Endoplasmic Reticulum Stress In Skeletal Muscle Research Articles

Long-chain ceramides are cell non-autonomous signals linking lipotoxicity to endoplasmic reticulum stress in skeletal muscle

The endoplasmic reticulum (ER) regulates cellular protein and lipid biosynthesis. ER dysfunction leads to protein misfolding and the unfolded protein response (UPR), which limits protein synthesis to prevent cytotoxicity. Chronic ER stress in skeletal muscle is a unifying mechanism linking lipotoxicity to metabolic disease. Unidentified signals from cells undergoing ER stress propagate paracrine and systemic UPR activation. Here, we induce ER stress and lipotoxicity in myotubes. We observe ER stress-inducing lipid cell non-autonomous signal(s). Lipidomics identifies that palmitate-induced cell stress induces long-chain ceramide 40:1 and 42:1 secretion. Ceramide synthesis through the ceramide synthase 2 de novo pathway is regulated by UPR kinase Perk. Inactivation of CerS2 in mice reduces systemic and muscle ceramide signals and muscle UPR activation. The ceramides are packaged into extracellular vesicles, secreted and induce UPR activation in naïve myotubes through dihydroceramide accumulation. This study furthers our understanding of ER stress by identifying UPR-inducing cell non-autonomous signals.

CO-Releasing Molecule-2 Prevents Acute Kidney Injury through Suppression of ROS-Fyn-ER Stress Signaling in Mouse Model

Acute kidney injury (AKI) most commonly appears in critically ill patients in hospitals. AKI is characterized as a quick deterioration of kidney function and has recently been identified to be tightly interlinked with chronic kidney diseases. The emerging major mediators of AKI include oxidative stress and endoplasmic reticulum (ER) stress. Carbon monoxide (CO) attenuates oxidative stress and ER stress in various cells, while Fyn, a member of the Src kinase family, is activated by oxidative stress and contributes to ER stress in skeletal muscle. Considering these, the objective of the current research was to determine (i) the involvement of Fyn in ER stress-mediated AKI and (ii) the effect of CO-releasing molecule-2 (CORM2) on reactive oxygen species- (ROS-) Fyn-ER stress-mediated AKI. Pretreatment with CORM2 (30 mg/kg) efficiently inhibited LPS (30 mg/kg)-induced oxidative stress, inflammation, and cellular apoptosis during AKI in C57BL/6J mice. Also, CORM2 efficiently suppressed the activation of Fyn and ER stress in AKI mice. Consistently, pretreatment with CORM2 inhibited oxidative stress, Fyn activation, ER stress, inflammation, and apoptosis in LPS- or H2O2-stimulated proximal epithelial tubular cells. Fyn inhibition using siRNA or an inhibitor (PP2) significantly attenuated ER stress responses in the cells. These data suggest that CORM2 may become a potential treatment option against ROS-Fyn-ER stress-mediated AKI.

Valdecoxib improves lipid-induced skeletal muscle insulin resistance via simultaneous suppression of inflammation and endoplasmic reticulum stress

Valdecoxib (VAL), a non-steroidal anti-inflammatory drug, has been widely used for treatment of rheumatoid arthritis, osteoarthritis, and menstrual pain. It is a selective cyclooxygenase-2 inhibitor. The suppressive effects of VAL on cardiovascular diseases and neuroinflammation have been documented; however, its impact on insulin signaling in skeletal muscle has not been studied in detail. The aim of this study was to investigate the effects of VAL on insulin resistance in mouse skeletal muscle. Treatment of C2C12 myocytes with VAL reversed palmitate-induced aggravation of insulin signaling and glucose uptake. Further, VAL attenuated palmitate-induced inflammation and endoplasmic reticulum (ER) stress in a concentration-dependent manner. Treatment with VAL concentration-dependently upregulated AMP-activated protein kinase (AMPK) and heat shock protein beta 1 (HSPB1) expression. In line with in vitro experiments, treatment with VAL augmented AMPK phosphorylation and HSPB1 expression, thereby alleviating high-fat diet-induced insulin resistance along with inflammation and ER stress in mouse skeletal muscle. However, small interfering RNA-mediated inhibition of AMPK abolished the effects of VAL on insulin resistance, inflammation, and ER stress. These results suggest that VAL alleviates insulin resistance through AMPK/HSPB1-mediated inhibition of inflammation and ER stress in skeletal muscle under hyperlipidemic conditions. Hence, VAL could be used as an effective pharmacotherapeutic agent for management of insulin resistance and type 2 diabetes.

Eukarion-134 Attenuates Endoplasmic Reticulum Stress-Induced Mitochondrial Dysfunction in Human Skeletal Muscle Cells.

Maladaptive endoplasmic reticulum (ER) stress is associated with modified reactive oxygen species (ROS) generation and mitochondrial abnormalities; and is postulated as a potential mechanism involved in muscle weakness in myositis, an acquired autoimmune neuromuscular disease. This study investigates the impact of ROS generation in an in vitro model of ER stress in skeletal muscle, using the ER stress inducer tunicamycin (24 h) in the presence or absence of a superoxide dismutase/catalase mimetic Eukarion (EUK)-134. Tunicamycin induced maladaptive ER stress, which was mitigated by EUK-134 at the transcriptional level. ER stress promoted mitochondrial dysfunction, described by substantial loss of mitochondrial membrane potential, as well as a reduction in respiratory control ratio, reserve capacity, phosphorylating respiration, and coupling efficiency, which was ameliorated by EUK-134. Tunicamycin induced ROS-mediated biogenesis and fusion of mitochondria, which, however, had high propensity of fragmentation, accompanied by upregulated mRNA levels of fission-related markers. Increased cellular ROS generation was observed under ER stress that was prevented by EUK-134, even though no changes in mitochondrial superoxide were noticeable. These findings suggest that targeting ROS generation using EUK-134 can amend aspects of ER stress-induced changes in mitochondrial dynamics and function, and therefore, in instances of chronic ER stress, such as in myositis, quenching ROS generation may be a promising therapy for muscle weakness and dysfunction.

Swimming Differentially Affects T2DM-Induced Skeletal Muscle ER Stress and Mitochondrial Dysfunction Related to MAM.

PurposeMitochondrial dysfunction and endoplasmic reticulum stress (ERS) are associated with metabolic diseases such as obesity and Type 2 diabetes mellitus (T2DM). Mitochondria and ER are connected via mitochondria-associated membranes (MAM) that are involved in glucose homeostasis and insulin resistance. We postulated that exercise might positively benefit T2DM-induced ER and mitochondrial dysfunction that might be associated with MAM.Materials and MethodsMice were fed a high-fat diet and injected with streptozotocin (STZ) to create T2DM models. Glucose tolerance, mitochondrial quality, MAM quality, and ERS were investigated in diabetic mice after six weeks of swimming.ResultsType 2 DM induced decreased MAM quantity, impaired mitochondrial quality, and deteriorated ERS in skeletal muscle that led to endoplasmic reticulum-associated degradation (ERAD). Swimming alleviated strong ERS caused by T2DM. Importantly, MAM quantity was positively associated with mitochondrial function and tended to negatively correlate with the ERS branch, ATF6. Moreover, both ATF6 branches of ERS and ERAD were positively associated with the pIRE1α branch of ERS.ConclusionType 2 DM induced glucose intolerance, powerful ERS, and mitochondrial dysfunction associated with decreased amounts of MAM. Swimming improved glucose intolerance and selectively mitigated the ERS in skeletal muscle. Therefore, MAM quality and ATF6 might be novel and important targets for T2DM treatment. Endoplasmic reticulum stress might be an effective target of swimming to improve diabetes.

Copper Induces Apoptosis Through Endoplasmic Reticulum Stress in Skeletal Muscle of Broilers.

The purpose of this research was to investigate whether copper (Cu) exposure could induce apoptosis via endoplasmic reticulum stress (ERS) in skeletal muscle of broilers. A total of 240 one-day-old chickens were randomly divided into four groups by free access; the diets are as follows: control diet (Cu 11mg/kg, control group) and high level of Cu diets (Cu 110mg/kg, group I; Cu 220mg/kg, group II; Cu 330mg/kg, group III). The skeletal muscle tissues were collected on day 49 for further examination. The content of Cu, histopathology, and the expression levels of the genes and proteins related to ERS and apoptosis were detected. Results showed that the Cu levels in skeletal muscle were increased in a dose-dependent manner. Meanwhile, the spaces between the muscle fibers were wider with the increase of Cu content, and the myolysis was observed in group III. Besides, the mRNA expression levels of GRP78, GRP94, eIF2α, ATF6, XBP1, CHOP, Caspase-12, and Caspase3 were markedly increased in treated groups compared with control group, and the protein expression levels of GRP78, Caspase3, Active-Caspase3 and JNK were significantly elevated with the increase of dietary Cu. In summary, these findings suggested that Cu could induce apoptosis through ERS in skeletal muscle of broilers.

SELENON (SEPN1) protects skeletal muscle from saturated fatty acid-induced ER stress and insulin resistance

Selenoprotein N (SELENON) is an endoplasmic reticulum (ER) protein whose loss of function leads to a congenital myopathy associated with insulin resistance (SEPN1-related myopathy). The exact cause of the insulin resistance in patients with SELENON loss of function is not known. Skeletal muscle is the main contributor to insulin-mediated glucose uptake, and a defect in this muscle-related mechanism triggers insulin resistance and glucose intolerance. We have studied the chain of events that connect the loss of SELENON with defects in insulin-mediated glucose uptake in muscle cells and the effects of this on muscle performance. Here, we show that saturated fatty acids are more lipotoxic in SELENON-devoid cells, and blunt the insulin-mediated glucose uptake of SELENON-devoid myotubes by increasing ER stress and mounting a maladaptive ER stress response. Furthermore, the hind limb skeletal muscles of SELENON KO mice fed a high-fat diet mirrors the features of saturated fatty acid-treated myotubes, and show signs of myopathy with a compromised force production. These findings suggest that the absence of SELENON together with a high-fat dietary regimen increases susceptibility to insulin resistance by triggering a chronic ER stress in skeletal muscle and muscle weakness.Importantly, our findings suggest that environmental cues eliciting ER stress in skeletal muscle (such as a high-fat diet) affect the pathological phenotype of SEPN1-related myopathy and can therefore contribute to the assessment of prognosis beyond simple genotype-phenotype correlations.

Adiponectin improves insulin sensitivity via activation of autophagic flux.

Skeletal muscle insulin resistance is known to play an important role in the pathogenesis of diabetes, and one potential causative cellular mechanism is endoplasmic reticulum (ER) stress. Adiponectin mediates anti-diabetic effects via direct metabolic actions and by improving insulin sensitivity, and we recently demonstrated an important role in stimulation of autophagy by adiponectin. However, there is limited knowledge on crosstalk between autophagy and ER stress in skeletal muscle and in particular how they are regulated by adiponectin. Here, we utilized the model of high insulin/glucose (HIHG)-induced insulin resistance, determined by measuring Akt phosphorylation (T308 and S473) and glucose uptake in L6 skeletal muscle cells. HIHG reduced autophagic flux measured by LC3 and p62 Western blotting and tandem fluorescent RFP/GFP-LC3 immunofluorescence (IF). HIHG also induced ER stress assessed by thioflavin T/KDEL IF, pIRE1, pPERK, peIF2α and ATF6 Western blotting and induction of a GRP78-mCherry reporter. Induction of autophagy by adiponectin or rapamycin attenuated HIHG-induced ER stress and improved insulin sensitivity. The functional significance of enhanced autophagy was validated by demonstrating a lack of improved insulin sensitivity in response to adiponectin in autophagy-deficient cells generated by overexpression of dominant negative mutant of Atg5. In summary, adiponectin-induced autophagy in skeletal muscle cells alleviated HIHG-induced ER stress and insulin resistance.

Endoplasmic reticulum stress regulates inflammation and insulin resistance in skeletal muscle from pregnant women

Sterile inflammation and infection are key mediators of inflammation and peripheral insulin resistance associated with gestational diabetes mellitus (GDM). Studies have shown endoplasmic reticulum (ER) stress to induce inflammation and insulin resistance associated with obesity and type 2 diabetes, however is paucity of studies investigating the effects of ER stress in skeletal muscle on inflammation and insulin resistance associated with GDM. ER stress proteins IRE1α, GRP78 and XBP-1s were upregulated in skeletal muscle of obese pregnant women, whereas IRE1α was increased in GDM women. Suppression of ER stress, using ER stress inhibitor tauroursodeoxycholic acid (TUDCA) or siRNA knockdown of IRE1α and GRP78, significantly downregulated LPS-, poly(I:C)- or IL-1β-induced production of IL-6, IL-8, IL-1β and MCP-1. Furthermore, LPS-, poly(I:C)- or TNF-α-induced insulin resistance was improved following suppression of ER stress, by increasing insulin-stimulated phosphorylation of IR-β, IRS-1, GLUT-4 expression and glucose uptake. In summary, our inducible obesity and GDM-like models suggests that the development of GDM may be involved in activating ER stress-induced inflammation and insulin resistance in human skeletal muscle.

Interferon‐γ is a Potent Activator of Caspase 12 in C2C12 cells HL112085, HL113494

We recently found that pharmacologic activation of endoplasmic reticulum (ER) stress in vivo induces skeletal muscle contractile dysfunction. However, it is not known if cytokines are capable of inducing ER stress in muscle. The purpose of the present study was to determine if interferon gamma (IFNγ), tumor necrosis factor alpha (TNFα), or interleukin1 beta (IL1β) activates ER stress and the unfolded protein response (UPR) in skeletal muscle. C2C12 myoblasts were grown to 80% confluence, differentiated for 5 days, and mature myotubes exposed to either PBS+0.1% BSA (control), IFNγ (25ng/ml),TNFα (20ng/ml), or IL1β (10ng/ml). Cells were harvested at 30 minutes, 1, 2, 4, 8, and 24 hours post‐exposure (n=5/condition/time point). Using Western blots, we assessed cleavage of caspase 12, an early marker of ER stress; we also measured BiP, Grp94, calreticulin, PDI and CHOP, proteins which are late markers of ER stress and the UPR. At 24 hours after exposure, caspase 12 cleavage robustly increased in C2C12 cells exposed to IFNγ, while TNFα and IL1β had little effect to activate caspase 12 (e.g. the 38 kDa caspase 12 degradation product increased 7‐fold with IFNγ, when compared to control, TNFα , or IL1β, p<0.001). We did not detect significant changes in late markers of ER stress at 24 hours. Our findings indicate that IFNγ is a potent activator of ER stress in skeletal muscle. Future studies are needed to determine the role of IFNγ –mediated ER stress activation in clinical conditions which produce skeletal muscle dysfunction (e.g. sepsis, cancer, and aging).

The unfolded protein response in human skeletal muscle is not involved in the onset of glucose tolerance impairment induced by a fat-rich diet

Endoplasmic reticulum (ER) stress in pancreas, liver, and adipose tissue is a key event in the pathogenesis of obesity-related metabolic disease. Lipid-induced ER stress in liver and adipose tissue leads to inhibition of insulin signaling. Whether this mechanism exists in skeletal muscle is currently unknown. The present study aimed at assessing the ER stress response in skeletal muscle of subjects receiving a hyper-caloric fat-rich diet (HFD). Seven healthy males (20.6 ± 0.5 years; 70.9 ± 3.4 kg) volunteered to participate in the study. They received a hyper-caloric (+30% kcal) fat-rich (50% kcal) diet for 6 weeks. An oral glucose tolerance test (OGTT) was performed, and muscle biopsies were taken before and after HFD. HFD increased body mass by ~3 kg (P = 0.007) and the sum of skinfolds by 15% (P = 0.003). After HFD, blood glucose concentrations were higher during OGTT (two-way ANOVA, P = 0.023; +45% at 20 min, P = 0.002), and fasting plasma insulin level tended to be higher (+20%). HFD increased intramyocellular lipids content by ~50 and 75% in type I (P = 0.0009) and IIa fibers (P = 0.002), respectively. The protein expression of inositol-requiring enzyme 1α, protein kinase R-like ER protein kinase, BiP and calnexin and the mRNA level of spliced X box binding protein-1, CCAAT/enhancer binding protein homologous protein and activating transcription factor 4 were not changed after HFD. Despite the increase in body mass, subcutaneous fat deposits, and intramyocellular lipids content, ER stress markers were unchanged in skeletal muscle of subjects receiving a HFD for 6 weeks. This suggests that the onset of glucose intolerance is not related to ER stress in skeletal muscle.