Insulin and IGF-1 are potent anabolic factors that maintain muscle growth and regulate protein synthesis in skeletal muscle (Yoshida and Delafontaine, 2020). Changes in IGF-1 signaling affect the size and function of muscle fibers, inducing protein synthesis with inhibition of its catabolism and promoting muscle mass gain through activation of the PI3K/Akt/mTOR pathway (Yoshida and Delafontaine, 2020; Canfora et al., 2022; Baht et al., 2020).

Insulin and IGF-1 are activated by binding to the insulin receptor on the plasma membrane of target cells, initiating a downstream signaling cascade that results in phosphorylation and activation of insulin receptor substrate 1 (IRS-1), PI3K, and Akt phosphorylation (Yoshida and Delafontaine, 2020; Sharlo et al., 2021; White, 2021). Akt activation phosphorylates and inactivates tuberous sclerosis complex 2 (TSC2), which promotes the activation of mammalian target of rapamycin complex 1 (mTORC1) by Ras homolog enriched in brain (Rheb), 70 kDa ribosomal protein S6 kinase (p70S6K) phosphorylation, and ribosomal protein S6, consequently promoting protein synthesis (Ji et al., 2022). Additionally, mTORC1 phosphorylates eukaryotic translation initiation factor 4E binding protein (4EBP1), releasing it from eukaryotic translation initiation factor 4E (EIF4E). Free EIF4E can bind with EIF4A and EIF4G to form a translation initiation complex, which initiates the binding of mRNAs to ribosomes, promotes translation initiation, and increases protein synthesis (Ji et al., 2022; Grüner et al., 2016) (Figure 1).

However, following hind limb suspension in rats, muscle ribosome degradation increases, biosynthesis decreases, and translational capacity diminishes (Figueiredo et al., 2021; Mathis et al., 2017). Furthermore, inhibition of mTORC1 causes a decrease in ribosomal DNA transcription, promotes ribose autophagy (Figueiredo et al., 2021), and rapidly dephosphorylates 4EBP1 and p70S6K, inhibiting the PI3K/Akt/mTOR pathway (Figueiredo et al., 2021). Additionally, E3 ubiquitin ligase Casitas B-cell lymphoma-b protein leads to diminished protein synthesis due to attenuated PI3K-dependent signaling and Akt dephosphorylation (Sharlo et al., 2021) (Figure 1). Another study has shown that Kr ü ppel like factor 15 (KLF15) and IL-6 are upregulated in skeletal muscle of mice with limb fixation. If KLF15 is deficient or IL-6 is systemically deficient, mice will be protected from fixation induced muscle atrophy (Hirata et al., 2022).

GSK3 is a serine/threonine kinase that regulates cytoskeletal composition and protein synthesis during the growth and development of the organism (Hajka et al., 2021). It has two isoforms, GSK3α and GSK3β, activated by autophosphorylation of tyrosine 279 and tyrosine 216, respectively (Patel and Werstuck, 2021). In response to insulin stimulation, Akt can lead to a decrease in GSK3α/β activity by phosphorylating serine 21 of GSK3α and serine 9 of GSK3β (Patel and Werstuck, 2021).

The dopaminergic D2 receptor activation promotes complex formation between Akt, GSK3, β-arrestin, and protein phosphatase 2A (PP2A). PP2A promotes dephosphorylation, which inactivates Akt and activates GSK3α at serine 21 and GSK3β at serine 9. GSK3 then facilitates complex formation, further activating GSK3α/β, inhibiting the PI3K/Akt/mTORC1 signaling pathway and reducing protein synthesis (Patel and Werstuck, 2021; Beurel et al., 2015; Papadopoli et al., 2021).

GSK3β isoforms are predominant in reducing protein synthesis (Hajka et al., 2021) through multiple mechanisms (Figure 1) (Figure 2):

(1) In cases of rat soleus muscle atrophy induced by dexamethasone and muscle disuse, dephosphorylation of GSK3β-Ser9 increases GSK3β activity, inducing TSC2 phosphorylation and subsequent inhibition of mTORC1, which reduces protein synthesis (Mirzoev et al., 2021);

Endogenous and exogenous glucocorticoid spillage can negatively affect skeletal muscle mass and function (Britto et al., 2014). Specifically, when glucocorticoids are present in excess, their receptors promote the upregulation of regulated in development and DNA damage response 1 (REDD1) and Kruppel-like Factor 15 (KLF15) (Britto et al., 2014). Upregulated REDD1 and KLF15 activate their upstream target, TSC2, inhibiting mTORC1. Upregulated KLF15 promotes branched-chain amino acid catabolism mediated by branched-chain transcarbamylase 2 with reduced mTORC1 activity (Nunes et al., 2022). Under diabetic conditions, downregulation of E3 ubiquitin ligase WWP1 and upregulation of KLF15 lead to muscle atrophy (Hirata et al., 2019). Glucocorticoids also induce insulin and IGF-1 resistance, upregulate MSTN expression, and culminate in reduced protein synthesis (Ji et al., 2022; Franco-Romero and Sandri, 2021) (Figure 1).

AMPK is a negative regulator of protein synthesis in skeletal muscle and is activated in the context of diabetes or triggered insulin resistance (such as glucocorticoid therapy, chronic inflammation, and lipid overload) (Franco-Romero and Sandri, 2021). Inhibition of the mTORC1 signaling pathway leads to reduced protein synthesis in skeletal muscle mainly through four pathways (White, 2021):

(1) Phosphorylation of mTOR-Thr2446, which prevents Akt-mediated phosphorylation of mTOR-Ser2448, thereby inhibiting translation initiation (Franco-Romero and Sandri, 2021);

Branched-chain amino acids, especially leucine, promote muscle protein synthesis and prevent catabolism by activating the mTOR signaling pathway. They can also serve as substrates for protein synthesis (Dimou et al., 2022). However, an increase in the expression of branched-chain amino acid sensors inhibits mTORC1 activity. For example, selenocysteine 2 protein (Sestrin2) is a leucine sensor that negatively regulates the mTORC1 pathway (Chantranupong et al., 2014). Sestrin2, leucine, and GTPase activating protein activity toward rags complex 2 (GATOR2) maintain an equilibrium (Wolfson and Sabatini, 2017). Under conditions of leucine deprivation or DNA damage, general control nonderepressible kinase 2 (GCN2) is activated, and ATF4 is upregulated, inducing upregulation of Sestrin2 expression in response to cellular stress, increased binding of Sestrin2 to GATOR2, disruption of the interaction of GATOR2 with GATOR1. Consequently, this increases free GATOR1 levels, enhances the interaction of Sestrin2 with Ras-related GTPase (Rag GTPase), inactivates Rag GTPase, and recruits TSC2 to the lysosome to inactivate Rheb. This process blocks leucine-induced lysosomal localization of mTORC1 and inhibits its kinase activity, ultimately decreasing the phosphorylation of p70S6K and 4EBP1, thereby inhibiting the mTORC1 signaling pathway (Wolfson and Sabatini, 2017; Ye et al., 2015; Budanov and Karin, 2008). Activated general control nonderepressible kinase 2 (GCN2) also phosphorylates eukaryotic initiation factor 2α (EIF2α), resulting in blocked GDP to GTP conversion on EIF2 and delayed translation initiation (Lehman et al., 2015). Leucine deprivation also increases the phosphorylation of eEF2α and slows the elongation rate, decreasing translation efficiency (Lehman et al., 2015). In leucine deficiency, Sestrin2 activates AMPK through direct interaction and induces phosphorylation of TSC2, which converts Rheb from a GTP-bound state to a GDP-bound state, inactivating Rheb and thus inhibiting the mTORC1 pathway (Xu et al., 2019). Under arginine depletion conditions, the cytosolic arginine sensor for mTORC1 subunit 1 (CASTOR1) can bind to GATOR2, disrupting the interaction between GATOR2 and GTPase activating protein activity toward rags complex 2 (GATOR1). This increase in free GATOR1 promotes the conversion of Rag GTPase to Rag GDPase, preventing the activation of the mTORC1 pathway (Franco-Romero and Sandri, 2021) (Figure 3).

MSTN, a transforming growth factor-β superfamily member, is produced and secreted mainly by skeletal muscle. It inhibits the proliferation and differentiation of myoblasts and is a negative regulator of skeletal muscle growth and development (Yoshida and Delafontaine, 2020; Chen et al., 2021). MSTN synthesis is increased by stimuli such as inflammation, oxidative stress, angiotensin II, and glucocorticoids (Ji et al., 2022). On the surface of myocytes, MSTN binds first to Activin Receptor Type IIB and then to Activin Receptor Like Kinase 4/5 (ALK4/5) to form a complex and activate it. Subsequently, phosphorylation of Smad2/3 induces the formation of a complex between Smad2/3 and Smad4, inactivation of Akt, blockage of Akt/mTORC1 signaling, and reduced protein synthesis (Sartori et al., 2021; Verzola et al., 2019). MSTN signaling can also be independent of Smad regulation (Chen et al., 2021). Activation of the nuclear transcription factor-κB (NF-κB) pathway can be induced by tumor necrosis factor-α (TNF-α), which stimulates the upregulation of MSTN expression in myoblasts and inhibits the proliferation of myoblasts through the activation of p38 mitogen-activated protein kinase (p38 MAPK) via the transforming growth factor beta-activated kinase 1-mitogen-activated protein kinase 6 (TAK1-MAPK6) pathway (Chen et al., 2021). In patients with chronic obstructive pulmonary emphysema, MSTN is elevated in skeletal muscle, which activates the NF-κB pathway, induces elevated reactive oxygen species (ROS) in skeletal muscle via the Smad2/3-TNF-α-NF-κB pathway, and inhibits the Akt/mTORC1 pathway, leading to decreased protein synthesis (Ji et al., 2022) (Figure 4).

In sepsis, cancer, chronic heart failure, and diabetes, high levels of pro-inflammatory cytokines such as IL-1β, IL-6, and TNF-α act directly on NF-κB, p38 mitogen-activated protein kinase, and Janus kinase-Signal Transducer and Activator of Transcription (JAK-STAT) pathways through the corresponding receptors, resulting in increased expression of suppressor of cytokine signaling 3, degradation of IRS1, and inhibition of the Akt/mTORC1 pathway, and decreased protein synthesis (Ji et al., 2022; Xu et al., 2019). During sepsis, activated IL-6 binds to cell surface α-receptors, leading to Glycoprotein 130 (GP130) recruitment and homodimerization, and the activated IL-6-α receptor binds to GP130 to form a complex, leading to GP130 phosphorylation and tyrosine kinase of the JAK family (mainly JAK1, JAK2, and tyrosine kinase 2) activation and binding to STAT, which phosphorylates STAT, leading to enhanced expression of CCAAT enhancer-binding protein δ, interference with insulin-induced IRS-1/Akt signaling, and inhibition of protein synthesis (Ji et al., 2022; Zanders et al., 2022) (Figure 4).

Oxidative stress will produce ROS, which is mainly produced in mitochondria, and accumulation of ROS can induce muscle atrophy under a variety of pathological conditions (such as aging, disuse, diabetes, denervation and cancer cachexia) (Xu et al., 2025; Eshima et al., 2025; Eshima et al., 2023). When the mitochondria of skeletal muscle are damaged due to ROS production, lipid peroxidation within the mitochondria intensifies, leading to increased mitochondrial damage (Xu et al., 2025; Hughes et al., 2022).

During the aging process, high levels of ROS induce oxidative damage to nucleic acids, proteins, and lipids, leading to a decrease in protein synthesis (Xu et al., 2025). Under high oxidative stress conditions, the activity or content of dihydropyridine receptors in muscles is significantly reduced, the expression of dihydropyridine receptors is decreased, the number of unconjugated ryanodine receptors is increased, the release of Ca²⁺in the sarcoplasmic reticulum is reduced, resulting in a significant decrease in the excitation contraction coupling of muscles, a decrease in muscle strength, and ultimately leading to muscle atrophy (Xu et al., 2025; Ahn et al., 2022).

Limb disuse and diabetes will increase the oxidative status of muscle tissue, reduce the level of antioxidant enzymes in the body, lead to excessive increase of ROS, reduce mitochondrial biology, and promote oxidative damage (Chen et al., 2023; Lv et al., 2022). When the load on muscles increases, neuronal nitric oxide synthase (nNOS) is activated, and nitric oxide reacts with superoxide to produce peroxynitrite, activating the transient receptor potential cation channel, subfamily V, member 1, inducing Ca²⁺ release in the sarcoplasmic reticulum, triggering mTOR activation, and inducing increased protein synthesis (Ito et al., 2013). On the contrary, disuse of limbs leads to a decrease in protein synthesis. Elevated ROS levels in patients with diabetes can directly damage β cells in pancreatic islets and promote apoptosis, induce insulin resistance, inhibit the PI3K/Akt/mTORC1 pathway, impede the initial stages of mRNA translation, and reduce protein synthesis (Zhang et al., 2023; Kubat et al., 2023). It is worth mentioning that deuterium-enhanced polyunsaturated fatty acids can resist the lipid peroxidation chain reaction triggered by ROS, inhibit lipid peroxidation, and prevent muscle atrophy and weakness caused by diabetes (Eshima et al., 2025).

In disuse muscular atrophy induced by denervation, skeletal muscle blood perfusion is reduced, resulting in ischemia and hypoxia. Long-term ischemia and hypoxia promote excessive production of ROS, resulting in oxidative damage, increase of inflammatory factors, activation of inflammatory response, and aggravation of skeletal muscle injury (Chen et al., 2023; Zhang et al., 2023).

In cancer cachexia patients, the expression of IL-6, TNF-α, and MSTN increases, leading to activation of the STAT3 and NF-κB pathways, increased inflammation, decreased activity and expression of peroxisome proliferator activated receptor-γ coactivator factor-1α, decreased antioxidant capacity, increased oxidative stress, reduced mitochondrial biogenesis, resulting in inhibition of the synthetic metabolic pathway (Abu et al., 2023; Carson et al., 2016) (Figure 4).

ATF4, a member of the CREB/ATF family, participates in anabolic cellular stress responses and acts as a downstream mediator of the EIF2α kinase and mTORC1 in response to anti-insulin/IGF-1 signaling. It is activated during oxidative stress and endoplasmic reticulum (ER) stress (Ebert et al., 2019). In patients with lateral sclerosis, unfolded and misfolded proteins accumulate in the lumen of the ER. Immunoglobulin-binding proteins bind to these proteins, activating the protein kinase RNA-like ER kinase, which leads to increased phosphorylation of EIF2α and the expression of ATF4 mRNA while inhibiting EIF2B activity. This process reduces ribosome assembly and protein synthesis (Sartori et al., 2021; Chen et al., 2015; Ebert et al., 2022). During fasting, immobilization, and aging, activated ATF4 promotes growth arrest and DNA damage-induced 45α (Gadd45α) protein expression. This protein binds to mitogen-activated protein kinase kinase kinase 4 (MAP3K4) to form a complex that activates downstream mitogen-activated protein kinases 3, 4, 6, and 7, leading to p38MAPK activation, which reduces mitochondrial production, inhibits protein synthesis, and promotes muscle atrophy (Ebert et al., 2019). In contrast, during denervation, growth arrest and DNA damage-induced 45α (Gadd45α) protein expression is induced by histone deacetylase 4 (Bongers et al., 2013; Bullard et al., 2016) (Figure 3).

Glycyl-tRNA synthetase (GARS) mediates protein synthesis by catalyzing the binding of glycine to tRNA to generate glycyl-tRNA, generating glycine residues to cognate tRNAs during mitochondrial and cytoplasmic protein translation (Galindo-Feria et al., 2022). GARS mutations can lead to type 2D Charcot Marie Tooth disease (Zhao et al., 2021). These mutations cause ribosomal arrest by inhibiting the regulation of glycyl-tRNA at the ribosomal A site, which induces phosphorylation of EIF2α and activates the integrated stress response, leading to inhibition of translational initiation (Mendonsa et al., 2021). The downregulation or complete deletion of GARS expression leads to mitochondrial translational defects in neuronal and myogenic cells, causing reduced protein synthesis (Wang et al., 2016; Zhang et al., 2021).