Diabetes mellitus (DM) represents a global metabolic dysregulation syndrome, where T2DM is pathophysiologically characterized by insulin resistance. T2DM is often accompanied by various complications, including skeletal muscle atrophy. Skeletal muscle, serving as the principal regulator of systemic glucose homeostasis, functions as the primary insulin-target tissue that mediates postprandial glucose disposal through GLUT4-dependent uptake mechanisms (Sylow et al., 2021). Skeletal muscle atrophy is pathologically associated with diverse clinical contexts spanning: (1) age-related sarcopenia; (2) chronic diseases; and (3) malignancy-associated cachexia (Ishii et al., 2008; Mendes et al., 2015; Zhang H. et al., 2023; Zhang et al., 2024). Emerging evidence demonstrates that dysregulated mROS production and caspase-3 activation constitute pathophysiological mechanisms driving proteolytic degradation in atrophying muscle (Plant et al., 2009; Min et al., 2011; Zhang et al., 2024).

The endoplasmic reticulum (ER) serves as a central organelle for cellular proteostasis, coordinating protein synthesis, post-translational modification, secretory trafficking, and folding quality control (Yoshida, 2007). ERS can be triggered by various pathological and physiological factors, including Ca2+ dysregulation, redox imbalance, defective protein folding, and other stressors (Jiang et al., 2021). In patients with T2DM, obesity is frequently associated with enhanced adipokine secretion, which subsequently promotes excessive reactive oxygen species (ROS) generation and induces systemic oxidative stress (Fernandez-Sanchez et al., 2011). As the primary defense mechanism against oxidative damage, the endogenous antioxidant system mobilizes crucial enzymes including superoxide dismutase (SOD) and catalase (CAT) to establish the first-line protective barrier against oxidative assault (Jomova et al., 2024). This compensatory upregulation represents a critical adaptive response to maintain redox homeostasis. Furthermore, the persistent oxidative imbalance contributes to the accumulation of misfolded proteins, thereby triggering endoplasmic reticulum (ER) stress through impaired protein folding capacity (Ong and Logue, 2023). Physiological Unfolded Protein Response (UPR) activation under moderate ERS orchestrates proteostatic adaptation through three transmembrane sensors. Conversely, sustained ERS drives terminal UPR signaling that activates caspase-dependent apoptotic cascades, culminating in programmed cell death (Park et al., 2019; Guo et al., 2024). The UPR is mediated by three evolutionarily conserved sensors: inositol-requiring enzyme 1α (IRE1α); protein kinase R (PKR)-like ER kinase (PERK); and activating transcription factor 6 (ATF6) (Casas-Martinez et al., 2024). Under proteostatic equilibrium, these sensors remain sequestered by the ER chaperone BiP/GRP78. During ERS, misfolded protein overload competitively dissociates BiP/GRP78 from the sensors, enabling their oligomerization and activation. PERK activation catalyzes eIF2α phosphorylation, which transiently attenuates global translation while paradoxically enabling ATF4-mediated transcriptional upregulation of pro-apoptotic factor CHOP. ATF4 and CHOP synergistically stimulate the expression of genes related to apoptosis, autophagy, and antioxidant responses (Lu et al., 2024). IRE1α endoribonuclease activity executes XBP1 mRNA splicing, generating sXBP1 for UPR gene activation, while concomitantly activating apoptosis signal-regulating kinase 1 (ASK1)-JNK1 signaling (Brozzi et al., 2015; Lu et al., 2024). Activated ATF6 translocates to the Golgi for proteolytic cleavage, yielding a transcriptionally active fragment that represses anti-apoptotic Bcl-2 proteins while inducing pro-apoptotic effectors. In myocytes, ATF6 hyperactivation provokes caspase-9-mediated intrinsic apoptosis via mitochondrial permeabilization (Kapuy, 2024).

Akt (protein kinase B, PKB), a pivotal node in insulin receptor substrate (IRS) signaling, coordinates cellular proliferation, survival, and metabolic homeostasis through phosphorylation-dependent regulation of downstream effectors (Viglietto et al., 2002; Bao et al., 2004; Li et al., 2007). Following insulin receptor activation, phosphatidylinositol-3,4,5-trisphosphate (PIP3) recruits Akt to the plasma membrane via pleckstrin homology (PH) domain interaction, enabling PDK1-mediated phosphorylation at Thr308 within the activation loop (Stephens et al., 1998). Concomitant mTOR complex 2 (mTORC2)-dependent phosphorylation at Ser473 in the hydrophobic motif confers full catalytic competence (Zeng et al., 2007). Activated Akt modulates downstream targets, including FoxOs, cell cycle regulators, mTOR, and GSK3, to regulate diverse physiological functions (Manning and Cantley, 2007; Vantler et al., 2023).

ERS-induced apoptosis may play a pivotal role in diabetic skeletal muscle atrophy. However, the specific mechanisms by which ERS regulates skeletal muscle atrophy in diabetes remain unclear. This research aims to explore the molecular mechanisms of ERS-mediated skeletal muscle atrophy in diabetic mice. By comparing skeletal muscle alterations, ERS marker expression, and apoptosis between diabetic and control groups, we seek to provide new insights into the pathogenesis of diabetic muscle atrophy and theoretical support for developing ERS-targeted therapies.

Maintaining redox homeostasis is crucial for normal physiological functions. Under pathological conditions, oxidative stress disrupts this balance and participates in the pathogenesis of multiple diseases including DM, inflammatory responses, and tumors (Martemucci et al., 2023). As a vital component of cellular stress responses, ERS interacts closely with oxidative stress, the UPS, and inflammatory signaling pathways through molecular mechanisms (Liu et al., 2023). SOD and CAT, serving as the first line of defense against oxidative stress, constitute the most robust protective barrier against oxidative damage (Jomova et al., 2024). Glutathione (GSH), a central antioxidant, maintains redox homeostasis in biological systems. Studies in the epididymis of T2DM rodents demonstrated significantly reduced glutathione peroxidase (GPx) activity and elevated glutathione reductase (GR) levels (Diniz et al., 2022). Another study further revealed that diminished GSH pools in diabetic systems exacerbate oxidative stress and inflammatory responses through redox imbalance (Dawi et al., 2024). These findings collectively imply that disrupted glutathione metabolism may underlie diabetes-associated tissue dysfunction. Research have revealed that oxidative stress disrupts nascent ER protein folding, triggering the UPR to handle misfolded proteins and activating downstream stress sensors that induce ER stress (Peserico et al., 2020). Our results demonstrated that diabetic mice exhibited elevated oxidative stress levels accompanied by reduced protein expression of antioxidant enzymes SOD and CAT. Concurrently, increased protein levels of ER chaperone Bip, UPR stress sensors ATF6 and IRE1α, along with their downstream regulators p-eIF2α, CHOP, and p-JNK were observed. Persistent ER stress ultimately leads to apoptosis-mediated cell death (Yang et al., 2008), while experimental evidence shows that knocking down CHOP or JNK protein levels attenuates apoptosis induction (Yang et al., 2021). The release of CytC into cytoplasm represents a critical event in the mitochondrial-dependent intrinsic apoptotic pathway, where cytosolic CytC combines with apoptotic protease-activating factors to initiate caspase cascade reactions, ultimately causing cellular damage (Wang et al., 2016). Our results showed upregulation of cleaved Caspase-3, Bax and CytC (pro-apoptotic) with concomitant downregulation of Bcl-2 (anti-apoptotic), indicating the occurrence of apoptosis that directly contributes to skeletal muscle atrophy.

Skeletal muscle atrophy primarily arises from three key mechanisms: increased proteolysis, reduced protein synthesis, and impaired myofiber regeneration. These processes are regulated by multiple molecular pathways, including the ubiquitin-proteasome system (UPS), autophagy-lysosomal pathway (ALP), calpain system, caspase pathway, IGF-1/Akt protein synthesis pathway, myostatin signaling, and muscle satellite cells (Ji et al., 2022). Among these, the PI3K/Akt/mTOR signaling pathway plays a central role in modulating protein synthesis in skeletal muscle. Activation of Akt inhibits FoxO-mediated transcription of muscle-specific E3 ubiquitin ligases, such as MuRF1 and Atrogin-1, while simultaneously promoting protein synthesis (Egerman and Glass, 2014). Chronic ER stress has been shown to impair the RTK/PI3K/AKT signaling pathway (Qin et al., 2010; Lu et al., 2017). For example, ER stress in aged rat livers suppresses insulin signaling, inhibits Akt, and upregulates FOXO expression (Kim et al., 2019). FoxOs are transcription factors that regulate E3 ubiquitin ligases. When inhibiting the Akt pathway, nuclear FoxOs promote the expression of MuRF1/atrogin-1, activating the UPS and driving protein degradation and skeletal muscle atrophy (Ji et al., 2022). Similarly, dietary selenium deficiency or excess in rainbow trout modulates the Akt/FoxO3a pathway, accelerating ubiquitin-mediated muscle protein degradation (Zhang et al., 2022).

In our research, we observed reduced Akt activity and elevated Atrogin-1 expression in the skeletal muscle of diabetic mice compared to control mice, accompanied by significant muscle atrophy. This muscular degeneration inevitably leads to impaired motor function, severely impacting the quality of life of affected individuals and imposing substantial personal, familial, and societal burdens.

While this study revealed significant dysregulation of ERS-related molecular markers in diabetic muscle atrophy, the causal relationships remain to be directly validated through functional interventions. Notably, previous investigations have demonstrated that 4-phenylbutyric acid (4-PBA), a chemical chaperone and ERS inhibitor, can ameliorate triptolide (TP)/lipopolysaccharide (LPS)-induced ERS-associated apoptosis and hepatic oxidative stress (Cheng et al., 2024). Furthermore, emerging evidence indicates that pharmacological inhibition of ERS with Salubrinal effectively attenuates ferroptosis and cellular damage in diabetic myocardial ischemia/reperfusion injury (Li et al., 2020). Based on these mechanistic insights, we recommend the following strategies to establish causal relationships: (1) Pharmacological validation using FDA-approved ERS inhibitors (e.g., 4-PBA or tauroursodeoxycholic acid [TUDCA]) in both Db/db mice and diabetic myotube models to assess potential reversal of muscle atrophy phenotypes; (2) Genetic validation through targeted knockdown of core ERS regulators (such as PERK, ATF6, or IRE1α) to systematically evaluate their regulatory effects on apoptotic pathways and ubiquitin-proteasome-mediated protein degradation. These investigations will advance the development of ERS-targeted therapies for diabetic sarcopenia.

As demonstrated in Figure 8, our research uncovers a dual regulatory mechanism underlying diabetic muscle atrophy. The diabetic skeletal muscle microenvironment exhibits elevated oxidative stress and significantly enhanced ER stress, which promotes direct muscle atrophy through ER stress sensor-mediated apoptosis. Concurrently, sustained ER stress suppresses Akt activity while upregulating the muscle-specific E3 ubiquitin ligase Atrogin-1, thereby accelerating proteolysis and inducing indirect muscle wasting. These findings provide crucial mechanistic insights into diabetic skeletal myopathy, highlighting the ER stress signaling network as a promising therapeutic target for mitigating muscle atrophy in diabetes. Future studies should elucidate the long-term consequences of ER stress modulation, identifying additional molecular players in this pathological cascade, and developing comprehensive intervention strategies to prevent and treat this debilitating complication.