The unfolded protein response is activated in skeletal muscle by high-fat feeding: potential role in the downregulation of protein synthesis

Endoplasmic Reticulum Stress Activation during Total Knee Arthroplasty

Total knee arthroplasty (TKA) is the most common remediation for knee pain from osteoarthritis (OA) and is performed 650,000 annually in the U.S. A tourniquet is commonly used during TKA which causes ischemia and reperfusion (I/R) to the lower limb but the effects of I/R on muscle are not fully understood. Previous reports suggest upregulation of cell stress and catabolism and downregulation of markers of cap-dependent translation during and after TKA. I/R has also been shown to cause endoplasmic reticulum (ER) stress and induce the unfolded protein response (UPR). We hypothesized that the UPR would be activated in response to ER stress during TKA. We obtained muscle biopsies from the vastus lateralis at baseline, before TKA; at maximal ischemia, prior to tourniquet deflation; and during reperfusion in the operating room. Phosphorylation of 4E-BP1 and AKT decreased during ischemia (−28%, P < 0.05; −20%, P < 0.05, respectively) along with an increase in eIF2α phosphorylation (64%, P < 0.05) suggesting decreased translation initiation. Cleaved ATF6 protein increased in ischemia (39%, P = 0.056) but returned to baseline during reperfusion. CASP3 activation increased during reperfusion compared to baseline (23%, P < 0.05). XBP1 splicing assays revealed an increase in spliced transcript during ischemia (31%, P < 0.05) which diminished during reperfusion. These results suggest that in response to I/R during TKA all three branches of the ER stress response are activated.

Effect of acute environmental hypoxia on protein metabolism in human skeletal muscle

Hypoxia-induced muscle wasting has been observed in several environmental and pathological conditions. However, the molecular mechanisms behind this loss of muscle mass are far from being completely elucidated, certainly in vivo. When studying the regulation of muscle mass by environmental hypoxia, many confounding factors have to be taken into account, such as decreased protein ingestion, sleep deprivation or reduced physical activity, which make difficult to know whether hypoxia per se causes a reduction in muscle mass.

AIM

We hypothesized that acute exposure to normobaric hypoxia (11% O2 ) would repress the activation of the mTOR pathway usually observed after a meal and would activate the proteolytic pathways in skeletal muscle.

METHODS

Fifteen subjects were exposed passively for 4 h to normoxic and hypoxic conditions in a random order after consumption of a light breakfast. A muscle biopsy and a blood sample were taken before, after 1 and 4 h of exposure.

RESULTS

After 4 h, plasma insulin concentration and the phosphorylation state of PKB and S6K1 in skeletal muscle were higher in hypoxia than in normoxia (P < 0.05). At the same time, Redd1 mRNA level was upregulated (P < 0.05), whilst MAFbx mRNA decreased (P < 0.05) in hypoxia compared with normoxia. Proteasome, cathepsin L and calpain activities were not altered by environmental hypoxia.

CONCLUSION

Contrary to our hypothesis and despite an increase in the mRNA level of Redd1, an inhibitor of the mTORC1 pathway, short-term acute environmental hypoxia induced a higher response of PKB and S6K1 to a meal, which may be due to increased plasma insulin concentration.

Toll-like receptor 4 knockout mice are protected against endoplasmic reticulum stress induced by a high-fat diet

The purpose of this study was to investigate whether toll-like receptor 4 (TLR4) is implicated in the development of endoplasmic reticulum stress (ER stress) observed after a high-fat diet (HFD) in liver, skeletal muscle and adipose tissue. TLR4^−/− and C57BL/6J wild-type mice (WT) were fed with chow or HFD (45% calories from fat) during 18 weeks. An oral glucose tolerance-test was performed. The animals were sacrificed in a fasted state and the tissues were removed. TLR4 deletion protected from body weight gain and glucose intolerance induced by HFD whereas energy intake was higher in transgenic mice suggesting larger energy expenditure. HFD induced an ER stress in skeletal muscle, liver and adipose tissue of WT mice as assessed by BiP, CHOP, spliced and unspliced XBP1 and phospho-eIF2α. TLR4^−/− mice were protected against HFD-induced ER stress. Then, we investigated the main signaling downstream of TLR4 namely the NF-κB pathway, expecting to identify the mechanism by which TLR4 is able to activate ER stress. The mRNA levels of cytokines regulated by NF-κB namely TNFα, IL-1β and IL-6, were not changed after HFD and phospho-IκB-α (ser 32) was not changed. Our results indicate that TLR4 is essential for the development of ER stress related to HFD. Nevertheless, the NFκ-B pathway does not seem to be directly implicated. The reduced fat storage in TLR4^−/− mice could explain the absence of an ER stress after HFD.

Mitochondrial DNA damage and dysfunction, and oxidative stress are associated with endoplasmic reticulum stress, protein degradation and apoptosis in high fat diet-induced insulin resistance mice

Background

Recent studies showed a link between a high fat diet (HFD)-induced obesity and lipid accumulation in non-adipose tissues, such as skeletal muscle and liver, and insulin resistance (IR). Although the mechanisms responsible for IR in those tissues are different, oxidative stress and mitochondrial dysfunction have been implicated in the disease process. We tested the hypothesis that HFD induced mitochondrial DNA (mtDNA) damage and that this damage is associated with mitochondrial dysfunction, oxidative stress, and induction of markers of endoplasmic reticulum (ER) stress, protein degradation and apoptosis in skeletal muscle and liver in a mouse model of obesity-induced IR.

Methodology/Principal Findings

C57BL/6J male mice were fed either a HFD (60% fat) or normal chow (NC) (10% fat) for 16 weeks. We found that HFD-induced IR correlated with increased mtDNA damage, mitochondrial dysfunction and markers of oxidative stress in skeletal muscle and liver. Also, a HFD causes a change in the expression level of DNA repair enzymes in both nuclei and mitochondria in skeletal muscle and liver. Furthermore, a HFD leads to activation of ER stress, protein degradation and apoptosis in skeletal muscle and liver, and significantly reduced the content of two major proteins involved in insulin signaling, Akt and IRS-1 in skeletal muscle, and Akt in liver. Basal p-Akt level was not significantly influenced by HFD feeding in skeletal muscle and liver.

Conclusions/Significance

This study provides new evidence that HFD-induced mtDNA damage correlates with mitochondrial dysfunction and increased oxidative stress in skeletal muscle and liver, which is associated with the induction of markers of ER stress, protein degradation and apoptosis.

'Sarcobesity': a metabolic conundrum

Two independent but inter-related conditions that have a growing impact on healthy life expectancy and health care costs in developed nations are an age-related loss of muscle mass (i.e., sarcopenia) and obesity. Sarcopenia is commonly exacerbated in overweight and obese individuals. Progression towards obesity promotes an increase in fat mass and a concomitant decrease in muscle mass, producing an unfavourable ratio of fat to muscle. The coexistence of diminished muscle mass and increased fat mass (so-called 'sarcobesity') is ultimately manifested by impaired mobility and/or development of life-style-related diseases. Accordingly, the critical health issue for a large proportion of adults in developed nations is how to lose fat mass while preserving muscle mass. Lifestyle interventions to prevent or treat sarcobesity include energy-restricted diets and exercise. The optimal energy deficit to reduce body mass is controversial. While energy restriction in isolation is an effective short-term strategy for rapid and substantial weight loss, it results in a reduction of both fat and muscle mass and therefore ultimately predisposes one to an unfavourable body composition. Aerobic exercise promotes beneficial changes in whole-body metabolism and reduces fat mass, while resistance exercise preserves lean (muscle) mass. Current evidence strongly supports the inclusion of resistance and aerobic exercise to complement mild energy-restricted high-protein diets for healthy weight loss as a primary intervention for sarcobesity.

Endoplasmic reticulum stress in skeletal muscle homeostasis and disease

Our appreciation of the role of endoplasmic reticulum (ER) stress pathways in both skeletal muscle homeostasis and the progression of muscle diseases is gaining momentum. This review provides insight into ER stress mechanisms during physiologic and pathological disturbances in skeletal muscle. The role of ER stress in the response to dietary alterations and acute stressors, including its role in autoimmune and genetic muscle disorders, has been described. Recent studies identifying ER stress markers in diseased skeletal muscle are noted. The emerging evidence for ER-mitochondrial interplay in skeletal muscle and its importance during chronic ER stress in activation of both inflammatory and cell death pathways (autophagy, necrosis, and apoptosis) have been discussed. Thus, understanding the ER stress-related molecular pathways underlying physiologic and pathological phenotypes in healthy and diseased skeletal muscle should lead to novel therapeutic targets for muscle disease.

Coupling nutrient sensing to metabolic homoeostasis: the role of the mammalian target of rapamycin complex 1 pathway

The mammalian target of rapamycin complex 1 (mTORC1) pathway is known to couple different environmental cues to the regulation of several energy-demanding functions within the cell, spanning from protein translation to mitochondrial activity. As a result, at the organism level, mTORC1 activity affects energy balance and general metabolic homoeostasis by modulating both the activity of neuronal populations that play key roles in the control of food intake and body weight, as well as by determining storage and use of fuel substrates in peripheral tissues. This review focuses on recent advances made in understanding the role of the mTORC1 pathway in the regulation of energy balance. More particularly, it aims at providing an overview of the status of knowledge regarding the mechanisms underlying the ability of certain amino acids, glucose and fatty acids, to affect mTORC1 activity and in turn illustrates how the mTORC1 pathway couples nutrient sensing to the hypothalamic regulation of the organisms' energy homoeostasis and to the control of intracellular metabolic processes, such as glucose uptake, protein and lipid biosynthesis. The evidence reviewed pinpoints the mTORC1 pathway as an integrator of the actions of nutrients on metabolic health and provides insight into the relevance of this intracellular pathway as a potential target for the therapy of metabolic diseases such as obesity and type-2 diabetes.

Endurance training in mice increases the unfolded protein response induced by a high-fat diet

Certain conditions, such as several weeks of high-fat diet, disrupt endoplasmic reticulum (ER) homeostasis and activate an adaptive pathway referred as the unfolded protein response. When the unfolded protein response fails, the result is the development of inflammation and insulin resistance. These two pathological states are known to be improved by regular exercise training but the mechanisms remain largely undetermined. As it has recently been shown that the unfolded protein response is regulated by exercise, we hypothesised that concomitant treadmill exercise training (HFD+ex) prevents ER homeostasis disruption and its downstream consequences induced by a 6-week high-fat diet (HFD) in mice by activating the protective unfolded protein response. Several well-documented markers of the unfolded protein response were measured in the soleus and tibialis anterior muscles as well as in the liver and pancreas. In HFD mice, an increase in these markers was observed (from 2- to 15-fold, P < 0.05) in all tissues studied. The combination of HFD+ex increased the expression of several markers further, up to 100 % compared to HFD alone (P < 0.05). HFD increased inflammatory markers both in the plasma (IL-6 protein, 2.5 ± 0.52-fold; MIP-1α protein, 1.3 ± 0.13-fold; P < 0.05) and in the tissues studied, and treadmill exercise attenuated the inflammatory state induced by HFD (P < 0.05). However, treadmill exercise could not reverse HFD-induced whole body glucose intolerance, assessed by OGTT (AUC, 1.8 ± 0.29-fold, P < 0.05). In conclusion, our results show that a HFD activated the unfolded protein response in mouse tissues in vivo, and that endurance training promoted this response. We speculate that the potentiation of the unfolded protein response by endurance training may represent a positive adaptation protecting against further cellular stress.

Exercise and nutritional interventions for improving aging muscle health

Skeletal muscle mass declines with age (i.e., sarcopenia) resulting in muscle weakness and functional limitations. Sarcopenia has been associated with physiological changes in muscle morphology, protein and hormonal kinetics, insulin resistance, inflammation, and oxidative stress. The purpose of this review is to highlight how exercise and nutritional intervention strategies may benefit aging muscle. It is well known that resistance exercise training increases muscle strength and size and evidence also suggests that resistance training can increase mitochondrial content and decrease oxidative stress in older adults. Recent findings suggest that fast-velocity resistance exercise may be an effective intervention for older adults to enhance muscle power and functional capacity. Aerobic exercise training may also benefit aging skeletal muscle by enhancing mitochondrial bioenergetics, improving insulin sensitivity, and/or decreasing oxidative stress. In addition to exercise, creatine monohydrate, milk-based proteins, and essential fatty acids all have biological effects which could enhance some of the physiological adaptations from exercise training in older adults. Additional research is needed to determine whether skeletal muscle adaptations to increased activity in older adults are further enhanced with effective nutritional interventions and whether this is due to enhanced muscle protein synthesis, improved mitochondrial function, and/or a reduced inflammatory response.

New insights into ER stress-induced insulin resistance

Insulin resistance is a major characteristic of obesity and type 2 diabetes (T2DM). During the last decade, endoplasmic reticulum (ER) stress has emerged as a new player in this field and a considerable number of recent studies have pointed out its role in the onset of insulin resistance (IR). ER stress appears to act directly as a negative modulator of the insulin signaling pathway but also indirectly by promoting lipid accumulation. This review aims to summarize and decipher the abundant new literature concerning the emerging and multifaceted involvement of ER stress in the development of metabolic dysfunctions in insulin target tissues.

The role of endoplasmic reticulum in hepatic lipid homeostasis and stress signaling

The endoplasmic reticulum (ER) is a critical site of protein, lipid, and glucose metabolism, lipoprotein secretion, and calcium homeostasis. Many of the sensing, metabolizing, and signaling mechanisms for these pathways exist within or on the ER membrane domain. Here, we review the cellular functions of ER, how perturbation of ER homeostasis contributes to metabolic dysregulation and potential causative mechanisms of ER stress in obesity, with a particular focus on lipids, metabolic adaptations of ER, and the maintenance of its membrane homeostasis. We also suggest a conceptual framework of metabolic roundabout to integrate key mechanisms of insulin resistance and metabolic diseases.

Arctigenin, a natural compound, activates AMP-activated protein kinase via inhibition of mitochondria complex I and ameliorates metabolic disorders in ob/ob mice

AIMS/HYPOTHESIS

Arctigenin is a natural compound that had never been previously demonstrated to have a glucose-lowering effect. Here it was found to activate AMP-activated protein kinase (AMPK), and the mechanism by which this occurred, as well as the effects on glucose and lipid metabolism were investigated.

METHODS

2-Deoxyglucose uptake and AMPK phosphorylation were examined in L6 myotubes and isolated skeletal muscle. Gluconeogenesis and lipid synthesis were evaluated in rat primary hepatocytes. The acute and chronic effects of arctigenin on metabolic abnormalities were observed in C57BL/6J and ob/ob mice. Changes in mitochondrial membrane potential were measured using the J-aggregate-forming dye, JC-1. Analysis of respiration of L6 myotubes or isolated mitochondria was conducted in a channel oxygen system.

RESULTS

Arctigenin increased AMPK phosphorylation and stimulated glucose uptake in L6 myotubes and isolated skeletal muscles. In primary hepatocytes, it decreased gluconeogenesis and lipid synthesis. The enhancement of glucose uptake and suppression of hepatic gluconeogenesis and lipid synthesis by arctigenin were prevented by blockade of AMPK activation. The respiration of L6 myotubes or isolated mitochondria was inhibited by arctigenin with a specific effect on respiratory complex I. A single oral dose of arctigenin reduced gluconeogenesis in C57BL/6J mice. Chronic oral administration of arctigenin lowered blood glucose and improved lipid metabolism in ob/ob mice.

CONCLUSIONS/INTERPRETATION

This study demonstrates a new role for arctigenin as a potent indirect activator of AMPK via inhibition of respiratory complex I, with beneficial effects on metabolic disorders in ob/ob mice. This highlights the potential value of arctigenin as a possible treatment of type 2 diabetes.

Endoplasmic reticulum stress impairs insulin signaling through mitochondrial damage in SH-SY5Y cells

Endoplasmic reticulum (ER) and mitochondrial stress are considered causal factors that induce neurodegenerative diseases. However, the relationship between these stresses remains poorly understood. To investigate the molecular mechanism underlying crosstalk between the ER and mitochondria in neurodegeneration, we treated SH-SY5Y human neuroblastoma cells with thapsigargin and tunicamycin, two inducers of ER stress, and atrazine, a promoter of mitochondrial stress. Each pharmacological agent caused mitochondrial dysfunction, which was characterized by reduced intracellular ATP, mitochondrial membrane potential, and endogenous cellular respiration as well as an augmentation of oxidative stress. Oligonucleotide microarray analysis followed by semiquantitative RT-PCR validation assays revealed that thapsigargin and tunicamycin downregulated the expression of most mitochondria-related genes in a manner similar to that induced by atrazine. In contrast, atrazine did not alter the expression of markers of ER stress. Three-dimensional principal component analysis showed that the gene expression profile produced by atrazine treatment was distinct from that generated by ER stress. However, all three agents impaired insulin receptor substrate-1 and Akt phosphorylation in the insulin signaling pathway. Ectopic overexpression of mitochondrial transcription factor A reversed the effects of thapsigargin on mitochondria and Akt signaling. We conclude that ER stress induces neuronal cell death through common perturbation of mitochondrial function and Akt signaling.

Endoplasmic reticulum stress does not mediate palmitate-induced insulin resistance in mouse and human muscle cells

AIMS/HYPOTHESIS

Recent experiments in liver and adipocyte cell lines indicate that palmitate can induce endoplasmic reticulum (ER) stress. Since it has been shown that ER stress can interfere with insulin signalling, our hypothesis was that the deleterious action of palmitate on the insulin signalling pathway in muscle cells could also involve ER stress.

METHODS

We used C2C12 and human myotubes that were treated either with palmitate or tunicamycin. Total lysates and RNA were prepared for western blotting or quantitative RT-PCR respectively. Glycogen synthesis was assessed by [¹⁴C]glucose incorporation.

RESULTS

Incubation of myotubes with palmitate or tunicamycin inhibited insulin-stimulated protein kinase B (PKB)/ v-akt murine thymoma viral oncogene homologue 1 (Akt). In parallel, an increase in ER stress markers was observed. Pre-incubation with chemical chaperones that reduce ER stress only prevented tunicamycin but not palmitate-induced insulin resistance. We hypothesised that ER stress activation levels induced by palmitate may not be high enough to induce insulin resistance, in contrast with tunicamycin-induced ER stress. Indeed, tunicamycin induced a robust activation of the inositol-requiring enzyme 1 (IRE-1)/c-JUN NH₂-terminal kinase (JNK) pathway, leading to serine phosphorylation of insulin receptor substrate 1 (IRS-1) and a decrease in IRS-1 tyrosine phosphorylation. In contrast, palmitate only induced a very weak activation of the IRE1/JNK pathway, with no IRS1 serine phosphorylation.

CONCLUSIONS/INTERPRETATION

These data show that insulin resistance induced by palmitate is not related to ER stress in muscle cells.

Reduction of endoplasmic reticulum stress using chemical chaperones or Grp78 overexpression does not protect muscle cells from palmitate-induced insulin resistance

Endoplasmic reticulum (ER) stress is proposed as a novel link between elevated fatty acids levels, obesity and insulin resistance in liver and adipose tissue. However, it is unknown whether ER stress also contributes to lipid-induced insulin resistance in skeletal muscle, the major tissue responsible of insulin-stimulated glucose disposal. Here, we investigated the possible role of ER stress in palmitate-induced alterations of insulin action, both in vivo, in gastrocnemius of high-palm diet fed mice, and in vitro, in palmitate-treated C(2)C(12) myotubes. We demonstrated that 8 weeks of high-palm diet increased the expression of ER stress markers in muscle of mice, whereas ex-vivo insulin-stimulated PKB phosphorylation was not altered in this tissue. In addition, exposure of C(2)C(12) myotubes to either tuncamycine or palmitate induced ER stress and altered insulin-stimulated PKB phosphorylation. However, alleviation of ER stress by either TUDCA or 4-PBA treatments, or by overexpressing Grp78, did not restore palmitate-induced reduction of insulin-stimulated PKB phosphorylation in C(2)C(12) myotubes. This work highlights that, even ER stress is associated with palmitate-induced alterations of insulin signaling, ER stress is likely not the major culprit of this effect in myotubes, suggesting that the previously proposed link between ER stress and insulin resistance is less important in skeletal muscle than in adipose tissue and liver.

Endoplasmic reticulum stress, obesity and diabetes

The endoplasmic reticulum (ER) stress response, also commonly known as the unfolded protein response (UPR), is an adaptive response used to align ER functional capacity with demand. It is activated in various tissues under conditions related to obesity and type 2 diabetes. Hypothalamic ER stress contributes to inflammation and leptin/insulin resistance. Hepatic ER stress contributes to the development of steatosis and insulin resistance, and components of the UPR regulate liver lipid metabolism. ER stress in enlarged fat tissues induces inflammation and modifies adipokine secretion, and saturated fats cause ER stress in muscle. Finally, prolonged ER stress impairs insulin synthesis and causes pancreatic β cell apoptosis. In this review, we discuss ways in which ER stress operates as a common molecular pathway in the pathogenesis of obesity and diabetes.

ER stress induces anabolic resistance in muscle cells through PKB-induced blockade of mTORC1

Background

Anabolic resistance is the inability to increase protein synthesis in response to an increase in amino acids following a meal. One potential mediator of anabolic resistance is endoplasmic reticulum (ER) stress. The purpose of the present study was to test whether ER stress impairs the response to growth factors and leucine in muscle cells.

Methods

Muscle cells were incubated overnight with tunicamycin or thapsigargin to induce ER stress and the activation of the unfolded protein response, mTORC1 activity at baseline and following insulin and amino acids, as well as amino acid transport were determined.

Results

ER stress decreased basal phosphorylation of PKB and S6K1 in a dose-dependent manner. In spite of the decrease in basal PKB phosphorylation, insulin (10–50 nM) could still activate both PKB and S6K1. The leucine (2.5–5 mM)-induced phosphorylation of S6K1 on the other hand was repressed by low concentrations of both tunicamycin and thapsigargin. To determine the mechanism underlying this anabolic resistance, several inhibitors of mTORC1 activation were measured. Tunicamycin and thapsigargin did not change the phosphorylation or content of either AMPK or JNK, both increased TRB3 mRNA expression and thapsigargin increased REDD1 mRNA. Tunicamycin and thapsigargin both decreased the basal phosphorylation state of PRAS40. Neither tunicamycin nor thapsigargin prevented phosphorylation of PRAS40 by insulin. However, since PKB is not activated by amino acids, PRAS40 phosphorylation remained low following the addition of leucine. Blocking PKB using a specific inhibitor had the same effect on both PRAS40 and leucine-induced phosphorylation of S6K1.

Conclusion

ER stress induces anabolic resistance in muscle cells through a PKB/PRAS40-induced blockade of mTORC1.

Deletion of Nck1 attenuates hepatic ER stress signaling and improves glucose tolerance and insulin signaling in liver of obese mice

Obesity has been shown to create stress in the endoplasmic reticulum (ER), and that initiates the activation of the unfolded protein response (UPR). This has been reported to cause insulin resistance in selective tissues through activation of the inositol-requiring enzyme 1α (IRE1α)-c-Jun NH(2)-terminal kinase (JNK) pathway, which results in the phosphorylation of the insulin receptor substrate-1 (IRS-1) at an inhibitory site and blocks insulin receptor signaling. In this study, we report that the Src homology domain-containing adaptor protein Nck1, previously shown to modulate the UPR, is of functional importance in obesity-induced ER stress signaling and inhibition of insulin actions. We have examined obese Nck1(-/-) and Nck1(+/+) mice for glucose tolerance, insulin sensitivity, and signaling as well as for ER stress markers and IRS-1 phosphorylation at Ser(307). Our findings show that obese Nck1-deficient mice display improved glucose disposal accompanied by enhanced insulin signaling in liver. This correlates with attenuated IRE1α and JNK activation and IRS-1 phosphorylation at Ser(307) compared with obese wild-type mice. Consistent with our in vivo data, we report that downregulation of Nck1 using siRNA in HepG2 cells results in decreased thapsigargin-induced IRE1α activation and signaling and IRS-1 phosphorylation at Ser(307), whereas it markedly enhances insulin signaling. Overall, in liver and in cultured cells, we show that depletion of Nck1 attenuates the UPR signal and its inhibitory action on insulin signaling. Taken all together, our findings implicate Nck1 in regulating the UPR, which secondary to obesity impairs glucose homeostasis and insulin actions.