Fenofibrate improves high-fat diet-induced and palmitate-induced endoplasmic reticulum stress and inflammation in skeletal muscle. Life Sci. 2016;157:158-167. [PMID: 27297630 DOI: 10.1016/j.lfs.2016.06.008]

Physiological 4-phenylbutyrate promotes mitochondrial biogenesis and metabolism in C2C12 myotubes

Type 2 diabetes is characterized by elevated circulating blood metabolites such as glucose, insulin, and branched chain amino acids (BCAA), which often coincide with reduced mitochondrial function. 4-Phenylbutyrate (PBA), an ammonia scavenger, has been shown to activate BCAA metabolism, resolve endoplasmic reticulum (ER) stress, and rescue BCAA-mediated insulin resistance. To determine the effect of PBA on the altered metabolic phenotype featured in type 2 diabetes, the present study investigated the effect of PBA on various metabolic parameters including mitochondrial metabolism and mitochondrial biogenesis. C2C12 myotubes were treated with PBA at 0.5 mM (representing physiologically attainable blood concentrations) or 10 mM (representing physiologically unattainable/proof-of-concept levels) for up to 24 h. Mitochondrial and glycolytic metabolism were assessed via oxygen consumption and extracellular acidification rate, respectively. Mitochondrial content, lipid content, and ER stress were measured by fluorescent staining. Metabolic gene expression was measured by qRT-PCR. Both doses of PBA increased expression of indicators of mitochondrial biogenesis, though only PBA at 0.5 mM increased mitochondrial function and content while 10 mM PBA reduced mitochondrial function and content. PBA at 0.5 mM also rescued reduced mitochondrial function during insulin resistance, though PBA also caused a reduced insulin stimulated pAkt expression during insulin resistance. PBA treatment also increased extracellular BCAA accumulation during insulin resistance despite unchanged pBCKDH expression. Taken together, PBA may increase mitochondrial biogenesis, content, and function in a dose-dependent fashion which may have implications for prevention or treatment of metabolic disease such as insulin resistance.

The PPARα agonist fenofibrate reduces the cytokine imbalance in a maternal immune activation model of schizophrenia

Maternal infections during pregnancy may increase the risk of psychiatric disorders in offspring. We recently demonstrated that activation of peroxisome proliferator-activate receptor-α (PPARα), with the clinically available agonist fenofibrate (FEN), attenuates the neurodevelopmental disturbances induced by maternal immune activation (MIA) in rat offspring. We hypothesized that fenofibrate might reduce MIA-induced cytokine imbalance using a MIA model based on the viral mimetic polyriboinosinic-polyribocytidilic acid [poly (I:C)]. By using the Bio-Plex Multiplex-Immunoassay-System, we measured cytokine/chemokine/growth factor levels in maternal serum and in the fetal brain of rats treated with fenofibrate, at 6 and 24 h after poly (I:C). We found that MIA induced time-dependent changes in the levels of several cytokines/chemokines/colony-stimulating factors (CSFs). Specifically, the maternal serum of the poly (I:C)/control (CTRL) group showed increased levels of (i) proinflammatory chemokine macrophage inflammatory protein 1-alpha (MIP-1α), (ii) tumor necrosis factor-alpha (TNF-α), the monocyte chemoattractant protein-1 (MCP-1), the macrophage (M-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Conversely, in the fetal brain of the poly (I:C)/CTRL group, interleukin 12p70 and MIP-1α levels were lower than in vehicle (veh)/CTRL group. Notably, MIP-1α, TNF-α, keratinocyte derived chemokine (GRO/KC), GM-CSF, and M-CSF levels were lower in the poly (I:C)/FEN than in poly (I:C)/CTRL rats, suggesting the protective role of the PPARα agonist. PPARα might represent a therapeutic target to attenuate MIA-induced inflammation.

The Selective LAT1 Inhibitor JPH203 Enhances Mitochondrial Metabolism and Content in Insulin-Sensitive and Insulin-Resistant C2C12 Myotubes

Population data have shown an association between higher circulating branched-chain amino acids (BCAA) and the severity of insulin resistance in people with diabetes. While several studies have assessed BCAA metabolism as a potential target for regulation, less attention has been paid to the role of L-type amino acid transporter 1 (LAT1), the primary transporter of BCAA in skeletal muscle. The aim of this study was to assess the impact of JPH203 (JPH), a LAT1 inhibitor, on myotube metabolism in both insulin-sensitive and insulin-resistant myotubes. C2C12 myotubes were treated with or without 1 μM or 2 μM JPH for 24 h with or without insulin resistance. Western blot and qRT-PCR were used to assess protein content and gene expression, respectively. Mitochondrial and glycolytic metabolism were measured via Seahorse Assay, and fluorescent staining was used to measure mitochondrial content. BCAA media content was quantified using liquid chromatography-mass spectrometry. JPH at 1 μM (but not 2 μM) increased mitochondrial metabolism and content without inducing changes in mRNA expression of transcripts associated with mitochondrial biogenesis or mitochondrial dynamics. Along with increased mitochondrial function, 1μM treatment also reduced extracellular leucine and valine. JPH at 2 μM reduced pAkt signaling and increased extracellular accumulation of isoleucine without inducing changes in BCAA metabolic genes. Collectively, JPH may increase mitochondrial function independent of the mitochondrial biogenic transcription pathway; however, high doses may reduce insulin signaling.

Insulin resistance promotes extracellular BCAA accumulation without altering LAT1 content, independent of prior BCAA treatment in a myotube model of skeletal muscle

PURPOSE

Type 2 diabetes is characterized by reduced insulin sensitivity which correlates with increased circulating BCAA. These experiments investigated the effects of insulin resistance with and without excess BCAA on myotube insulin sensitivity and L-type amino acid transporter-1 (LAT1).

METHODS

C2C12 myotubes were treated with or without excess BCAA for 1 or 6 days, both with and without insulin resistance. Western blot was used to assess insulin sensitivity and LAT1 content. Liquid chromatography-mass spectrometry was used to evaluate BCAA media content.

RESULTS

Insulin resistance was associated with significantly increased extracellular BCAA accumulation independent of LAT1 content. Conversely, prior BCAA treatment was not associated with extracellular BCAA accumulation regardless of level of insulin sensitivity.

CONCLUSION

These data suggest insulin resistance, but not BCAA treatment, promotes extracellular BCAA accumulation independent of changes in LAT1 content, implicating insulin resistance as a causal agent of extracellular BCAA accumulation.

Fenofibrate Increases the Population of Non-Classical Monocytes in Asymptomatic Chagas Disease Patients and Modulates Inflammatory Cytokines in PBMC

Chronic Chagas disease cardiomyopathy (CCC) is the most important clinical manifestation of infection with Trypanosma cruzi (T. cruzi) due to its frequency and effects on morbidity and mortality. Peripheral blood mononuclear cells (PBMC) infiltrate the tissue and differentiate into inflammatory macrophages. Advances in pathophysiology show that myeloid cell subpopulations contribute to cardiac homeostasis, emerging as possible therapeutic targets. We previously demonstrated that fenofibrate, PPARα agonist, controls inflammation, prevents fibrosis and improves cardiac function in a murine infection model. In this work we investigated the spontaneous release of inflammatory cytokines and chemokines, changes in the frequencies of monocyte subsets, and fenofibrate effects on PBMC of seropositive patients with different clinical stages of Chagas disease. The results show that PBMC from Chagas disease patients display higher levels of IL-12, TGF-β, IL-6, MCP1, and CCR2 than cells from uninfected individuals (HI), irrespectively of the clinical stage, asymptomatic (Asy) or with Chagas heart disease (CHD). Fenofibrate reduces the levels of pro-inflammatory mediators and CCR2 in both Asy and CHD patients. We found that CHD patients display a significantly higher percentage of classical monocytes in comparison with Asy patients and HI. Besides, Asy patients have a significantly higher percentage of non-classical monocytes than CHD patients or HI. However, no difference in the intermediate monocyte subpopulation was found between groups. Moreover, monocytes from Asy or CHD patients exhibit different responses upon stimulation in vitro with T. cruzi lysates and fenofibrate treatment. Stimulation with T. cruzi significantly increases the percentage of classical monocytes in the Asy group whereas the percentage of intermediate monocytes decreases. Besides, there are no changes in their frequencies in CHD or HI. Notably, stimulation with T. cruzi did not modify the frequency of the non-classical monocytes subpopulation in any of the groups studied. Moreover, fenofibrate treatment of T. cruzi-stimulated cells, increased the frequency of the non-classical subpopulation in Asy patients. Interestingly, fenofibrate restores CCR2 levels but does not modify HLA-DR expression in any groups. In conclusion, our results emphasize a potential role for fenofibrate as a modulator of monocyte subpopulations towards an anti-inflammatory and healing profile in different stages of chronic Chagas disease.

A new therapeutic effect of fenofibrate in Duchenne muscular dystrophy: The promotion of myostatin degradation

BACKGROUND AND PURPOSE

Duchenne muscular dystrophy (DMD) is a degenerative muscle disease with no effective drug treatment. This study investigated the positive effects of fenofibrate on dystrophic muscles.

EXPERIMENTAL APPROACH

Myostatin expression in serum and muscle tissue of DMD patients and mdx mice were tested. Primary myoblasts isolated from mdx mice were challenged with an inflammatory stimulus and treated with fenofibrate. In animal experiments, 6-week-old male mdx mice were treated with fenofibrate (100 mg/kg) administered orally once per day for 6 weeks. Tests of muscle function plus histology and biochemical analyses of serum were conducted to evaluate the effects of fenofibrate. The expressions of myostatin, MuRF1, and atrogin-1 in skeletal muscle were evaluated by Western blotting and real-time PCR. Total and oxidative myosin heavy chain (MHC) were assessed via immunofluorescence.

KEY RESULTS

Increased expression of myostatin protein was found in dystrophic muscle of DMD patients and mdx mice. Fenofibrate enhanced myofibre differentiation by downregulating the expression of myostatin protein but not mRNA in primary myoblasts of mdx mice. Fenofibrate significantly improved muscle function while ameliorating muscle damage in mdx mice. These benefits are accompanied by an anti-inflammatory effect. Fenofibrate treatment returned myofibre function by inhibiting the expressions of myostatin, MuRF1, and atrogin-1 protein in the gastrocnemius muscle and diaphragm, while leaving the mRNA level of myostatin unaffected.

CONCLUSIONS AND IMPLICATIONS

Fenofibrate substantially slows muscle dystrophy by promoting the degradation of myostatin protein, which may indicate a new therapeutic focus for DMD patients.

Lipotoxicity suppresses the synthesis of growth hormone in pituitary somatotrophs via endoplasmic reticulum stress

Lipotoxicity has been shown to cause dysfunction of many organs and tissues. However, it is unclear whether lipotoxicity is harmful to the somatotrophs, a kind of cell that synthesize growth hormone (GH) in the pituitary. In this study, we performed an epidemiological study, serum levels of triglyceride (TG) and GH showed a negative correlation, even after adjustment for potential confounders. In an animal study, male Sprague‐Dawley rats were fed a high‐fat diet (HFD) or a control diet for 28 weeks. HFD rats showed impaired GH synthesis, resulting in a decrease in circulating GH levels. The expression of pituitary Pit‐1, a key transcription factor of GH, was inhibited. We found that the inositol‐requiring enzyme 1α (IRE1α) pathway of endoplasmic reticulum (ER) stress was triggered in HFD rat pituitary glands and palmitic acid‐treated GH3 cells, respectively. On the contrary, applying 4‐phenyl butyric acid (4‐PBA) to alleviate ER stress or 4µ8c to specifically block the IRE1α pathway attenuated the impairment of both Pit‐1 and GH expression. In conclusion, we demonstrated that lipotoxicity directly inhibits the synthesis of GH, probably by reducing Pit‐1 expression. The IRE1α signaling pathway of ER stress may play an important role in this process.

Caloric restriction following early-life high fat-diet feeding represses skeletal muscle TNF in male rats

Chronic metabolic diseases are on the rise worldwide and their etiology is multifactorial. Among them, inflammatory components like Tumor Necrosis Factor (TNF), contribute to whole-body metabolic impairment. Caloric Restriction (CR) combats metabolic diseases, but how it reduces inflammation remains understudied. We aimed to evaluate the impact of chronic CR on muscle inflammation, in particular TNF. In our study, 4-week old male Sprague-Dawley rats were fed a high-fat diet (HF, 45% Kcal of fat from lard) ad libitum for 3 months. After estimation of their energy requirement (1 month), they were then divided into three groups: HF ad libitum (OL), weight maintenance with AIN93M (9.5% Kcal from fat; ML, 100% of energy requirement), and caloric restriction (CR, AIN93M with 75% of energy requirement). This dietary intervention continued for six months. At this point, rats were sacrificed and gastrocnemius muscle was collected. CR induced a profound shift in fat and lean mass, and decreased growth factor IGF-1. Muscle qPCR analysis showed a marked decrease in inflammation and TNF (premRNA, mRNA, and protein) by CR, accompanied by Tnf promoter DNA hypermethylation. CR increased expression of histone deacetylase Sirt6 and decreased methyltransferase Suv39h1, together with decreased Tnf promoter and coding region binding of NF- κB and C/EBP-β. Following miRNA database mining, qPCR analysis revealed that CR downregulated the proinflammatory miR-19b and increased the anti-inflammatory miR-181a and its known targets. Chronic CR is able to regulate muscle-specific inflammation by targeting the NF-κB pathway as well as transcriptional and post-transcriptional regulation of Tnf gene.

1,2-Dilinoleoyl-sn-glycero-3-phosphocholine increases insulin sensitivity in palmitate-treated myotubes and induces lipolysis in adipocytes

Obesity causes the development of insulin resistance and type 2 diabetes. Phosphatidylcholine (PPC) has been reported to increase hepatic insulin sensitivity and lipolysis in adipose tissue to resolve local obesity. In this study, we proposed 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC), the main active species of PPC, as an effective substance for the treatment of obesity-mediated disorders such as impaired fat metabolism and insulin resistance. Therefore, we investigated the potential lipolytic effects of DLPC on adipocytes and insulin signaling in muscle cells. In this study, DLPC-treated 3T3-L1 adipocytes showed enhanced tumor necrosis factor α (TNF-α) release. Suppression of TNF-α by short interfering RNA (siRNA) mitigated DLPC-induced lipolysis and apoptosis. DLPC treatment increased peroxisome proliferator-activated receptor α (PPARα) expression levels in C2C12 myocytes. siRNA-mediated suppression of PPARα abrogated the suppressive effects of DLPC on palmitate-induced inflammation and insulin resistance. In conclusion, DLPC enhanced lipolysis and apoptosis via a TNFα-dependent pathway in adipocytes and attenuated palmitate-induced insulin resistance through PPARα-mediated suppression of inflammation in myocytes.

Dietary fenofibrate attenuated high-fat-diet-induced lipid accumulation and inflammation response partly through regulation of pparα and sirt1 in juvenile black seabream (Acanthopagrus schlegelii)

An 8-week feeding trail was conducted in Acanthopagrus schlegelii with an initial body weight of 8.34 ± 0.01g. Three isonitrogenous diets were formulated, (1) Control: medium-fat diet (12%); (2) HFD: high-fat diet (18%); (3) HFD + FF: high-fat diet with fenofibrate (0.15%). Liver histological analysis revealed that, compared to HFD, vacuolar fat drops were smaller and fewer in fish fed fenofibrate. Expression of lipid catabolism regulator peroxisome proliferator-activated receptor alpha (pparα) was up-regulated by fenofibrate compared with HFD. In addition, fenofibrate significantly increased the expression level of silent information regulator 1 (sirt1). Meanwhile, the expression level of anti-inflammatory cytokine interleukin 10 (il-10) in intestine was up-regulated, while pro-inflammatory cytokine interleukin 1β (il-1β) in liver and intestine were down-regulated by dietary fenofibrate supplementation. Overall, the present study indicated that fenofibrate reduced fat deposition and attenuated inflammation response caused by HFD partly through a pathway involving regulation of pparα and sirt1.

Roles of sarcoplasmic reticulum Ca2+ ATPase pump in the impairments of lymphatic contractile activity in a metabolic syndrome rat model

The intrinsic lymphatic contractile activity is necessary for proper lymph transport. Mesenteric lymphatic vessels from high-fructose diet-induced metabolic syndrome (MetSyn) rats exhibited impairments in its intrinsic phasic contractile activity; however, the molecular mechanisms responsible for the weaker lymphatic pumping activity in MetSyn conditions are unknown. Several metabolic disease models have shown that dysregulation of sarcoplasmic reticulum Ca2+ ATPase (SERCA) pump is one of the key determinants of the phenotypes seen in various muscle tissues. Hence, we hypothesized that a decrease in SERCA pump expression and/or activity in lymphatic muscle influences the diminished lymphatic vessel contractions in MetSyn animals. Results demonstrated that SERCA inhibitor, thapsigargin, significantly reduced lymphatic phasic contractile frequency and amplitude in control vessels, whereas, the reduced MetSyn lymphatic contractile activity was not further diminished by thapsigargin. While SERCA2a expression was significantly decreased in MetSyn lymphatic vessels, myosin light chain 20, MLC20 phosphorylation was increased in these vessels. Additionally, insulin resistant lymphatic muscle cells exhibited elevated intracellular calcium and decreased SERCA2a expression and activity. The SERCA activator, CDN 1163 partially restored lymphatic contractile activity in MetSyn lymphatic vessel by increasing phasic contractile frequency. Thus, our data provide the first evidence that SERCA2a modulates the lymphatic pumping activity by regulating phasic contractile amplitude and frequency, but not the lymphatic tone. Diminished lymphatic contractile activity in the vessels from the MetSyn animal is associated with the decreased SERCA2a expression and impaired SERCA2 activity in lymphatic muscle.

Oleic acid ameliorates palmitic acid-induced ER stress and inflammation markers in naive and cerulein-treated exocrine pancreas cells

Dietary fat overload (typical to obesity) increases the risk of pancreatic pathologies through mechanisms yet to be defined. We previously showed that saturated dietary fat induces pancreatic acinar lipotoxicity and cellular stress. The endoplasmic reticulum (ER) of exocrine pancreas cells is highly developed and thus predisposed to stress. We studied the combination of saturated and unsaturated FAs in metabolic and pancreatitis like cerulein (CER)-induced stress states on cellular ER stress.

Exocrine pancreas AR42J and rat primary exocrine acinar cells underwent acute (24 h) challenge with different FAs (saturated, monounsaturated) at different concentrations (250 and 500 µM) and in combination with acute CER-induced stress, and were analyzed for fat accumulation, ER stress unfolded protein response (UPR) and immune and enzyme markers. Acute exposure of AR42J and pancreatic acinar cells to different FAs and their combinations increased triglyceride accumulation. Palmitic acid significantly dose-dependently enhanced the UPR, immune factors and pancreatic lipase (PL) levels, as demonstrated by XBP1 splicing and elevation in UPR transcripts and protein levels (Xbp1,Atf6, Atf4, Chop, Tnfα, Tgfβ and Il-6). Exposure to high palmitic levels in a CER-induced stress state synergistically increased ER stress and inflammation marker levels. Exposure to oleic acid did not induce ER stress and PL levels and significantly decreased immune factors in an acute CER-induced stress state. Combination of oleic and palmitic acids significantly reduced the palmitic-induced ER stress, but did not affect the immune factor response. We show that combination of monounsaturated and saturated FAs protects from exocrine pancreatic cellular ER stress in both metabolic and CER-induced stress.

ER stress in skeletal muscle remodeling and myopathies

Skeletal muscle is a highly plastic tissue in the human body that undergoes extensive adaptation in response to environmental cues, such as physical activity, metabolic perturbation, and disease conditions. The endoplasmic reticulum (ER) plays a pivotal role in protein folding and calcium homeostasis in many mammalian cell types, including skeletal muscle. However, overload of misfolded or unfolded proteins in the ER lumen cause stress, which results in the activation of a signaling network called the unfolded protein response (UPR). The UPR is initiated by three ER transmembrane sensors: protein kinase R-like endoplasmic reticulum kinase, inositol-requiring protein 1α, and activating transcription factor 6. The UPR restores ER homeostasis through modulating the rate of protein synthesis and augmenting the gene expression of many ER chaperones and regulatory proteins. However, chronic heightened ER stress can also lead to many pathological consequences including cell death. Accumulating evidence suggests that ER stress-induced UPR pathways play pivotal roles in the regulation of skeletal muscle mass and metabolic function in multiple conditions. They have also been found to be activated in skeletal muscle under catabolic states, degenerative muscle disorders, and various types of myopathies. In this article, we have discussed the recent advancements toward understanding the role and mechanisms through which ER stress and individual arms of the UPR regulate skeletal muscle physiology and pathology.

Treatment with Fenofibrate plus a low dose of Benznidazole attenuates cardiac dysfunction in experimental Chagas disease

Trypanosoma cruzi induces serious cardiac alterations during the chronic infection. Intense inflammatory response observed from the beginning of infection, is critical for the control of parasite proliferation and evolution of Chagas disease. Peroxisome proliferator-activated receptors (PPAR)-α, are known to modulate inflammation. In this study we investigated whether a PPAR-α agonist, Fenofibrate, improves cardiac function and inflammatory parameters in a murine model of T. cruzi infection. BALB/c mice were sequentially infected with two T. cruzi strains of different genetic background. Benznidazole, commonly used as trypanocidal drug, cleared parasites but did not preclude cardiac pathology, resembling what is found in human chronic chagasic cardiomyopathy. Fenofibrate treatment restored to normal values the ejection and shortening fractions, left ventricular end-diastolic, left ventricular end-systolic diameter, and isovolumic relaxation time. Moreover, it reduced cardiac inflammation and fibrosis, decreased the expression of pro-inflammatory (IL-6, TNF-α and NOS2) and heart remodeling mediators (MMP-9 and CTGF), and reduced serum creatine kinase activity. The fact that Fenofibrate partially inhibited NOS2 expression and NO release in the presence of a PPAR-α non-competitive inhibitor, suggested it also acted through PPAR-α-independent pathways. Since IκBα cytosolic degradation was inhibited by Fenofibrate, it can be concluded that the NFκB pathway has a role in its effects. Thus, we demonstrate that Fenofibrate acts through PPAR-α-dependent and -independent pathways. Our study shows that combined treatment with Fenofibrate plus Benznidazole is able both to reverse the cardiac dysfunction associated with the ongoing inflammatory response and fibrosis and to attain parasite clearance in an experimental model of Chagas disease.

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.

Fermented Moringa oleifera Decreases Hepatic Adiposity and Ameliorates Glucose Intolerance in High-Fat Diet-Induced Obese Mice

Metabolic diseases, such as glucose intolerance and nonalcoholic fatty-liver disease (NAFLD), are primary risk factors for life-threatening conditions such as diabetes, heart attack, stroke, and hepatic cancer. Extracts from the tropical tree Moringa oleifera show antidiabetic, antioxidant, anti-inflammatory, and anticancer effects. Fermentation can further improve the safety and nutritional value of certain foods. We investigated the efficacy of fermented M. oleifera extract (FM) against high-fat diet (HFD)-induced glucose intolerance and hepatic lipid accumulation and investigated the underlying mechanisms by analyzing expression of proteins and genes involved in glucose and lipid regulation. C57BL/6 mice were fed with normal chow diet (ND) or HFD supplemented with distilled water (DW, control), nonfermented M. oleifera extract (NFM), or FM for 10 weeks. Although body weights were similar among HFD-fed treatment groups, liver weight was decreased, and glucose tolerance test (GTT) results improved in the FM group compared with DW and NFM groups. Hepatic lipid accumulation was also lower in the FM group, and expressions of genes involved in liver lipid metabolism were upregulated. In addition, HFD-induced endoplasmic reticulum (ER) stress, oxidative stress, and lipotoxicity in quadriceps muscles were decreased by FM. Finally, proinflammatory cytokine mRNA expression was decreased by FM in the liver, epididymal adipose tissue, and quadriceps of HFD-fed mice. FMs may decrease glucose intolerance and NAFLD under HFD-induced obesity by decreasing ER stress, oxidative stress, and inflammation.

Emerging roles of ER stress and unfolded protein response pathways in skeletal muscle health and disease

Skeletal muscle is the most abundant tissue in the human body and can adapt its mass as a consequence of physical activity, metabolism, growth factors, and disease conditions. Skeletal muscle contains an extensive network of endoplasmic reticulum (ER), called sarcoplasmic reticulum, which plays an important role in the regulation of proteostasis and calcium homeostasis. In many cell types, environmental and genetic factors that disrupt ER function cause an accumulation of misfolded and unfolded proteins in the ER lumen that ultimately leads to ER stress. To alleviate the stress and restore homeostasis, the ER activates a signaling network called the unfolded protein response (UPR). The UPR has three arms, which regulate protein synthesis and expression of many ER chaperone and regulatory proteins. However, the role of individual UPR pathways in skeletal muscle has just begun to be investigated. Recent studies suggest that UPR pathways play pivotal roles in muscle stem cell homeostasis, myogenic differentiation, and regeneration of injured skeletal muscle. Moreover, markers of ER stress and the UPR are activated in skeletal muscle in diverse conditions such as exercise, denervation, starvation, high fat diet, cancer cachexia, and aging. Accumulating evidence also suggests that ER stress may have important roles in the pathogenesis of inflammatory myopathies and genetic muscle disorders. The purpose of this review article is to discuss the role and potential mechanisms by which ER stress and the individual arms of the UPR regulate skeletal muscle formation, plasticity, and function in various physiological and pathophysiological conditions.

High fat diet dysregulates microRNA-17-5p and triggers retinal inflammation: Role of endoplasmic-reticulum-stress

AIM

To elucidate how high diet-induced endoplasmic reticulum-stress upregulates thioredoxin interacting protein expression in Müller cells leading to retinal inflammation.

METHODS

Male C57Bl/J mice were fed either normal diet or 60% high fat diet for 4-8 wk. During the 4 wk study, mice received phenyl-butyric acid (PBA); endoplasmic reticulum-stress inhibitor; for 2 wk. Insulin resistance was assessed by oral glucose tolerance. Effects of palmitate-bovine serum albumin (BSA) (400 μmol/L) were examined in retinal Müller glial cell line and primary Müller cells isolated from wild type and thioredoxin interacting protein knock-out mice. Expression of thioredoxin interacting protein, endoplasmic reticulum-stress markers, miR-17-5p mRNA, as well as nucleotide-binding oligomerization domain-like receptor protein (NLRP3) and IL1β protein was determined.

RESULTS

High fat diet for 8 wk induced obesity and insulin resistance evident by increases in body weight and impaired glucose tolerance. By performing quantitative real-time polymerase chain reaction, we found that high fat diet triggered the expression of retinal endoplasmic reticulum-stress markers (P < 0.05). These effects were associated with increased thioredoxin interacting protein and decreased miR-17-5p expression, which were restored by inhibiting endoplasmic reticulum-stress with PBA (P < 0.05). In vitro, palmitate-BSA triggered endoplasmic reticulum-stress markers, which was accompanied with reduced miR-17-5p and induced thioredoxin interacting protein mRNA in retinal Müller glial cell line (P < 0.05). Palmitate upregulated NLRP3 and IL1β expression in primary Müller cells isolated from wild type. However, using primary Müller cells isolated from thioredoxin interacting protein knock-out mice abolished palmitate-mediated increase in NLRP3 and IL1β.

CONCLUSION

Our work suggests that targeting endoplasmic reticulum-stress or thioredoxin interacting protein are potential therapeutic strategies for early intervention of obesity-induced retinal inflammation.