Role of the mTOR-autophagy-ER stress pathway in high fructose-induced metabolic-associated fatty liver disease

Ileum Proteomics Identifies Distinct Pathways Associated with Different Dietary Doses of Copper-Fructose Interactions: Implications for the Gut-Liver Axis and MASLD

The interactions of different dietary doses of copper with fructose contribute to the development of metabolic dysfunction-associated steatotic liver disease (MASLD) via the gut-liver axis. The underlying mechanisms remain elusive. The aim of this study was to identify the specific pathways leading to gut barrier dysfunction in the ileum using a proteomics approach in a rat model. Male weanling Sprague Dawley rats were fed diets with adequate copper (CuA), marginal copper (CuM), or supplemented copper (CuS) in the absence or presence of fructose supplementation (CuAF, CuMF, and CuSF) for 4 weeks. Ileum protein was extracted and analyzed with an LC-MS. A total of 2847 differentially expressed proteins (DEPs) were identified and submitted to functional enrichment analysis. As a result, the ileum proteome and signaling pathways that were differentially altered were revealed. Of note, the CuAF is characterized by the enrichment of oxidative phosphorylation and ribosome as analyzed with the KEGG; the CuMF is characterized by an enriched arachidonic acid metabolism pathway; and focal adhesion, the regulation of the actin cytoskeleton, and tight junction were significantly enriched by the CuSF. In conclusion, our proteomics analysis identified the specific pathways in the ileum related to the different dietary doses of copper-fructose interactions, suggesting that distinct mechanisms in the gut are involved in the development of MASLD.

AMPK/autophagy-mediated alleviation of tendinopathy by IL-38: A novel strategy for the treatment of obesity-related tendinopathy

The effect of interleukin-38 (IL-38), a recently identified member of the IL-1 family with potential applications in various inflammation-related conditions, on ER stress has not been explored. Furthermore, its role in obesity-associated tendinopathy has not been investigated. In this study, human primary tenocytes were treated with palmitate (200 or 400 μM) and palmitate plus IL-38 (0-50 ng/mL) for 24 h. Western blotting was used to assess ER stress and tendinopathogenic markers in tenocytes. Monodansylcadaverine (MDC) staining was used to evaluate autophagosomes. Apoptosis was determined by cell viability assays, caspase 3 activity assays and TUNEL assays. Cell migration was evaluated by a cell scratch assay. Small interfering (si) RNA transfection was used for target gene silencing. Treatment of tenocytes with IL-38 attenuated apoptosis, restored the balance between MMPs and TIMP-1, and alleviated ER stress under palmitate conditions. IL-38 treatment enhanced AMPK phosphorylation and promoted the expression of autophagy markers related to LC3 conversion, p62 degradation, and autophagosome formation in cultured tenocytes. The effects of IL-38 on ER stress, apoptosis, and MMP-9, MMP-13, and TIMP-1 expression in palmitate-treated tenocytes were abrogated by AMPK siRNA or 3-methyladenine (3MA). These results suggest that IL-38 alleviates ER stress through the AMPK/autophagy pathway, thereby reducing apoptosis and preventing extracellular matrix (ECM) degradation in tenocytes under hyperlipidemic conditions. This study provides a promising therapeutic avenue for treating obesity-related tendinopathy using an endogenous compound such as IL-38.

Astragaloside-IV promotes autophagy via the Akt/mTOR pathway to improve cellular lipid deposition

The current study aimed to investigate the potential role of astragaloside IV (AS-IV) in improving cellular lipid deposition and its underlying mechanism. A fatty liver cell model was established by treating hepatoma cells with palmitic acid. AS-IV and SC79 were used for treatment. Oil Red O staining was applied to detect intracellular lipid deposition, and transmission electron microscopy was utilized to assess autophagosome formation. Immunofluorescence double staining was applied to determine microtubule-associated proteins 1A/1B light chain 3 (LC3) expression. Western blot analysis was performed to detect the expression of LC3, prostacyclin, Beclin-1, V-akt murine thymoma viral oncogene homolog (Akt), phosphorylated Akt, mTOR, and phosphorylated mTOR. Oil Red O staining revealed that AS-IV reduced intracellular lipid accumulation. Further, it increased autophagosome synthesis and the expression of autophagy proteins LC3 and Beclin-1 in the cells. It also reduced the phosphorylation levels of Akt and mTOR and the levels of prostacyclin. However, the effects of AS-IV decreased with SC79 treatment. In addition, LC3B + BODIPY493/503 fluorescence double staining showed that AS-IV reduced intracellular lipid deposition levels by enhancing autophagy. AS-IV can reduce lipid aggregation in fatty liver cells, which can be related to enhanced hepatocyte autophagy by inhibiting the Akt/mTOR signaling pathway.

Fructose induces hepatic steatosis in adolescent mice linked to the disorders of lipid metabolism, bile acid metabolism, and autophagy

The effects of excessive fructose intake on the development and progression of metabolic disorders have received widespread attention. However, the deleterious effects of fructose on the development of hepatic metabolic disease in adolescents and its potential mechanisms are not fully understood. In this study, we investigated the effects of isocaloric fructose-rich diets on the liver of adolescent mice. The results showed that fructose-rich diets had no effect on the development of obesity in the adolescent mice, but did induce hepatic lipid accumulation. Besides, we found that fructose-rich diets promoted hepatic inflammatory responses and oxidative stress in adolescent mice, which may be associated with activation of the NLRP3 inflammasome and inhibition of the Nrf2 pathway. Furthermore, our results showed that fructose-rich diets caused disturbances in hepatic lipid metabolism and bile acid metabolism, as well as endoplasmic reticulum stress and autophagy dysfunction. Finally, we found that the intestinal barrier function was impaired in the mice fed fructose-rich diets. In conclusion, our study demonstrates that dietary high fructose induces hepatic metabolic disorders in adolescent mice. These findings provide a theoretical foundation for fully understanding the effects of high fructose intake on the development of hepatic metabolic diseases during adolescence.

Insights into the influence of three types of sugar on goose fatty liver formation from endoplasmic reticulum stress (ERS)

This study analyzed the formation of goose fatty liver due to endoplasmic reticulum stress (ERS) caused by 3 types of sugar. Transcriptome analysis was performed for liver tissues from geese fed a traditional diet (maize flour), geese overfed with traditional diet, and geese overfed with diet supplemented with glucose, fructose, or sucrose. Correlation analysis of the liver tissue transcriptomes showed that differentially expressed genes (DEGs) involved in ERS were significantly negatively correlated with DEGs involved in inflammation response in the sucrose overfeeding group, and significantly positively correlated with the DEGs involved in lipid metabolism in fructose overfeeding group. Goose primary hepatocytes were isolated in vitro and then treated with glucose or fructose. Some were also treated with ERS inhibitor 4-phenylbutyric acid (4-PBA). In the hepatocytes, mRNA expression of X-Box Binding Protein 1 (XBP1), activating transcription factor 6 (AFT6) and glucose-regulated protein 78 (GRP78) genes increased in the two sugar groups (glucose and fructose), but were suppressed by adding 4-PBA. The mRNA expression data, protein kinase contents, and triglyceride (TG) and very low-density lipoprotein (VLDL) concentrations all suggest that ERS regulates lipid deposition induced by glucose and fructose via elevating lipid synthesis, inhibiting fatty acid oxidation, and decreasing lipid transportation. In conclusion, glucose, or fructose cause ERS and then ERS causes lipid deposition in goose primary hepatocytes. Three types of sugar cause lipid accumulation and then lipid accumulation prevents ERS during goose fatty liver formation, which suggests a potential mechanism protects goose livers from ERS. The different sugars may induce lipid deposition in different ways.

Fructose-induced perturbation in cellular proteostasis via RPS6KB1 promotes hepatic steatosis

Dietary fructose intake through increased consumption of refined sugar induces hepatic de novo lipogenesis (DNL), a major contributor to hepatic steatosis in NAFLD, however, it's mechanism is not completely understood. Using HepG2 cells, we show that fructose induced DNL involves ribosomal protein S6 kinase B1 (RPS6KB1) driven augmentation of hepatic protein synthesis. This consequently results in endoplasmic reticulum (ER)-stress induced expression of pro-lipogenic gene, fatty acid synthase (FASN). Additionally, the inhibition of fructose induced protein synthesis by either cycloheximide (CHX) or an RPS6KB1 inhibitor significantly reduced both ER-stress and FASN expression. Additionally, corroborating with our in vitro results, the analysis of human NAFLD transcriptomic datasets showed significant upregulation of protein synthesis pathways in the liver of patients with hepatic steatosis, thus linking protein synthesis to lipid accumulation during the early stages of NAFLD. Our results, therefore, demonstrate that RPS6KB1 driven "translation overdrive" coupled with ER-stress contributes to lipogenic gene transcription, and propose RPS6KB1 inhibition as a therapeutic strategy to counter fructose induced hepatic steatosis in NAFLD.

The Beneficial and Adverse Effects of Autophagic Response to Caloric Restriction and Fasting

Each cell is equipped with a conserved housekeeping mechanism, known as autophagy, to recycle exhausted materials and dispose of injured organelles via lysosomal degradation. Autophagy is an early-stage cellular response to stress stimuli in both physiological and pathological situations. It is thought that the promotion of autophagy flux prevents host cells from subsequent injuries by removing damaged organelles and misfolded proteins. As a correlate, the modulation of autophagy is suggested as a therapeutic approach in diverse pathological conditions. Accumulated evidence suggests that intermittent fasting or calorie restriction can lead to the induction of adaptive autophagy and increase longevity of eukaryotic cells. However, prolonged calorie restriction with excessive autophagy response is harmful and can stimulate a type II autophagic cell death. Despite the existence of a close relationship between calorie deprivation and autophagic response in different cell types, the precise molecular mechanisms associated with this phenomenon remain unclear. Here, we aimed to highlight the possible effects of prolonged and short-term calorie restriction on autophagic response and cell homeostasis.

Glyphosate-induced autophagy inhibition results in hepatic steatosis via mediating epigenetic reprogramming of PPARα in roosters

Glyphosate (Gly) is the most widely used herbicide with well-defined hepatotoxic effects, but the underlying mechanisms of Gly-induced hepatic steatosis remain largely unknown. In this study, a rooster model combined with primary chicken embryo hepatocytes was established to dissect the progresses and mechanisms of Gly-induced hepatic steatosis. Data showed that Gly exposure caused liver injury with disrupted lipid metabolism in roosters, manifested by significant serum lipid profile disorder and hepatic lipid accumulation. Transcriptomic analysis revealed that PPARα and autophagy-related pathways played important roles in Gly-induced hepatic lipid metabolism disorders. Further experimental results suggested that autophagy inhibition was involved in Gly-induced hepatic lipid accumulation, which was confirmed by the effect of classic autophagy inducer rapamycin (Rapa). Moreover, data substantiated that Gly-mediated autophagy inhibition caused nuclear increase of HDAC3, which altered epigenetic modification of PPARα, leading to fatty acid oxidation (FAO) inhibition and subsequently lipid accumulation in the hepatocytes. In summary, this study provides novel evidence that Gly-induced autophagy inhibition evokes the inactivation of PPARα-mediated FAO and concomitant hepatic steatosis in roosters by mediating epigenetic reprogramming of PPARα.

Integrative analysis of transcriptome and lipidome reveals fructose pro-steatosis mechanism in goose fatty liver

To further explore the fructose pro-steatosis mechanism, we performed an integrative analysis of liver transcriptome and lipidome as well as peripheral adipose tissues transcriptome analysis using samples collected from geese overfed with maize flour (control group) and geese overfed with maize flour supplemented with 10% fructose (treatment group). Overfeeding period of the treatment group was significantly shorter than that of the control group (p < 0.05). Dietary supplementation with 10% fructose induced more severe steatosis in goose liver. Compared with the control group, the treatment group had lower in ceramide levels (p < 0.05). The key differentially expressed genes (DEGs) (control group vs. treatment group) involved in liver fatty acid biosynthesis and steroid biosynthesis were downregulated. The conjoint analysis between DEGs and different lipids showed that fatty acid biosynthesis and steroid biosynthesis were the highest impact score pathways. In conclusion, fructose expedites goose liver lipid accumulation maximization during overfeeding.

Nutri-stress, mitochondrial dysfunction, and insulin resistance-role of heat shock proteins

Excess nutrient flux into the cellular energy system results in a scenario of cellular metabolic stress in diseases involving insulin resistance, such as type 2 diabetes, referred to as nutri-stress and results in cellular bioenergetic imbalance, which leads to insulin resistance and disease. Under nutri-stress, the heat shock response system is compromised due to metabolic abnormalities that disturb energy homeostasis. Heat shock proteins (HSPs) are the chief protectors of intracellular homeostasis during stress. Heat shock response (HSR) impairment contributes to several metabolic pathways that aggravate chronic hyperglycaemia and insulin resistance, highlighting a central role in disease pathogenesis. This article discusses the role of nutri-stress-related molecular events in causing insulin resistance and the nature of the roles played by heat shock proteins in some of the crucial checkpoints of the molecular networks involved in insulin resistance. Ample evidence suggests that the heat shock machinery regulates critical pathways in mitochondrial function and energy metabolism and that cellular energy status highly influences it. Weakening of HSPs, therefore, leads to loss of their vital cytoprotective functions, propagating nutri-stress in the system. Further research into the mechanistic roles of HSPs in metabolic homeostasis will help widen our understanding of lifestyle diseases, their onset, and complications. These inducible proteins may be crucial to attenuating lifestyle risk factors and disease management.

The involvement of oxidative stress mediated endoplasmic reticulum pathway in apoptosis of Golden Pompano (Trachinotus blochii) liver under PS-MPs stress

Globally, microplastics (MPs) are highly prevalent, especially in coastal areas. Unfortunately, golden pompano as a major marine fish in China is typically raised in floating marine cages near coasts, facing these MPs sources. However, toxicological studies on Golden Pompano which farm in coastal areas and face actual microplastic exposure are rare. Therefore, golden pompano were exposed to 10.0 μg/L, 100.0 μg/L, and 1000.0 μg/L polystyrene MPs (PS-MPs) for 14 days to study the potential impact of the microplastics on the Golden Pompano. Fish show slowed growth after 14 days of exposure. Histopathology shows irregular shaped nuclei and nuclear and cytoplasmic vacuolation traits in liver. Oxidative stress-related enzyme activity and gene expression data show that oxidative damage occurs in the high-concentrations (100.0 μg/L and 1000.0 μg/L) of PS-MPs exposures. Up-regulation of Grp78, Xbp-1, Eif-2α and chop gene expression indicates the occurrence of endoplasmic reticulum stress, and the western blot results also confirmed this. Severe oxidative stress also caused ERS, which ultimately increased BAX/Bcl-2 ratios and induces apoptosis. Furthermore, up-regulated anaerobic respiration, altered lipid metabolism, and immune disturbance were exhibited during PS-MPs stress. Therefore, oxidative stress appeared to be the main toxicity issue caused by MPs, while ERS-mediated apoptosis, metabolic alterations, and immune responses were induced by this stress. Notably, endoplasmic reticulum stress and apoptosis are a self-protective mechanism, which may be an intermediate link in the toxicity of microplastics. This study highlights the role of endoplasmic reticulum stress in MPs toxicology and assesses the adverse effects of microplastics on Golden Pompano.

CRELD2, endoplasmic reticulum stress, and human diseases

CRELD2, a member of the cysteine-rich epidermal growth factor-like domain (CRELD) protein family, is both an endoplasmic reticulum (ER)-resident protein and a secretory factor. The expression and secretion of CRELD2 are dramatically induced by ER stress. CRELD2 is ubiquitously expressed in multiple tissues at different levels, suggesting its crucial and diverse roles in different tissues. Recent studies suggest that CRELD2 is associated with cartilage/bone metabolism homeostasis and pathological conditions involving ER stress such as chronic liver diseases, cardiovascular diseases, kidney diseases, and cancer. Herein, we first summarize ER stress and then critically review recent advances in the knowledge of the characteristics and functions of CRELD2 in various human diseases. Furthermore, we highlight challenges and present future directions to elucidate the roles of CRELD2 in human health and disease.

Utility of Human Relevant Preclinical Animal Models in Navigating NAFLD to MAFLD Paradigm

Fatty liver disease is an emerging contributor to disease burden worldwide. The past decades of work established the heterogeneous nature of non-alcoholic fatty liver disease (NAFLD) etiology and systemic contributions to the pathogenesis of the disease. This called for the proposal of a redefinition in 2020 to that of metabolic dysfunction-associated fatty liver disease (MAFLD) to better reflect the current understanding of the disease. To date, several clinical cohort studies comparing NAFLD and MAFLD hint at the relevancy of the new nomenclature in enriching for patients with more severe hepatic injury and extrahepatic comorbidities. However, the underlying systemic pathogenesis is still not fully understood. Preclinical animal models have been imperative in elucidating key biological mechanisms in various contexts, including intrahepatic disease progression, interorgan crosstalk and systemic dysregulation. Furthermore, they are integral in developing novel therapeutics against MAFLD. However, substantial contextual variabilities exist across different models due to the lack of standardization in several aspects. As such, it is crucial to understand the strengths and weaknesses of existing models to better align them to the human condition. In this review, we consolidate the implications arising from the change in nomenclature and summarize MAFLD pathogenesis. Subsequently, we provide an updated evaluation of existing MAFLD preclinical models in alignment with the new definitions and perspectives to improve their translational relevance.

Sclerostin aggravates insulin signaling in skeletal muscle and hepatic steatosis via upregulation of ER stress by mTOR-mediated inhibition of autophagy under hyperlipidemic conditions

Recently, sclerostin (SCL), a circulating glycoprotein, was proposed to be a novel myokine involved in developing metabolic disorders. The association between SCL levels and insulin resistance in skeletal muscle, liver, and adipose tissue was studied in individuals with aggravated glucose tolerance. Thus, we hypothesized that elevated circulating SCL might affect skeletal muscle insulin signaling and hepatic lipid metabolism, and aimed to investigate the effects of SCL on skeletal muscle insulin resistance and hepatic steatosis in obesity using in vitro and in vivo experimental models under hyperlipidemic conditions. In the current study, we found elevated SCL messenger RNA expression levels in myocytes in obese patients. In addition to a higher blood level, SCL was expressed at an elevated level in the skeletal muscle of mice fed a high-fat diet (HFD). Higher SCL release levels and expression were also noticed in palmitate-treated C2C12 myocytes. SCL suppression by in vivo transfection improves skeletal muscle insulin resistance and hepatic steatosis in HFD-fed mice. The treatment of C2C12 myocytes with recombinant SCL aggravated insulin signaling. Furthermore, treatment with SCL augmented lipogenic lipid deposition in human primary hepatocytes. Treatment with SCL upregulated mammalian target of rapamycin (mTOR) phosphorylation and suppressed autophagy markers, thereby causing endoplasmic reticulum (ER) stress. 4-Phenylbutyric acid, a pharmacological ER stress inhibitor, abolished the effects of SCL on insulin signaling in C2C12 myocytes and lipid accumulation in primary hepatocytes. In conclusion, SCL promotes skeletal muscle insulin resistance and hepatic steatosis by upregulating ER stress via the mTOR/autophagy-mediated pathway. The present study suggests that antagonizing SCL might be a novel therapeutic strategy for simultaneously managing insulin resistance and hepatic steatosis in obesity.

Luteolin alleviates cognitive impairment in Alzheimer's disease mouse model via inhibiting endoplasmic reticulum stress-dependent neuroinflammation

Luteolin is a flavonoid in a variety of fruits, vegetables, and herbs, which has shown anti-inflammatory, antioxidant, and anti-cancer neuroprotective activities. In this study, we investigated the potential beneficial effects of luteolin on memory deficits and neuroinflammation in a triple-transgenic mouse model of Alzheimer's disease (AD) (3 × Tg-AD). The mice were treated with luteolin (20, 40 mg · kg^-1 · d^-1, ip) for 3 weeks. We showed that luteolin treatment dose-dependently improved spatial learning, ameliorated memory deficits in 3 × Tg-AD mice, accompanied by inhibiting astrocyte overactivation (GFAP) and neuroinflammation (TNF-α, IL-1β, IL-6, NO, COX-2, and iNOS protein), and decreasing the expression of endoplasmic reticulum (ER) stress markers GRP78 and IRE1α in brain tissues. In rat C6 glioma cells, treatment with luteolin (1, 10 µM) dose-dependently inhibited LPS-induced cell proliferation, excessive release of inflammatory cytokines, and increase of ER stress marker GRP78. In conclusion, luteolin is an effective agent in the treatment of learning and memory deficits in 3 × Tg-AD mice, which may be attributable to the inhibition of ER stress in astrocytes and subsequent neuroinflammation. These results provide the experimental basis for further research and development of luteolin as a therapeutic agent for AD.

Eicosapentaenoic Acid-Induced Autophagy Attenuates Intervertebral Disc Degeneration by Suppressing Endoplasmic Reticulum Stress, Extracellular Matrix Degradation, and Apoptosis

Intervertebral disc degeneration (IDD) is a major cause of low back pain (LBP), but there is still a lack of effective therapy. Multiple studies have reported that endoplasmic reticulum (ER) stress and extracellular matrix (ECM) degradation exert an enormous function on the occurrence and development of IDD. Autophagy can effectively repair ER stress and maintain ECM homeostasis. Eicosapentaenoic acid (EPA) can specifically induce autophagy. The purpose of this study is to demonstrate that EPA can promote autophagy, reduce ECM degradation and ER stress in vitro, thereby reducing cell apoptosis, and the protective effects of EPA in an IDD-rat model in vivo. Western blot and immunofluorescence were used to detect the autophagic flux, ER stress, ECM degradation, and apoptosis in nucleus pulposus cells (NPCs) treated by EPA. We also used puncture-induced IDD rats as experimental subjects to observe the therapeutic effect of EPA on IDD. Our findings indicated that EPA can effectively improve the autophagy activity in NPCs, inhibit the endoplasmic reticulum stress process, reduce the degree of cell apoptosis, and exert protective effects on the anabolism and catabolism of ECM. In addition, in vivo investigations demonstrated that EPA ameliorated the progression of puncture-induced IDD in rats. In conclusion, this study revealed the intrinsic mechanisms of EPA's protective role in NPCs and its potential therapeutic significance for the treatment of IDD.

Cryptotanshinone Inhibits the Growth of HCT116 Colorectal Cancer Cells Through Endoplasmic Reticulum Stress-Mediated Autophagy

Among cancers, colorectal cancer (CRC) has one of the highest annual incidence and death rates. Considering severe adverse reactions associated with classical chemotherapy medications, traditional Chinese medicines have become potential drug candidates. In the current study, the effects of cryptotanshinone (CPT), a major component of Salvia miltiorrhiza Bunge (Danshen) on CRC and underlying mechanism were explored. First of all, data from in vitro experiments and in vivo zebrafish models indicated that CPT selectively inhibited the growth and proliferation of HCT116 and SW620 cells while had little effect on SW480 cells. Secondly, both ER stress and autophagy were associated with CRC viability regulation. Interestingly, ER stress inhibitor and autophagy inhibitor merely alleviated cytotoxic effects on HCT116 cells in response to CPT stimulation, while have little effect on SW620 cells. The significance of apoptosis, autophagy and ER stress were verified by clinical data from CRC patients. In summary, the current study has revealed the anti-cancer effects of CPT in CRC by activating autophagy signaling mediated by ER stress. CPT is a promising drug candidate for CRC treatment.