ATF6 and IRE1α in rat podocytes is linked to protein arginine methyltransferase 1 (PRMT1) (34). Concurrently, the activation of XBP1s in glomerular mesangial cells triggers histone-lysine N-methyltransferase SETD7-mediated histone methylation, leading to the upregulation of DKD-associated monocyte chemoattractant protein 1 (35,36). These findings indicate that ERS influences the expression of UPR-induced transcription factors, which in turn contribute to fibrotic and inflammatory pathological changes, thereby regulating the balance between adaptive and apoptotic states in podocytes.

The depletion of terminally differentiated podocytes, which form the glomerular filtration barrier, is characteristic of chronic kidney diseases, including DKD. Numerous studies have revealed an association between ERS and podocyte injury in DKD, as ERS markers are upregulated in human and murine podocytes exposed to high glucose; this process is induced by the dysregulation of hyperglycemia, lipid metabolism and insulin signaling (37,38). In mouse podocytes, the accumulation of advanced glycation end-products (AGEs) and high-glucose exposure activate GRP78, CHOP and caspase-12 through distinct pathways, ultimately driving podocyte apoptotic processes (39). PERK-eIF2α activation induces CHOP-mediated podocyte apoptosis, and PERK phosphorylation triggers a CHOP-reticulon 1A (RTN1A) positive feedback loop for synergistic apoptotic enhancement (40). Dysfunction in the insulin PI3K/Akt pathway may underlie GRP78/CHOP-mediated ERS and podocyte apoptosis. Reduced activity of PTEN, a downstream inhibitory regulator, exacerbates ERS, while MEK/ERK signaling counteracts this pathological effect (41). The GRP78-mediated modulation of the MEK kinase 1 (MEKK1)/JNK signaling cascade induces podocyte apoptosis through MEKK1-dependent Ser280 phosphorylation (42).

The ERAD pathway is a specialized mechanism within the ER that employs the ubiquitin-proteasome system to degrade misfolded or aberrantly modified proteins, serving an important role in eliminating these faulty proteins from cells (43). Derlin-2 expression is elevated in podocytes from patients with DKD biopsies, triggering ERAD to sustain the cellular balance (44,45). Similarly, ERS serves an important role in balancing podocyte survival and apoptosis through autophagy, given that compared with DKD, podocytes inherently exhibit a higher basal autophagic activity (46) High glucose levels increase eIF2α, CHOP, caspase-3/12, GRP78, ATF6 and PERK levels in podocytes through the inhibition of autophagy, whereas sarcoendoplasmic reticulum calcium-ATPase (SERCA)2b-mediated ERS attenuation is associated with AMPKα-induced autophagy (46,47). The intercellular transmission of ERS signals, coupled with podocyte-intraglomerular cell interactions, suppresses ERAD through derlin-2 downregulation, inducing podocyte apoptosis in DKD (48). Additional evidence has shown that GRP78/CHOP activation in endothelial cells drives epithelial-mesenchymal transition (EMT) (49), implying a potential role for glomerular endothelial cells in DKD pathogenesis, although the underlying mechanisms remain ambiguous.

UPR-activated transcription factors drive fibrosis and inflammation in glomerular podocytes at the early stage of diabetic kidney injury. Three ERS pathways collectively influence disease progression, including: i) Regulation of podocyte apoptosis through the PI3K/Akt, PTEN and MEK/ERK signaling pathways; ii) maintenance of cellular homeostasis through the activation of ERAD; and iii) modulation of podocyte autophagy via AMPK (50,51). In addition, key ERS factors also contribute to the DKD process by affecting intraglomerular mesangial cells and podocytes (52).

Composed of Henle's loop, proximal and distal tubules and collecting ducts, renal tubules reabsorb small molecules and lipids, with their epithelium serving as a key DKD-targeted cell type susceptible to injury (53,54). Extensive research has established that ERS contributes to DKD-related renal tubular injury, as evidenced by the upregulation of differential ERS markers in human biopsies, animal tissues and in vitro models (15,55–57). This process is driven by the AGE/receptor for AGEs (RAGE) axis, PRMT1 and histone H4 deacetylation (15,58,59), while it is alleviated by calbindin-D28k, transmembrane BAX inhibitor motif-containing protein 6, sestrin 2 and dapagliflozin (60,61). Renal tubular ERS markers (notably including GRP78) can modulate the autophagic processes and drive inflammatory responses by regulating mitochondrial function and reactive oxygen species (ROS) generation (62,63). Saturated fatty acid-associated factors, such as carbohydrate-responsive element-binding protein, cholesterol and palmitic acid (PA), promote ERS marker upregulation in renal tubular cells, causing lipid accumulation and apoptotic cell death (64,65). Therefore, the comprehensive role of ERS in renal tubules is closely intertwined with autophagy, inflammation and lipid peroxidation.

In the renal tubular system, the proximal tubule reabsorbs glucose, amino acids, HCO₃⁻, Cl⁻ and Na⁺, while secreting H⁺. By contrast, the distal tubule and collecting ducts mainly mediate Na⁺ and water reabsorption alongside K⁺ secretion. The proximal and distal tubule of the kidney functional distinction partially explain the structural variations in ERS signaling, with GRP78-governed ATF4/p16/p21 axes promoting AGE/RAGE-driven premature cellular senescence in mouse proximal tubular cells (66). GRP78 and protein disulfide isomerase exhibit distinct regulatory dynamics between proximal and distal tubules, varying across early and late stages of diabetes-induced ERS. Early-stage diabetes triggers PERK activation, while late-stage disease promotes ATF6 activation in renal tubules, resulting in stage-dependent differential ERS responses between proximal and distal segments in DKD (67).

ERS notably regulates renal cell function by suppressing inflammation, inhibiting abnormal proliferation and fibrosis, and combating oxidative damage through the precise modulation of DKD pathways, including the PERK/CHOP and caspase-12 pathways (53,68,69). The mechanistic exploration of ERS in DKD pathway regulation sheds light on DKD pathogenesis and facilitates the identification of novel therapeutic targets (Fig. 2).

The PERK/CHOP pathway regulates ERS and contributes to DKD. The core aspects of the PERK/CHOP pathway comprise the following mechanisms: Initially, ERS is triggered when high glucose levels, oxidative stress and inflammation lead to misfolded protein accumulation in the ER, prompting GRP78/BiP dissociation from PERK and subsequent kinase activation (70). Subsequently, the PERK/eIF2α/ATF4 axis is activated; PERK phosphorylates eIF2α, globally inhibiting translation to reduce ERS burden. Simultaneously, p-eIF2α selectively promotes ATF4 translation, leading to CHOP expression (71) Finally, CHOP-mediated apoptotic signaling occurs (72) CHOP, a transcription factor, upregulates pro-apoptotic genes, such as Bcl-2-interacting mediator of cell death (Bim), p53 upregulated modulator of apoptosis and Bax, and downregulates Bcl-2, ultimately inducing apoptosis (73).

The PERK/CHOP pathway mediates DKD podocyte injury through high glucose-activated PERK/eIF2α/ATF4/CHOP signaling, inducing apoptosis, foot process effacement, proteinuria and glomerulosclerosis, which are exacerbated by pathway hyperactivation (74). The PERK/CHOP pathway drives mesangial cell proliferation, fibrosis and ECM deposition, mediated via pathways such as the TGF-β1/connective tissue growth factor pathway, in DKD, additionally serving as a central mediator of proteinuria and glomerulosclerosis (75). In DKD, the CHOP pathway mediates renal tubular injury. High glucose-induced ERS promotes tubular apoptosis through the PERK/CHOP axis and activates pro-inflammatory cytokines such as IL-6 and TNF-α to exacerbate renal interstitial inflammation (68). Renal tubulointerstitial fibrosis is an important step in the progression of DKD to end-stage renal disease. Sustained PERK activation enhances ATF4 transcription to upregulate CHOP, which downregulates Bcl-2 and promotes apoptosis through ROS production and ATP depletion (76). Concurrently, CHOP triggers growth arrest and DNA-damage-inducible 34 activation to facilitate eIF2α dephosphorylation, a negative feedback mechanism that restores transiently-arrested protein synthesis in the PERK pathway (77).

The PERK/CHOP regulatory axis balances adaptive and apoptotic responses to ERS through integrated positive and negative feedback mechanisms (78). PERK and its downstream effector CHOP exhibit segment-specific regulation across different regions of the renal tubule. Renal tubular cells secrete CHOP-induced fibronectin, driving mesenchymal fibrosis in DKD, a process implying that PERK/CHOP signaling controls the synthesis and release of ECM proteins in these cells (79). The RTN1A/PERK/CHOP positive feedback loop exacerbates ERS-induced podocyte apoptosis, highlighting the dual role of the PERK/CHOP pathway in maintaining ERS balance and promoting pathological cell death (80).

Caspase-12 acts as a key executor of apoptotic signaling, with its activation closely linked to ERS-induced cell death (81). Upon ER misfolded protein accumulation, for example, due to high blood glucose levels, oxidative stress or calcium dyshomeostasis, GRP78/BiP dissociates from ER membrane sensors such as PERK, IRE1 and ATF6 to bind misfolded proteins, triggering the activation of caspase-12 as a downstream responder to ERS signals (82,83). In non-stressed cells, GRP78 associates with pro-caspase-12 to suppress its activity. During ERS, GRP78 dissociation exposes the active site for proteolytic processing by upstream enzymes, including IRE1α and calpain, generating active caspase-12 (p35/p12 subunits) that cleaves caspase-3 to initiate apoptosis (84,85).

In patients with DKD, high glucose induces ERS, which activates caspase-12 and promotes the apoptosis of podocytes and tubular epithelial cells, accelerating proteinuria and renal interstitial fibrosis (86). In the DKD mouse model, CHOP overexpression upregulates caspase-12 to promote apoptosis, whereas caspase-12 inhibition attenuates high glucose-induced renal cell injury (87). ERS also induces calcium dyshomeostasis and mitochondrial calcium overload, indirectly activating the mitochondrial apoptotic pathway via the ER-mitochondrial axis (88). ER-released Ca²⁺ activates peripheral calpains, prompting their translocation to the cytoplasm. Calpain activation results in translocation to the ER outer membrane, triggering caspase-7 translocation and activation (87,89). Both factors process pro-caspase-12 into an active form that translocates to the cytoplasm, activates caspase-9 and −3 through the cytochrome c (CytC)-independent pathway and induces apoptosis (90). In addition, the IRE1α-TRAF2 complex activates caspase-12 through calmodulin decarboxylase, thereby initiating the caspase-mediated apoptotic pathway (91). Caspase-12-dependent signaling contributes to severe podocyte and tubular cell injury in DKD. Therapeutic targeting of this pathway mitigates ERS-mediated apoptosis, slowing DKD progression (92,93). Therefore, caspase-12 signaling is important for ERS-induced apoptosis. Despite genetic polymorphisms limiting its activity in humans, caspase-12 retains diagnostic and therapeutic value for ERS (94). Targeting the caspase-12 pathway may provide novel targets for the treatment of DKD.

JNK, originally identified in its capacity to phosphorylate the amino terminus of the transcription factor c-Jun (95), is also referred to as stress-activated protein kinase. Extracellular stimuli, such as stress signals, cytokines and growth factors, activate JNK, with the activation relying on the cellular microenvironment. JNK inhibition reduces podocyte apoptosis and matrix deposition, and combination with PERK/CHOP inhibitors may enhance therapeutic efficacy (96,97). As an important branch of the MAPK cascade, the JNK pathway orchestrates multiple physiological processes (98). During ERS, activated IRE1α engages TRAF2 to trigger apoptosis signal-regulating kinase 1 (ASK1) activation, generating a trimeric complex that sequentially activates JNK, CHOP and pro-apoptotic factors such as BH3-interacting domain death agonist and Bim, a process that suppresses anti-apoptotic genes, including Bcl-2 and Bcl-xL, and initiates mitochondrial apoptotic pathways (99–102). In addition, phosphorylation at the MEKK1 Ser280 residue activates the JNK signaling pathway, thereby promoting ERS-induced podocyte apoptosis (38,103). High glucose triggers JNK activation in renal cells, inducing apoptosis and fibrosis. These processes are alleviated by JNK inhibitors, which reduce CHOP/caspase-3-dependent apoptosis in ERS-impaired podocytes to mitigate DKD-related renal injury (33). Acting as a central stress response hub, the JNK pathway integrates external stimuli with intracellular signaling to govern inflammation and disease progression, with functional complexity emerging from stimulus subtype specificity, stimulus intensity and cellular context (104,105). Despite the challenges of drug development targeting JNK (such as poor specificity and high concentrations required to inhibit c-Jun phosphorylation), its potential applications in areas such as DKD and neurodegenerative diseases make it a promising therapeutic target (106–109).

The PERK/eIF2α signaling axis, a key branch of ERS signaling, maintains intracellular proteostasis by regulating protein synthesis and gene expression (110). Following the accumulation of misfolded or unfolded proteins in the ER, this pathway is activated to promote either stress adaptation or apoptosis through global translation inhibition and induction of stress-responsive genes (111). The key component of this pathway, PERK, is an ER transmembrane protein with a kinase domain on the cytoplasmic side, while only its stress-sensing domain is present within the lumen of the ER) (112). In unstressed cells, PERK stays inactive by associating with the ER chaperone BiP that has dissociated from GRP78 (113). By contrast, eIF2α is a key factor in the initiation of protein translation, and phosphorylation at its Ser51 site is a central event in the activation of the PERK/eIF2α pathway. This pathway coordinates cellular homeostasis through multifaceted mechanisms, such as translation repression for energy conservation, amino acid transporter induction for survival and proteostasis regulation through reduced synthesis, enhanced folding through mechanisms such as BiP upregulation, or misfolded protein degradation, for example, via ERAD (72).

The PERK/eIF2α branch of the UPR signaling pathway induces autophagy-associated gene expression through the transcriptional regulation of ATF4, thereby promoting the transcription of autophagy-associated genes (114). Downstream genetic effectors include beclin-1, which mediates autophagosome biogenesis and maturation, the ubiquitin-like system and transport receptor genes such as p62 and next to BRCA1 gene 1 protein (115). These transport receptor genes undergo selective degradation via ubiquitinated cargoes. Vaspin, a serine protease inhibitor, suppresses p62 aggregation by forming complexes with GRP78 and heat shock 70 kDa protein 1L (116,117). This mechanism protects organelles from metabolic stress-induced damage through endocytosis-driven autophagy facilitation, maintaining cellular homeostasis under stress (118). The PERK/eIF2α pathway is a ‘regulatory hub’ for ERS through precise regulation at the translational level and reprogramming of gene expression to maintain a balance between cell adaptation and death (119).

The IRE1 signaling cascade acts as a core ERS effector, governing adaptive or apoptotic cell fates predominantly by modulating mRNA splicing and gene transcription (120,121). The pathway is highly evolutionarily conserved from yeast to mammals and is a key mechanism for maintaining ER protein homeostasis. The IRE1/XBP1 axis preserves cellular homeostasis during acute stress by boosting protein folding, degradation and transport, while governing the activity of secretory cells, such as plasma and pancreatic β-cells, for efficient antibody and insulin production (122,123). Conversely, chronic ERS in pancreatic β-cells engages the IRE1/JNK pathway, causing β-cell death and diminishing insulin release (124,125).

The IRE1 pathway engages in crosstalk with the other two UPR arms: The PERK/eIF2α and ATF6 pathways. The PERK branch suppresses global translation through eIF2α phosphorylation to reduce protein synthesis load, while ATF6 activates BiP, ERAD and other chaperone genes through nuclear translocation (126,127). The IRE1 pathway promotes protein folding and degradation through XBP1 splicing, concurrently regulating cell fate through JNK activation and the regulated IRE1-dependent decay (RIDD) pathway (111,128). Collectively, these three pathways dictate cellular outcomes during stress, promoting survival under acute stress conditions and triggering apoptosis under prolonged or severe stress (129). In the ERS state, IRE1/XBP1 signaling directly promotes the expression of Bcl-2, increases the binding of Bcl-2 to beclin-1 and downregulates the activity of beclin-1, thus inhibiting autophagy (2). During early ERS, IRE1α assembles with TRAF2 and ASK1 to activate the downstream JNK signaling cascade, which phosphorylates Bcl-2 to promote its dissociation from beclin-1 and encourages subsequent autophagy initiation (92,93). In addition, XBP1, a downstream effector of IRE1α, promotes autophagy through beclin-1 activation (43). Blocking the kinase and RNase activities of IRE1α has been shown to intensify PA-induced cytotoxicity in renal tubular cells, indicating that IRE1α protects against PA-mediated kidney injury in DKD (130). The IRE1 signaling pathway is a ‘multifunctional hub’ of the ERS response, responsible for shutting down and splicing of the mRNA that encodes secretory proteins in a process called regulated RIDD to decrease ER protein load and spliced mRNA of XBP1 that activated the transcription of ER hemostatic factors as chaperone and endoplasmic reticulum-associated degradation components that degraded unfolded and misfolded proteins in an attempt to resolve endoplasmic reticulum stress; if this stress was unresolved, XBP1 upregulated the expression of JNK that activated podocytes, tubular apoptosis, and inflammation (131), and its aberrant activation is closely associated with a variety of diseases(such as Sepsis, DKD, breast cancer et.) (131–133).

Calcium channels are the core molecular machinery regulating the intracellular flow of Ca²⁺ across the membrane (129). DKD marked by glomerular mesangial matrix dilation, basement membrane thickening, tubulointerstitial fibrosis and renal unit loss involves Ca²⁺ as a key signaling molecule; calcium homeostatic imbalance markedly drives disease pathogenesis (134,135). The ER functions as a primary intracellular Ca²⁺ store, regulating calcium homeostasis (136). During stress, Ca²⁺ influx sustains cellular homeostasis by activating the MAPK pathway, thereby promoting intracellular protein folding (137,138). Excessive release of Ca²⁺ into the cytoplasm initiates ERS, thereby activating caspase-12-dependent autophagy (139). The transient ER surface calcium concentration is a key signal that determines the activation of ER autophagy (140). In addition, ATP and other Ca²⁺ mobilizers antagonize mTOR-mediated autophagy inhibition via Ca²⁺-dependent pathways, thereby promoting beclin-1- or autophagy-related gene 7-dependent autophagosome biogenesis (141,142). In addition, Ca²⁺ can activate both protein kinase C (PKC) and death-associated protein kinase (DAPK). PKC induces autophagy through an mTOR-independent pathway, whereas DAPK promotes beclin-1 phosphorylation and disrupts the beclin-1/Bcl-2 complex, thereby activating autophagy (143,144). The ERS-associated inhibition of PERK and IRE1 protects against kidney injury, mitochondrial dysfunction and renal cell apoptosis by maintaining Ca²⁺ homeostasis, which blocks cytosolic release of CytC and apoptosis-inducing factor, inhibits caspase-9 activation and restores mitochondrial membrane integrity, thereby suppressing pathological cell death pathways (145). Calcium channels are ‘signaling hubs’ for cellular functions, and their diversity and sophisticated regulatory mechanisms serve key roles in physiological and pathological processes (129,134).