IRE1α-XBP1s induces PDI expression to increase MTP activity for hepatic VLDL assembly and lipid homeostasis

Bacteroides uniformis-generated hexadecanedioic acid ameliorates metabolic-associated fatty liver disease

Gut microbiota exerts a pivotal influence on the development of Metabolic Associated Fatty Liver Disease (MAFLD), although the specific contributions of individual bacterial strains and their metabolites remain poorly defined. We conducted stool shotgun metagenomic sequencing and plasma untargeted metabolomics in a large prospective cohort comprising 120 MAFLD patients and 120 matched healthy controls. The mechanisms and microbial-derived metabolites involved in MAFLD were further investigated through multi-omics analyses in vitro and in vivo. Distinct differences were identified in both the microbial community structure and metabolomic profiles between MAFLD patients and healthy controls. Bacteroides uniformis (B. uniformis) was the most significantly depleted species in MAFLD and negatively correlated with hepatic steatosis and BMI. MAFLD was characterized by marked disruptions in fatty acid and amino acid metabolism. Combined analysis of metabolomic and metagenomic data achieved high diagnostic accuracy for MAFLD and hepatic steatosis severity (AUC = 0.93). Transplantation of fecal microbiota from MAFLD subjects into ABX mice led to the onset of MAFLD-like symptoms, whereas B. uniformis administration alleviate disease progression by inhibiting intestinal fat absorption, FFA from eWAT influx into liver via the gut-liver axis, and IRE1α-XBP1s-mediated flipogenesis and ferroptosis, as confirmed by hepatic transcriptomic and proteomic analyses. Hexadecanedioic acid (HDA), potentially identified as a key metabolite produced by B. uniformis, ameliorated MAFLD symptoms. Mechanistically, B. uniformis-derived HDA also inhibited fat absorption and transported, and entered the liver via the portal vein to suppress IRE1α-XBP1s-mediated flipogenesis and ferroptosis. B. uniformis and its potential putative metabolite HDA may contribute to MAFLD progression modulation, through regulation of the IRE1α-XBP1s axis. This study provides new insights into the gut-liver axis in MAFLD and offers promising therapeutic targets based on specific microbes and their metabolites.

Development of new genipin derivatives as potential NASH treatments: Design, synthesis and action mechanism

Nonalcoholic steatohepatitis (NASH) is a multifaceted liver disease. Endoplasmic reticulum stress (ERS), a key driver in NASH pathogenesis, triggers metabolic irregularities, liver steatosis, and inflammation. Genipin, an iridoid from the traditional Chinese medicine Gardenia jasminoides, has demonstrated significant effects against ERS. In the current work, 33 new genipin derivatives were designed and synthesized to evaluate their potential to treat NASH. Notably, G15 emerged as the most potent candidate, significantly attenuating lipid accumulation induced by free fatty acids (FFAs) in L-02 cells. Further investigation revealed that G15's mitigation of ERS was primarily achieved by suppressing the levels of inositol-requiring enzyme 1 (IRE1). Western blot analysis confirmed that G15 effectively down-regulated IRE1 protein expression and decreased the expression levels of its downstream X-box binding protein 1 (XBP1) and signal transducer and activator of transcription 3 (STAT3) proteins, thereby reducing cellular lipid accumulation. In addition, G15 treatment inhibited FFA-induced nitric oxide (NO) production in a concentration-dependent manner and suppressed the secretion of pro-inflammatory cytokines, including IL-1β, IL-6, and TNF-α. Collectively, these findings underscore that G15 has the potential to be a leading candidate for the treatment of NASH by down-regulating the IRE1/XBP1/STAT3 signaling pathway.

Therapeutic potential of fisetin in hepatic steatosis: Insights into autophagy pathway regulation and endoplasmic reticulum stress alleviation in high-fat diet-fed mice

Non-alcoholic fatty liver disease (NAFLD) is a common condition with limited FDA-approved treatments due to its complex pathogenesis. Metabolic stress-induced lipotoxicity triggers the unfolded protein response, leading to the development of NAFLD through inflammation and apoptosis. Moreover, metabolic dysregulation compromises autophagic capacity, impairing effective ERphagy and lipophagy in the liver. Fisetin (FSN), a flavonoid present in various fruits and vegetables, has demonstrated the ability to regulate the processes mentioned above and possesses a range of biological properties. In this study using a high-fat diet-induced NAFLD mouse model, treatment with FSN at a dosage of 80 mg/kg per day for eight weeks resulted in reduced hepatic lipid accumulation. This effect was mediated by modulating ER stress through enhancing autophagic activity, as indicated by decreased expression of GRP78, elf2a, ATF4, and CHOP genes, along with increased AMPK phosphorylation, decreased mTOR expression, and elevated levels of ULK1, ATG5, and Beclin1. Additionally, there was an increase in the LCII/LC3I ratio and a reduction in p62 levels in hepatic tissue. Our findings suggest that FSN exerts its effects by activating the AMPK/mTOR signaling pathway and its downstream targets, underscoring its potential therapeutic advantages in managing NAFLD by targeting autophagy and ER stress pathways.

Ceramide mediates cell-to-cell ER stress transmission by modulating membrane fluidity

Under endoplasmic reticulum (ER) stress (ERS), cells initiate the unfolded protein response (UPR) to maintain ER homeostasis. Recent studies revealed ERS transmission between cells and tissues, by activating the cell-nonautonomous UPR in cells that do not experience ERS directly. Here, we report that ERS triggers a rapid release of ceramide independent of the UPR, but requiring the acid sphingomyelinase activity. Carried by lipoproteins, ceramide is delivered to receiving cells to induce the UPR and regulate cell functions at multiple aspects, including lipid accumulation, cell death, and cytokine production. Mechanistically, extracellular ceramide stimulates ceramide synthesis at the transcription level in receiving cells, leading to ceramide accumulation in the ER so as to reduce membrane fluidity to disrupt ER calcium homeostasis, thus activating the UPR. Sphingomyelin counterbalanced the effect of ceramide. UPR induction is the frontline response to protect cells from ceramide insult. Our study suggests ceramide-mediated ERS transmission as a universal cell-cell communication model regulating a wide range of physiological events.

XBP1: A key regulator in breast cancer development and treatment

X-box binding protein 1 (XBP1), as a transcription factor, plays pivotal role in unfolded protein response (UPR), which is activated in response to endoplasmic reticulum (ER) stress to restore ER homeostasis. IRE1α/XBP1 pathway is a key component of UPR, and the expression levels of XBP1 can dictate the fate of cells under ER stress, either promoting survival or driving apoptosis. High expression of XBP1 in breast tumors is closely associated with poor prognosis. The paper elucidates the biological functions of XBP1 and its involvement in UPR, while also surveying the latest research on how XBP1 influences immunity, metabolism, apoptosis, angiogenesis, and the invasive and migratory behaviors of breast cancer cells. Moreover, it contemplates the potential of XBP1 as a therapeutic target for breast cancer treatment.

Endoplasmic reticulum stress: A key player in immune cell regulation and autoimmune disorders

The endoplasmic reticulum (ER) is a large organelle, found in all eukaryotes, that is essential for normal cellular function. This function encompasses protein folding and quality control, post-translational modifications, lipid regulation, and the storage of intracellular calcium, among others. These diverse processes are essential for maintaining proteome stability. Therefore, a robust surveillance system is established under stress to ensure cell homeostasis. Sources of stress can originate from the cellular environment, including nutrient deprivation, hypoxia, and low pH, as well as from endogenous signals within the cell, such as metabolic challenges and increased demands for protein production. When cellular homeostasis is altered by one of these triggers, ER primary functions are altered which leads to the accumulation of misfolded proteins. These impaired proteins trigger the activation of the Unfolded Protein Response (UPR) pathway. This response aims at reducing ER stress by implementing the induction of complex programs to restore cell homeostasis. However, extended ER stress can modify the UPR response, shifting its signals from promoting survival to triggering pathways that reprogram or eliminate affected cells.

Distinct yet Overlapping Functions of VMP1 and TMEM41B in Modulating Hepatic Lipoprotein Secretion and Autophagy in MASH

Background

Transmembrane protein 41B (TMEM41B) and vacuolar membrane protein 1 (VMP1) are endoplasmic reticulum (ER) transmembrane scramblase proteins that have been recently identified to have important roles in autophagy and hepatic lipoprotein secretion. While TMEM41B and VMP1 are structurally and functionally similar, the nature of their interactions and how they coordinately regulate hepatic lipoprotein secretion and autophagy in metabolic-associated steatotic liver disease (MASLD) and metabolic-associated steatohepatitis (MASH) remains unclear.

Methods

Liver-specific and hepatocyte-specific Tmem41b knockout (KO) mice as well as Tmem41b knock-in (KI) mice were generated from Tmem41b flox or Tmem41b KI mice by crossing with albumin-Cre mice or by injecting AAV8-TBG-cre, respectively. Lipid metabolism in these mice was characterized by lipidomic analyses. Mice with hepatic overexpression of TMEM41B that were fed a MASH diet were also characterized. To explore the relationship between TMEM41B and VMP1, Tmem41b/Vmp1 double KO (DKO), Tmem41b KO/ Vmp1 KI, and Vmp1 KO/ Tmem41b KI mice were generated, and steatosis and autophagy were characterized.

Results

The loss of hepatic Tmem41b severely impaired very low-density lipoprotein (VLDL) secretion, resulting in significant microsteatosis, increased hepatic triglycerides, inflammation, fibrosis, and ultimately the MASH development. TMEM41B protein was decreased in human MASLD livers. Overexpression of TMEM41B mitigated the effects of diet-induced MASLD. Mice lacking both Vmp1 and Tmem41b (DKO) showed further impairment in VLDL secretion compared to single Tmem41b KO, but were similar that of Vmp1 KO mice. Lipidomic analysis of liver tissues revealed decreased levels of phosphatidylcholine and phosphatidylethanolamine, along with increased neutral lipids. Cellular fractionation studies indicated that VMP1 and TMEM41B localize at the mitochondrial-associated membrane (MAM). Electron microscopy analysis showed reduced contact between mitochondria and the ER in hepatocytes deficient in either VMP1 or TMEM41B. The loss of hepatic VMP1 or TMEM41B led to markedly increased levels of LC3B-II and p62/SQSTM1, which were not further affected by double deletion of VMP1 and TMEM41B. Restoring VMP1 in Tmem41b KO mice partially improved defective VLDL secretion, though autophagy was only partially corrected by overexpression of VMP1 at a low but not high level. In contrast, restoring TMEM41B in Vmp1 KO mice dose-dependently improved both defective VLDL secretion and autophagy.

Conclusion

Loss of hepatic VMP1 or TMEM41B decreases MAM and phospholipid content and reduces VLDL secretion, resulting in the development of MASH. TMEM41B and VMP1 may have overlapping but distinct mechanisms in regulating lipoprotein secretion and autophagy.

Binding of Selected Ligands to Human Protein Disulfide Isomerase and Microsomal Triglyceride Transfer Protein Complex and the Associated Conformational Changes: A Computational Molecular Modelling Study

Human protein disulfide isomerase (PDI) is a multifunctional protein, and also serves as the β subunit of the human microsomal triglyceride transfer protein (MTP) complex, a lipid transfer machinery. Dysfunction of the MTP complex is associated with certain disease conditions such as abetalipoproteinemia and cardiovascular diseases. It is known that the functions of PDI or the MTP complex can be regulated by the binding of a small-molecule ligand to either of these two proteins. In the present study, the conformational changes of the MTP complex upon the binding of three selected small-molecule ligands (17β-estradiol, lomitapide and a phospholipid) are investigated based on the available biochemical and structural information by using the protein-ligand docking method and molecular dynamics (MD) simulation. The ligand-binding sites, the binding poses and binding strengths, the key binding site residues, and the ligand binding-induced conformational changes in the MTP complex are analyzed based on the MD trajectories. The open-to-closed or closed-to-open transitions of PDI is found to occur in both reduced and oxidized states of PDI and also independent of the presence or absence of small-molecule ligands. It is predicted that lomitapide and 1,2-diacyl-sn-glycero-3-phosphocholine (a phospholipid) can bind inside the lipid-binding pocket in the MTP complex with high affinities, whereas 17β-estradiol interacts with the lipid-binding pocket in addition to its binding to the interface region of the MTP complex. Additionally, lomitapide can bind to the b' domain of PDI as reported earlier for E2. Key residues for the ligand-binding interactions are identified in this study. It will be of interest to further explore whether the binding of small molecules can facilitate the conformational transitions of PDI in the future. The molecular and structural insights gained from the present work are of value for understanding some of the important biological functions of PDI and the MTP complex.

Unfolded protein responses: Dynamic machinery in wound healing

Skin wound healing is a dynamic process consisting of multiple cellular and molecular events that must be tightly coordinated to repair the injured tissue efficiently. The healing pace is decided by the type of injuries, the depth and size of the wounds, and whether wound infections occur. However, aging, comorbidities, genetic factors, hormones, and nutrition also impact healing outcomes. During wound healing, cells undergo robust processes of synthesizing new proteins and degrading multifunctional proteins. This imposes an increasing burden on the endoplasmic reticulum (ER), causing ER stress. Unfolded protein response (UPR) represents a collection of highly conserved stress signaling pathways originated from the ER to maintain protein homeostasis and modulate cell physiology. UPR is known to be beneficial for tissue healing. However, when excessive ER stress exceeds ER's folding potential, UPR pathways trigger cell apoptosis, interrupting tissue regeneration. Understanding how UPR pathways modulate the skin's response to injuries is critical for new interventions toward the control of acute and chronic wounds. Herein, in this review, we focus on the participation of the canonical and noncanonical UPR pathways during different stages of wound healing, summarize the available evidence demonstrating UPR's unique position in balancing homeostasis and pathophysiology of healing tissues, and highlight the understudied areas where therapeutic opportunities may arise.

Endoplasmic reticulum stress and unfolded protein response in renal lipid metabolism

The endoplasmic reticulum (ER) is a crucial cellular organelle involved in protein synthesis, folding, modification, and transport. Exposure to internal and external stressors can induce endoplasmic reticulum stress (ERS), leading to abnormal protein folding and ER malfunction. This stress can disrupt lipid synthesis, metabolism, and transport processes. Fatty acid oxidation is the primary energy source for the renal system. When energy intake exceeds the storage capacity of adipose tissue, lipids accumulate abnormally in non-adipose tissues, including kidneys, liver, and pancreas. Lipids accumulate in the kidneys of nearly all cell types, including thylakoid membranous, pedunculated, and proximal renal tubular epithelial cells. Intracellular free fatty acids can significantly disrupt renal lipid metabolism, contributing to ischemia-reperfusion acute kidney injury, diabetic nephropathy, renal fibrosis, and lupus nephritis. Consequently, this study delineated the primary signaling pathways and mechanisms of the ERS-induced unfolded protein response, explored the mechanistic link between ERS and lipid metabolism, and elucidated its role in renal lipid metabolism. This study aimed to offer new perspectives on managing and treating renal disorders.

Differential transport pathways of saturated and unsaturated fatty acid esters in male mouse hepatocytes

Saturated fatty acid (SFA) and unsaturated fatty acid (UFA) have distinct impacts on health. Whether SFA and UFA are differentially transported in liver remains elusive. Here, we find the secretion of UFA but not SFA esters is retarded in a male mouse hepatic endoplasmic reticulum (ER) stress model. Among 13 members of protein disulfide isomerase (PDI) family, only PDIA1 (PDI) deficiency leads to hepatosteatosis and hypolipidemia. In PDI-deficient male mouse liver, there is a severe accumulation but secretory blockade of UFA esters, whereas the accumulation and secretion of SFA esters remain normal. PDI catalyzes the oxidative folding of microsomal triglyceride transfer protein (MTP). In addition, PDI deficiency in hepatocytes abolishes Apolipoprotein B-100 (ApoB-100) very low-density lipoprotein (VLDL) secretion while maintaining partial ApoB-48 VLDL secretion. In summary, we find that the secretion of UFA esters is PDI-MTP indispensable, while SFA esters could be transferred out of liver via ApoB-48 VLDL through a PDI-MTP-independent pathway.

Roles of X-box binding protein 1 in liver pathogenesis

The prevalence of drug-induced liver injury (DILI) and viral liver infections presents significant challenges in modern healthcare and contributes to considerable morbidity and mortality worldwide. Concurrently, metabolic dysfunctionassociated steatotic liver disease (MASLD) has emerged as a major public health concern, reflecting the increasing rates of obesity and leading to more severe complications such as fibrosis and hepatocellular carcinoma. X-box binding protein 1 (XBP1) is a distinct transcription factor with a basic-region leucine zipper structure, whose activity is regulated by alternative splicing in response to disruptions in endoplasmic reticulum (ER) homeostasis and the unfolded protein response (UPR) activation. XBP1 interacts with a key signaling component of the highly conserved UPR and is critical in determining cell fate when responding to ER stress in liver diseases. This review aims to elucidate the emerging roles and molecular mechanisms of XBP1 in liver pathogenesis, focusing on its involvement in DILI, viral liver infections, MASLD, fibrosis/cirrhosis, and liver cancer. Understanding the multifaceted functions of XBP1 in these liver diseases offers insights into potential therapeutic strategies to restore ER homeostasis and mitigate liver damage.

A stress paradox: the dual role of the unfolded protein response in the placenta

The placenta is a temporary organ that forms during pregnancy and is essential for fetal development and maternal health. As an endocrine organ, proper placental function requires continual production, folding, and transport of proteins and lipids. Central to these processes is the endoplasmic reticulum (ER), a dynamic organelle responsible for maintaining cellular protein and lipid synthesis and processing. ER stress occurs when there is an accumulation of unfolded or misfolded proteins, which triggers the activation of cellular pathways collectively called the unfolded protein response. Unfolded protein response pathways act to alleviate the misfolded protein burden and restore ER homeostasis, or if unresolved, initiate cell death. While prolonged ER stress has been linked to deficient placental function and adverse pregnancy outcomes, basal activation of unfolded protein response pathways is required for placental development and function. This review explores the importance of ER homeostasis in placental development and function, examining how disruptions in ER stress responses may contribute to adverse pregnancy outcomes.

Zhimu-Huangbai herb-pair ameliorates hepatic steatosis in mice by regulating IRE1α/XBP1s pathway to inhibit SREBP-1c

BACKGROUND

Currently, there is a lack of validated pharmacological interventions for non-alcoholic fatty liver disease (NAFLD), which is characterized by the accumulation of hepatic triglyceride. Zhimu-Huangbai (ZH) herb-pair is a traditional Chinese medicine that regulates glucose and lipid metabolism disorders. However, the precise mechanisms underlying the preventive effects of hepatic triglyceride induced by high-fat diet (HFD) remain elusive.

PURPOSE

The study aimed to examine the impact of ZH herb-pair on NAFLD in mice and explore the underlying mechanisms, particularly its effects on endoplasmic reticulum (ER) stress and lipid metabolism.

METHODS

NAFLD was induced in mice using HFD, and the treated mice were orally administered ZH, metformin (Glucophage) or lovastatin. The lipid metabolism factors, ER stress markers, and the unfolded protein response (UPR) branch factors were measured using immunohistochemistry, western blotting or qRT-PCR. Co-Immunoprecipitation (CoIP) was performed to reveal the connection between SCAP and SREBP-1c. Tunicamycin (TM) and plasmid delivery were used to induce acute ER stress or crease XBP1 gain function models. The main compounds in ZH binding to IRE1α protein were studied by molecular docking and cellular thermal shift assay (CETSA).

RESULTS

Treatment with ZH significantly ameliorated hepatic steatosis and reduced lipid synthesis process mainly inhibiting the expression of mature active form of SREBP-1c through relieving ER stress. The expression of IRE1α and XBP1s was inhibited after treatment with ZH. In addition, ZH improved the fatty liver phenotype caused by XBP1 overexpression via decreasing srebp1c transcription. In vitro experimental results suggested that the main compounds in ZH decreased cellular TG contents. Mechanistically, ZH targeted IRE1α and inhibited XBP1s mRNA expression to relieve ER stress and inhibit SREBP-1c production.

CONCLUSIONS

ZH herb-pair can protect against NAFLD by reducing the expression of SREBP-1c, in part, via regulating IRE1α/XBP1s pathway.

Insulin-degrading enzyme inhibition increases the unfolded protein response and favours lipid accumulation in the liver

BACKGROUND AND PURPOSE

Nonalcoholic fatty liver disease refers to liver pathologies, ranging from steatosis to steatohepatitis, with fibrosis ultimately leading to cirrhosis and hepatocellular carcinoma. Although several mechanisms have been suggested, including insulin resistance, oxidative stress, and inflammation, its pathophysiology remains imperfectly understood. Over the last decade, a dysfunctional unfolded protein response (UPR) triggered by endoplasmic reticulum (ER) stress emerged as one of the multiple driving factors. In parallel, growing evidence suggests that insulin-degrading enzyme (IDE), a highly conserved and ubiquitously expressed metallo-endopeptidase originally discovered for its role in insulin decay, may regulate ER stress and UPR.

EXPERIMENTAL APPROACH

We investigated, by genetic and pharmacological approaches, in vitro and in vivo, whether IDE modulates ER stress-induced UPR and lipid accumulation in the liver.

KEY RESULTS

We found that IDE-deficient mice display higher hepatic triglyceride content along with higher inositol-requiring enzyme 1 (IRE1) pathway activation. Upon induction of ER stress by tunicamycin or palmitate in vitro or in vivo, pharmacological inhibition of IDE, using its inhibitor BDM44768, mainly exacerbated ER stress-induced IRE1 activation and promoted lipid accumulation in hepatocytes, effects that were abolished by the IRE1 inhibitors 4μ8c and KIRA6. Finally, we identified that IDE knockout promotes lipolysis in adipose tissue and increases hepatic CD36 expression, which may contribute to steatosis.

CONCLUSION AND IMPLICATIONS

These results unravel a novel role for IDE in the regulation of ER stress and development of hepatic steatosis. These findings pave the way to innovative strategies modulating IDE to treat metabolic diseases.

The lipid side of unfolded protein response

Although our current knowledge of the molecular crosstalk between the ER stress, the unfolded protein response (UPR), and lipid homeostasis remains limited, there is increasing evidence that dysregulation of either protein or lipid homeostasis profoundly affects the other. Most research regarding UPR signaling in human diseases has focused on the causes and consequences of disrupted protein folding. The UPR itself consists of very complex pathways that function to not only maintain protein homeostasis, but just as importantly, modulate lipid biogenesis to allow the ER to adjust and promote cell survival. Lipid dysregulation is known to activate many aspects of the UPR, but the complexity of this crosstalk remains a major research barrier. ER lipid disequilibrium and lipotoxicity are known to be important contributors to numerous human pathologies, including insulin resistance, liver disease, cardiovascular diseases, neurodegenerative diseases, and cancer. Despite their medical significance and continuous research, however, the molecular mechanisms that modulate lipid synthesis during ER stress conditions, and their impact on cell fate decisions, remain poorly understood. Here we summarize the current view on crosstalk and connections between altered lipid metabolism, ER stress, and the UPR.

Regulation mechanism of endoplasmic reticulum stress on metabolic enzymes in liver diseases

The endoplasmic reticulum (ER) plays a pivotal role in protein folding and secretion, Ca2+ storage, and lipid synthesis in eukaryotic cells. When the burden of protein synthesis and folding required to be handled exceeds the processing capacity of the ER, the accumulation of misfolded/unfolded proteins triggers ER stress. In response to short-term ER stress, the unfolded protein response (UPR) is activated to allow cells to survive. When ER stress is severe and sustained, it typically provokes cell death through multiple approaches. It is well documented that ER stress and metabolic deregulation are functionally intertwined, both are considered contributing factors to the pathogenesis of liver diseases, including non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease (ALD), ischemia/reperfusion (I/R) injury, viral hepatitis, liver fibrosis, and hepatocellular carcinoma (HCC). Hepatocytes are rich in smooth and rough ER, which harbor metabolic enzymes that are capable of sensing alterations in various nutritional status and external stimuli. Extensive research has focused on the molecular mechanism linking ER stress with metabolic enzymes. The purpose of this review is to summarize the current knowledge regarding the effects of ER stress on metabolic enzymes in various liver diseases and to provide potential therapeutic strategies for chronic liver diseases via targeting UPR.

Comparative effects of lifelong moderate-intensity continuous training and high-intensity interval training on blood lipid levels and mental well-being in naturally ageing mice

OBJECTIVE

This study aimed to investigate the impact of lifelong exercise, including both moderate-intensity continuous training and high-intensity interval training, on blood lipid levels and mental behaviour in naturally ageing mice to identify effective exercise strategies for ageing-related health issues.

METHODS

Six-week-old male BALB/c mice were randomly assigned to one of four groups: young control (YC), natural ageing control (OC), lifelong moderate-intensity continuous exercise (EM), and lifelong high-intensity interval exercise (EH) groups. The EM group was trained at a speed corresponding to 70 % of the maximum running speed, while the EH group was trained at a running speed alternating between 50 % of the maximum running speed, 70 % of the maximum running speed, and 90 % of the maximum running speed. All exercise sessions were conducted three times per week, with each session lasting 50 min. Behavioural tests and blood sample collection were conducted at 72 weeks of age.

RESULTS

Ageing in mice led to changes in muscle and fat mass. Both the EM and EH groups showed greater muscle mass and lower fat mass than did the OC group. Ageing was associated with elevated anxiety (fewer open arm entries, time spent in the central region) and depression (lower sucrose preference) indicators. However, these changes were reversed in both exercise groups, with no differences between the two exercise groups. Blood lipid levels, including total cholesterol (TC), total triglycerides (TGs), low-density lipoprotein (LDL), and free fatty acid (FFA) levels, were greater in the OC group than in the YC group. Additionally, the OC group exhibited lower high-density lipoprotein (HDL) levels. However, both the EM and EH groups exhibited improved lipid profiles compared to those of the YC group.

CONCLUSION

Lifelong exercise, whether moderate-intensity continuous or high-intensity interval training, can preserve body health during ageing, prevent anxiety and depression, and maintain stable blood lipid levels. Both exercise types are equally effective, suggesting that exercise intensity may not be the critical factor underlying these beneficial adaptations.

Endoplasmic reticulum stress in diseases

The endoplasmic reticulum (ER) is a key organelle in eukaryotic cells, responsible for a wide range of vital functions, including the modification, folding, and trafficking of proteins, as well as the biosynthesis of lipids and the maintenance of intracellular calcium homeostasis. A variety of factors can disrupt the function of the ER, leading to the aggregation of unfolded and misfolded proteins within its confines and the induction of ER stress. A conserved cascade of signaling events known as the unfolded protein response (UPR) has evolved to relieve the burden within the ER and restore ER homeostasis. However, these processes can culminate in cell death while ER stress is sustained over an extended period and at elevated levels. This review summarizes the potential role of ER stress and the UPR in determining cell fate and function in various diseases, including cardiovascular diseases, neurodegenerative diseases, metabolic diseases, autoimmune diseases, fibrotic diseases, viral infections, and cancer. It also puts forward that the manipulation of this intricate signaling pathway may represent a novel target for drug discovery and innovative therapeutic strategies in the context of human diseases.

Pemafibrate abrogates SLD in a rat experimental dietary model, inducing a shift in fecal bile acids and microbiota composition

BACKGROUND AND AIMS

Drugs resolving steatotic liver disease (SLD) could prevent the evolution of metabolic dysfunction associated SLD (MASLD) to more aggressive forms but must show not only efficacy, but also a high safety profile. Repurposing of drugs in clinical use, such as pemafibrate and mirabegron, could facilitate the finding of an effective and safe drug-treatment for SLD.

APPROACH AND RESULTS

The SLD High Fat High Fructose (HFHFr) rat model develops steatosis without the influence of other metabolic disturbances, such as obesity, inflammation, or type 2 diabetes. Further, liver fatty acids are provided, as in human pathology, both from dietary origin and de novo lipid synthesis. We used the HFHFr model to evaluate the efficacy of pemafibrate and mirabegron, alone or in combination, in the resolution of SLD, analyzing zoometric, biochemical, histological, transcriptomic, fecal metabolomic and microbiome data. We provide evidence showing that pemafibrate, but not mirabegron, completely reverted liver steatosis, due to a direct effect on liver PPARα-driven fatty acid catabolism, without changes in total energy consumption, subcutaneous, perigonadal and brown fat, blood lipids and body weight. Moreover, pemafibrate treatment showed a neutral effect on whole-body glucose metabolism, but deeply modified fecal bile acid composition and microbiota.

CONCLUSIONS

Pemafibrate administration reverts liver steatosis in the HFHFr dietary rat SLD model without altering parameters related to metabolic or organ toxicity. Our results strongly support further clinical research to reposition pemafibrate for the treatment of SLD/MASLD.

Protein disulfide isomerase is essential for spermatogenesis in mice

Spermatogenesis requires precise posttranslational control in the endoplasmic reticulum (ER), but the mechanism remains largely unknown. The protein disulfide isomerase (PDI) family is a group of thiol oxidoreductases responsible for catalyzing the disulfide bond formation of nascent proteins. In this study, we generated 14 strains of KO mice lacking the PDI family enzymes and found that only PDI deficiency caused spermatogenesis defects. Both inducible whole-body PDI-KO (UBC-Cre/Pdifl/fl) mice and premeiotic PDI-KO (Stra8-Cre/Pdifl/fl) mice experienced a significant decrease in germ cells, testicular atrophy, oligospermia, and complete male infertility. Stra8-Cre/Pdifl/fl spermatocytes had significantly upregulated ER stress-related proteins (GRP78 and XBP1) and apoptosis-related proteins (Cleaved caspase-3 and BAX), together with cell apoptosis. PDI deletion led to delayed DNA double-strand break repair and improper crossover at the pachytene spermatocytes. Quantitative mass spectrometry indicated that PDI deficiency downregulated vital proteins in spermatogenesis such as HSPA4L, SHCBP1L, and DDX4, consistent with the proteins' physical association with PDI in normal testes tissue. Furthermore, PDI served as a thiol oxidase for disulfide bond formation of SHCBP1L. Thus, PDI plays an essential role in protein quality control for spermatogenesis in mice.

Crosstalk between protein misfolding and endoplasmic reticulum stress during ageing and their role in age-related disorders

Maintaining the proteome is crucial to retaining cell functionality and response to multiple intrinsic and extrinsic stressors. Protein misfolding increased the endoplasmic reticulum (ER) stress and activated the adaptive unfolded protein response (UPR) to restore cell homeostasis. Apoptosis occurs when ER stress is prolonged or the adaptive response fails. In healthy young cells, the ratio of protein folding machinery to quantities of misfolded proteins is balanced under normal circumstances. However, the age-related deterioration of the complex systems for handling protein misfolding is accompanied by ageing-related disruption of protein homeostasis, which results in the build-up of misfolded and aggregated proteins. This ultimately results in decreased cell viability and forms the basis of common age-related diseases called protein misfolding diseases. Proteins or protein fragments convert from their ordinarily soluble forms to insoluble fibrils or plaques in many of these disorders, which build up in various organs such as the liver, brain, or spleen. Alzheimer's, Parkinson's, type II diabetes, and cancer are diseases in this group commonly manifest in later life. Thus, protein misfolding and its prevention by chaperones and different degradation paths are becoming understood from molecular perspectives. Proteodynamics information will likely affect future interventional techniques to combat cellular stress and support healthy ageing by avoiding and treating protein conformational disorders. This review provides an overview of the diverse proteostasis machinery, protein misfolding, and ER stress involvement, which activates the UPR sensors. Here, we will discuss the crosstalk between protein misfolding and ER stress and their role in developing age-related diseases.

Dietary conjugated linoleic acid downregulates the AlCl3-induced hyperactivation of compensatory and maladaptive signalling in the mouse brain cortex

Oxidative stress, hyperactivation of compensatory mechanisms (unfolded protein response, UPR; nuclear factor erythroid 2-related factor 2, Nrf2) and the stimulation of maladaptive response (inflammation/apoptosis) are interconnected pathogenic processes occurring during Alzheimer's disease (AD) progression. The neuroprotective ability of dietary Conjugated linoleic acid (CLAmix) in a mouse model of AlCl3-induced AD was recently described but, the effects of AlCl3 or CLAmix intake on these pathogenic processes are still unknown. The effects of dietary AlCl3 or CLAmix - alone and in combination - were examined in the brain cortex of twenty-eight BalbC mice divided into 4 groups (n = 7 each). The neurotoxic effects of AlCl3 were investigated in animals treated for 5 weeks with 100 mg/kg/day (AL). CLAmix supplementation (600 mg/kg bw/day) for 7 weeks (CLA) was aimed at evaluating its modulatory effects on the Nrf2 pathway while its co-treatment with AlCl3 during the last 5 weeks of CLAmix intake (CLA + AL) was used to investigate its neuroprotective ability. Untreated mice were used as controls. In the CLA group, the NADPH oxidase (NOX) activation in the brain cortex was accompanied by the modulation of the Nrf2 pathway. By contrast, in the AL mice, the significant upregulation of oxidative stress markers, compensatory pathways (UPR/Nrf2), proinflammatory cytokines (IL-6, TNFα) and the proapoptotic protein Bax levels were found as compared with control. Notably, in CLA + AL mice, the marked decrease of oxidative stress, UPR/Nrf2 markers and proinflammatory cytokines levels were associated with the significant increase of the antiapoptotic protein Bcl2. The involvement of NOX in the adaptive response elicited by CLAmix along with its protective effects against the onset of several pathogenic processes triggered by AlCl3, broadens the knowledge of the mechanism underlying the pleiotropic activity of Nrf2 activators and sheds new light on their potential therapeutic use against neurodegenerative disorders.

Conditional hepatocyte ablation of PDIA1 uncovers indispensable roles in both APOB and MTTP folding to support VLDL secretion

OBJECTIVES

The assembly and secretion of hepatic very low-density lipoprotein (VLDL) plays pivotal roles in hepatic and plasma lipid homeostasis. Protein disulfide isomerase A1 (PDIA1/P4HB) is a molecular chaperone whose functions are essential for protein folding in the endoplasmic reticulum. Here we investigated the physiological requirement in vivo for PDIA1 in maintaining VLDL assembly and secretion.

METHODS

Pdia1/P4hb was conditionally deleted in adult mouse hepatocytes and the phenotypes characterized. Mechanistic analyses in primary hepatocytes determined how PDIA1 ablation alters MTTP synthesis and degradation as well as altering synthesis and secretion of Apolipoprotein B (APOB), along with complementary expression of intact PDIA1 vs a catalytically inactivated PDIA1 mutant.

RESULTS

Hepatocyte-specific deletion of Pdia1/P4hb inhibited hepatic MTTP expression and dramatically reduced VLDL production, leading to severe hepatic steatosis and hypolipidemia. Pdia1-deletion did not affect mRNA expression or protein stability of MTTP but rather prevented Mttp mRNA translation. We demonstrate an essential role for PDIA1 in MTTP synthesis and function and show that PDIA1 interacts with APOB in an MTTP-independent manner via its molecular chaperone function to support APOB folding and secretion.

CONCLUSIONS

PDIA1 plays indispensable roles in APOB folding, MTTP synthesis and activity to support VLDL assembly. Thus, like APOB and MTTP, PDIA1 is an obligatory component of hepatic VLDL production.

Endoplasmic Reticulum Stress and Its Impact on Adipogenesis: Molecular Mechanisms Implicated

Endoplasmic reticulum (ER) stress plays a pivotal role in adipogenesis, which encompasses the differentiation of adipocytes and lipid accumulation. Sustained ER stress has the potential to disrupt the signaling of the unfolded protein response (UPR), thereby influencing adipogenesis. This comprehensive review illuminates the molecular mechanisms that underpin the interplay between ER stress and adipogenesis. We delve into the dysregulation of UPR pathways, namely, IRE1-XBP1, PERK and ATF6 in relation to adipocyte differentiation, lipid metabolism, and tissue inflammation. Moreover, we scrutinize how ER stress impacts key adipogenic transcription factors such as proliferator-activated receptor γ (PPARγ) and CCAAT-enhancer-binding proteins (C/EBPs) along with their interaction with other signaling pathways. The cellular ramifications include alterations in lipid metabolism, dysregulation of adipokines, and aged adipose tissue inflammation. We also discuss the potential roles the molecular chaperones cyclophilin A and cyclophilin B play in adipogenesis. By shedding light on the intricate relationship between ER stress and adipogenesis, this review paves the way for devising innovative therapeutic interventions.

The Role of Endoplasmic Reticulum in Lipotoxicity during Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) Pathogenesis

Perturbations in lipid and protein homeostasis induce endoplasmic reticulum (ER) stress in metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as nonalcoholic fatty liver disease. Lipotoxic and proteotoxic stress can activate the unfolded protein response (UPR) transducers: inositol requiring enzyme1α, PKR-like ER kinase, and activating transcription factor 6α. Collectively, these pathways induce expression of genes that encode functions to resolve the protein folding defect and ER stress by increasing the protein folding capacity of the ER and degradation of misfolded proteins. The ER is also intimately connected with lipid metabolism, including de novo ceramide synthesis, phospholipid and cholesterol synthesis, and lipid droplet formation. Following their activation, the UPR transducers also regulate lipogenic pathways in the liver. With persistent ER stress, cellular adaptation fails, resulting in hepatocyte apoptosis, a pathological marker of liver disease. In addition to the ER-nucleus signaling activated by the UPR, the ER can interact with other organelles via membrane contact sites. Modulating intracellular communication between ER and endosomes, lipid droplets, and mitochondria to restore ER homeostasis could have therapeutic efficacy in ameliorating liver disease. Recent studies have also demonstrated that cells can convey ER stress by the release of extracellular vesicles. This review discusses lipotoxic ER stress and the central role of the ER in communicating ER stress to other intracellular organelles in MASLD pathogenesis.

Endoplasmic Reticulum Stress and Metabolism in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer, accounting for 85% to 90% of all liver cancer cases. It is a hepatocyte-derived primary tumor, causing 550,000 deaths per year, ranking it as one of the most common cancers worldwide. The liver is a highly metabolic organ with multiple functions, including digestion, detoxification, breakdown of fats, and production of bile and cholesterol, in addition to storage of vitamins, glycogen, and minerals, and synthesizing plasma proteins and clotting factors. Due to these fundamental and diverse functions, the malignant transformation of hepatic cells can have a severe impact on the liver's metabolism. Furthermore, tumorigenesis is often accompanied by activation of the endoplasmic reticulum (ER) stress pathways, which are known to be highly intertwined with several metabolic pathways. Because HCC is characterized by changes in the metabolome and by an aberrant activation of the ER stress pathways, the aim of this review was to summarize the current knowledge that links ER stress and metabolism in HCC, thereby focusing on potential therapeutic targets.

More than meets the eye in Parkinson's disease and other synucleinopathies: from proteinopathy to lipidopathy

The accumulation of proteinaceous inclusions in the brain is a common feature among neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease (PD), and dementia with Lewy bodies (DLB). The main neuropathological hallmark of PD and DLB are inclusions, known as Lewy bodies (LBs), enriched not only in α-synuclein (aSyn), but also in lipid species, organelles, membranes, and even nucleic acids. Furthermore, several genetic risk factors for PD are mutations in genes involved in lipid metabolism, such as GBA1, VSP35, or PINK1. Thus, it is not surprising that mechanisms that have been implicated in PD, such as inflammation, altered intracellular and vesicular trafficking, mitochondrial dysfunction, and alterations in the protein degradation systems, may be also directly or indirectly connected through lipid homeostasis. In this review, we highlight and discuss the recent evidence that suggests lipid biology as important drivers of PD, and which require renovated attention by neuropathologists. Particularly, we address the implication of lipids in aSyn accumulation and in the spreading of aSyn pathology, in mitochondrial dysfunction, and in ER stress. Together, this suggests we should broaden the view of PD not only as a proteinopathy but also as a lipidopathy.

A Metabolic Enhancer Protects against Diet-Induced Obesity and Liver Steatosis and Corrects a Pro-Atherogenic Serum Profile in Mice

OBJECTIVE

Metabolic Syndrome (MetS) affects hundreds of millions of individuals and constitutes a major cause of morbidity and mortality worldwide. Obesity is believed to be at the core of metabolic abnormalities associated with MetS, including dyslipidemia, insulin resistance, fatty liver disease and vascular dysfunction. Although previous studies demonstrate a diverse array of naturally occurring antioxidants that attenuate several manifestations of MetS, little is known about the (i) combined effect of these compounds on hepatic health and (ii) molecular mechanisms responsible for their effect.

METHODS

We explored the impact of a metabolic enhancer (ME), consisting of 7 naturally occurring antioxidants and mitochondrial enhancing agents, on diet-induced obesity, hepatic steatosis and atherogenic serum profile in mice.

RESULTS

Here we show that a diet-based ME supplementation and exercise have similar beneficial effects on adiposity and hepatic steatosis in mice. Mechanistically, ME reduced hepatic ER stress, fibrosis, apoptosis, and inflammation, thereby improving overall liver health. Furthermore, we demonstrated that ME improved HFD-induced pro-atherogenic serum profile in mice, similar to exercise. The protective effects of ME were reduced in proprotein convertase subtilisin/kexin 9 (PCSK9) knock out mice, suggesting that ME exerts it protective effect partly in a PCSK9-dependent manner.

CONCLUSIONS

Our findings suggest that components of the ME have a positive, protective effect on obesity, hepatic steatosis and cardiovascular risk and that they show similar effects as exercise training.

β-sitosterol attenuates high- fat diet-induced hepatic steatosis in rats by modulating lipid metabolism, inflammation and ER stress pathway

Nonalcoholic fatty liver disease (NAFLD) is the most prevalent chronic hepatic disorder. The naturally occurring phytosterol; β-sitosterol has antiobesogenic and anti-diabetic properties. The purpose of this study was to explore the role of β-sitosterol in preventing hepatic steatosis induced by a high-fat diet (HFD) in rats. In the current study, to induce NAFLD in the female Wister rats, an HFD was administered to them for 8 weeks. The pathogenic severity of steatosis in rats receiving an HFD diet was dramatically decreased by oral administration of β-sitosterol. After administering β-sitosterol to HFD-induced steatosis for three weeks, several oxidative stress-related markers were then assessed. We showed that β-sitosterol reduced steatosis and the serum levels of triglycerides, transaminases (ALT and AST) and inflammatory markers (IL-1β and iNOS) compared to HFD-fed rats. Additionally, β-sitosterol reduced endoplasmic reticulum stress by preventing the overexpression of inositol-requiring enzyme-1 (IRE-1α), X-box binding protein 1(sXBP1) and C/EBP homologous protein (CHOP) genes which, showing a function in the homeostatic regulation of protein folding. Also, it was found that the expression of the lipogenic factors; peroxisome proliferator-activated receptor (PPAR-α), sterol regulatory element binding protein (SREBP-1c) and carnitine palmitoyltransferase-1(CPT-1), which are involved in the regulation of the fatty acid oxidation process, may be regulated by β-sitosterol. It can be concluded that β-sitosterol may prevent NAFLD by reducing oxidative stress, endoplasmic reticulum stress and inflammatory responses, which supports the possibility of using β-sitosterol as an alternative therapy for NAFLD. Together, β-sitosterol may be an option for NAFLD prevention.

The endoplasmic reticulum stress sensor IRE1 regulates collagen secretion through the enforcement of the proteostasis factor P4HB/PDIA1 contributing to liver damage and fibrosis

Collagen is one the most abundant proteins and the main cargo of the secretory pathway, contributing to hepatic fibrosis and cirrhosis due to excessive deposition of extracellular matrix. Here we investigated the possible contribution of the unfolded protein response, the main adaptive pathway that monitors and adjusts the protein production capacity at the endoplasmic reticulum, to collagen biogenesis and liver disease. Genetic ablation of the ER stress sensor IRE1 reduced liver damage and diminished collagen deposition in models of liver fibrosis triggered by carbon tetrachloride (CCl ₄ ) administration or by high fat diet. Proteomic and transcriptomic profiling identified the prolyl 4-hydroxylase (P4HB, also known as PDIA1), which is known to be critical for collagen maturation, as a major IRE1-induced gene. Cell culture studies demonstrated that IRE1 deficiency results in collagen retention at the ER and altered secretion, a phenotype rescued by P4HB overexpression. Taken together, our results collectively establish a role of the IRE1/P4HB axis in the regulation of collagen production and its significance in the pathogenesis of various disease states.

RASAL2 Deficiency Attenuates Hepatic Steatosis by Promoting Hepatic VLDL Secretion via the AKT/TET1/MTTP Axis

BACKGROUND AND AIMS

RAS protein activator like 2 (RASAL2) is a newly discovered metabolic regulator involved in energy homeostasis and adipogenesis. However, whether RASAL2 is involved in hepatic lipid metabolism remains undetermined. This study explored the function of RASAL2 and elucidated its potential mechanisms in nonalcoholic fatty liver disease (NAFLD).

METHODS

NAFLD models were established either by feeding mice a high-fat diet or by incubation of hepatocytes with 1 mM free fatty acids (oleic acid:palmitic acid=2:1). Pathological changes were observed by hematoxylin and eosin staining. Lipid accumulation was assessed by Oil Red O staining, BODIPY493/503 staining, and triglyceride quantification. The in vivo secretion rate of very low-density lipoprotein was determined by intravenous injection of tyloxapol. Gene regulation was analyzed by chromatin immunoprecipitation assays and hydroxymethylated DNA immunoprecipitation combined with real-time polymerase chain reaction.

RESULTS

RASAL2 deficiency ameliorated hepatic steatosis both in vivo and in vitro. Mechanistically, RASAL2 deficiency upregulated hepatic TET1 expression by activating the AKT signaling pathway and thereby promoted MTTP expression by DNA hydroxymethylation, leading to increased production and secretion of very low-density lipoprotein, which is the major carrier of triglycerides exported from the liver to distal tissues.

CONCLUSIONS

Our study reports the first evidence that RASAL2 deficiency ameliorates hepatic steatosis by regulating lipid metabolism through the AKT/TET1/MTTP axis. These findings will help understand the pathogenesis of NAFLD and highlight the potency of RASAL2 as a new molecular target for NAFLD.

PDIA4, a novel ER stress chaperone, modulates adiponectin expression and inflammation in adipose tissue

Increasing evidence supporting a causal link between obesity and endoplasmic reticulum (ER) stress in adipose tissue is being reported. Protein disulfide isomerase 4 (PDIA4) is a novel ER chaperone involved in the pancreatic β-cells pathogenesis in diabetes. However, the role of PDIA4 in obesity progression remains poorly understood. To assess the relationship between PDIA4, adiponectin, and metformin, we used the palmitate-induced inflammation in hypertrophic adipocytes and the high-fat diet-induced obesity mouse model. Our results revealed that palmitate-induced hypertrophic adipocytes exhibit obesity-associated conditions such as increased lipid accumulation, inflammation, and reduced glucose uptake. Pharmacological and genetic inhibition of PDIA4 significantly reverses these obesity-associated conditions in adipocytes. PDIA4 mechanistically promotes obesity progression via adiponectin downregulation. Furthermore, metformin modulates PDIA4 and adiponectin expression and improves obesity-associated conditions in both in vitro adipocytes and in vivo mouse models. Serum PDIA4 concentrations are also associated with body mass index, adiponectin, triglycerides, and inflammatory cytokines in humans. This is the first study demonstrating that PDIA4 modulates adipocytes by downregulating adiponectin. Moreover, metformin may serve as a potential therapeutic for preventing obesity via PDIA4-targeting.

Lack of VMP1 impairs hepatic lipoprotein secretion and promotes non-alcoholic steatohepatitis

BACKGROUND & AIMS

Vacuole membrane protein 1 (VMP1) is an endoplasmic reticulum (ER) transmembrane protein that regulates the formation of autophagosomes and lipid droplets. Recent evidence suggests that VMP1 plays a critical role in lipoprotein secretion in zebra fish and cultured cells. However, the pathophysiological roles and mechanisms by which VMP1 regulates lipoprotein secretion and lipid accumulation in non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are unknown.

METHODS

Liver-specific and hepatocyte-specific Vmp1 knockout mice as well as Vmp1 knock-in mice were generated by crossing Vmp1flox or Vmp1KI mice with albumin-Cre mice or by injecting AAV8-TBG-cre, respectively. Lipid and energy metabolism in these mice were characterized by metabolomic and transcriptome analyses. Mice with hepatic overexpression of VMP1 who were fed a NASH diet were also characterized.

RESULTS

Hepatocyte-specific deletion of Vmp1 severely impaired VLDL secretion resulting in massive hepatic steatosis, hepatocyte death, inflammation and fibrosis, which are hallmarks of NASH. Mechanistically, loss of Vmp1 led to decreased hepatic levels of phosphatidylcholine and phosphatidylethanolamine as well as to changes in phospholipid composition. Deletion of Vmp1 in mouse liver also led to the accumulation of neutral lipids in the ER bilayer and impaired mitochondrial beta-oxidation. Overexpression of VMP1 ameliorated steatosis in diet-induced NASH by improving VLDL secretion. Importantly, we also showed that decreased liver VMP1 is associated with NAFLD/NASH in humans.

CONCLUSIONS

Our results provide novel insights on the role of VMP1 in regulating hepatic phospholipid synthesis and lipoprotein secretion in the pathogenesis of NAFLD/NASH.

LAY SUMMARY

Non-alcoholic fatty liver disease and its more severe form, non-alcoholic steatohepatitis, are associated with a build-up of fat in the liver (steatosis). However, the exact mechanisms that underly steatosis in patients are not completely understood. Herein, the authors identified that the lack of a protein called VMP1 impairs the secretion and metabolism of fats in the liver and could therefore contribute to the development and progression of non-alcoholic fatty liver disease.

Metabolic Injury of Hepatocytes Promotes Progression of NAFLD and AALD

Nonalcoholic liver disease is a component of metabolic syndrome associated with obesity, insulin resistance, and hyperlipidemia. Excessive alcohol consumption may accelerate the progression of steatosis, steatohepatitis, and fibrosis. While simple steatosis is considered a benign condition, nonalcoholic steatohepatitis with inflammation and fibrosis may progress to cirrhosis, liver failure, and hepatocellular cancer. Studies in rodent experimental models and primary cell cultures have demonstrated several common cellular and molecular mechanisms in the pathogenesis and regression of liver fibrosis. Chronic injury and death of hepatocytes cause the recruitment of myeloid cells, secretion of inflammatory and fibrogenic cytokines, and activation of myofibroblasts, resulting in liver fibrosis. In this review, we discuss the role of metabolically injured hepatocytes in the pathogenesis of nonalcoholic steatohepatitis and alcohol-associated liver disease. Specifically, the role of chemokine production and de novo lipogenesis in the development of steatotic hepatocytes and the pathways of steatosis regulation are discussed.

Endoplasmic reticulum stress in innate immune cells - a significant contribution to non-alcoholic fatty liver disease

Liver disease and its complications affect millions of people worldwide. NAFLD (non-alcoholic fatty liver disease) is the liver disease associated with metabolic dysfunction and consists of four stages: steatosis with or without mild inflammation (NAFLD), non-alcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. With increased necroinflammation and progression of liver fibrosis, NAFLD may progress to cirrhosis or even hepatocellular carcinoma. Although the underlying mechanisms have not been clearly elucidated in detail, what is clear is that complex immune responses are involved in the pathogenesis of NASH, activation of the innate immune system is critically involved in triggering and amplifying hepatic inflammation and fibrosis in NAFLD/NASH. Additionally, disruption of endoplasmic reticulum (ER) homeostasis in cells, also known as ER stress, triggers the unfolded protein response (UPR) which has been shown to be involved to inflammation and apoptosis. To further develop the prevention and treatment of NAFLD/NASH, it is imperative to clarify the relationship between NAFLD/NASH and innate immune cells and ER stress. As such, this review focuses on innate immune cells and their ER stress in the occurrence of NAFLD and the progression of cirrhosis.

The role of the immune system in the pathogenesis of NAFLD and potential therapeutic impacts of mesenchymal stem cell-derived extracellular vesicles

Non-Alcoholic Fatty Liver Disease (NAFLD) is characterized by intra-hepatocyte triglyceride accumulation and concomitant involvement of the immune system with subsequent histological changes, tissue damage, and clinical findings. There are various molecular pathways involved in the progression of NAFLD including lipotoxicity, endoplasmic reticulum stress, and the immune response. Both innate and adaptive immune systems are involved in the NAFLD pathogenesis, and crosstalk between the immune cells and liver cells participates in its initiation and progression. Among the various treatments for this disease, new cell based therapies have been proposed. Extracellular vesicles (EVs) derived from mesenchymal stem cells (MSC) (MSC-EVs) are new cell-free vehicles with low immunogenicity, which can suppress detrimental immune responses in inflamed tissues. This review aimed to express the immune system's molecular pathways associated with the initiation and progression of NAFLD. Then, the possible role of MSC-EVs in the treatment of this entity through immune response modulation was discussed. Finally, engineered EVs enhanced by specific therapeutic miRNA were suggested for alleviating the pathological cellular events in liver disease.

Regulated IRE1α-dependent decay (RIDD)-mediated reprograming of lipid metabolism in cancer

IRE1α is constitutively active in several cancers and can contribute to cancer progression. Activated IRE1α cleaves XBP1 mRNA, a key step in production of the transcription factor XBP1s. In addition, IRE1α cleaves select mRNAs through regulated IRE1α-dependent decay (RIDD). Accumulating evidence implicates IRE1α in the regulation of lipid metabolism. However, the roles of XBP1s and RIDD in this process remain ill-defined. In this study, transcriptome and lipidome profiling of triple negative breast cancer cells subjected to pharmacological inhibition of IRE1α reveals changes in lipid metabolism genes associated with accumulation of triacylglycerols (TAGs). We identify DGAT2 mRNA, encoding the rate-limiting enzyme in TAG biosynthesis, as a RIDD target. Inhibition of IRE1α, leads to DGAT2-dependent accumulation of TAGs in lipid droplets and sensitizes cells to nutritional stress, which is rescued by treatment with the DGAT2 inhibitor PF-06424439. Our results highlight the importance of IRE1α RIDD activity in reprograming cellular lipid metabolism.

IRE1α cleaves several mRNAs upon accumulation of misfolded proteins. Here the authors show that active IRE1α cleaves DGAT2 mRNA encoding the rate-limiting enzyme in the synthesis of triacylglycerols, suggesting a role of IRE1α in reprogramming lipid metabolism in cancer cells.

Phosphorylation at Ser724 of the ER stress sensor IRE1α governs its activation state and limits ER stress-induced hepatosteatosis

Inositol-requiring enzyme 1 (IRE1) is an evolutionarily conserved sensor of endoplasmic reticulum (ER) stress and mediates a key branch of the unfolded protein response in eukaryotic cells. It is an ER-resident transmembrane protein that possesses Ser/Thr protein kinase and endoribonuclease (RNase) activities in its cytoplasmic region. IRE1 is activated through dimerization/oligomerization and autophosphorylation at multiple sites, acting through its RNase activity to restore the functional capacity of the ER. However, it remains poorly defined in vivo how the autophosphorylation events of endogenous IRE1 govern its dynamic activation and functional output. Here, we generated a mouse model harboring a S724A knock-in mutation (Ern1^S724A/S724A) and investigated the importance of phosphorylation at Ser⁷²⁴ within the kinase activation loop of murine IRE1α. We found that in mouse embryonic fibroblast cells and in primary hepatocytes, S724A mutation resulted in markedly reduced IRE1α autophosphorylation in parallel with blunted activation of its RNase activity to catalyze X-box binding protein 1 (Xbp1) mRNA splicing. Furthermore, ablation of IRE1α phosphorylation at Ser⁷²⁴ exacerbated ER stress–induced hepatic steatosis in tunicamycin-treated Ern1^S724A/S724A mice. This was accompanied by significantly decreased hepatic production of spliced XBP1 protein but increased CCAAT-enhancer–binding protein homologous protein (CHOP) level, along with suppressed expression of key metabolic regulators of fatty acid β-oxidation and lipid secretion. These results demonstrate a critical role of phosphorylation at Ser⁷²⁴ of IRE1α in dynamically controlling its kinase activity, and thus its autophosphorylation state, which is coupled to activation of its RNase activity in counteracting hepatic steatosis under ER stress conditions.

ChREBP-driven DNL and PNPLA3 Expression Induced by Liquid Fructose are Essential in the Production of Fatty Liver and Hypertriglyceridemia in a High-Fat Diet-Fed Rat Model

SCOPE

The aim of this study is to delineate the contribution of dietary saturated fatty acids (FA) versus liquid fructose to fatty liver and hypertriglyceridemia.

METHODS AND RESULTS

Three groups of female rats are maintained for 3 months in standard chow (CT); High-fat diet (46.9% of fat-derived calories, rich in palmitic and stearic FA, HFD); and HFD with 10% w/v fructose in drinking water (HFHFr). Zoometric parameters, plasma biochemistry, and liver Oil-Red O (ORO) staining, lipidomics, and expression of proteins involved in FA metabolism are analyzed. Both diets increase ingested calories without modifying body weight. Only the HFHFr diet increases liver triglycerides (x11.0), with hypertriglyceridemia (x1.7) and reduces FA β-oxidation (x0.7), and increases liver FA markers of DNL (de novo lipogenesis). Whereas HFD livers show a high content of ceramides, HFHFr samples show unchanged ceramides, and an increase in diacylglycerols. Only the HFHFr diet leads to a marked increase in the expression of enzymes involved in DNL and triglyceride metabolism, such as carbohydrate response element binding protein β (ChREBPβ, x3.2), a transcription factor that regulates DNL, and patatin-like phospholipase domain-containing 3 (PNPLA3, x2.6), a lipase that mobilizes stored triglycerides for VLDL secretion.

CONCLUSION

The addition of liquid-fructose to dietary FA is determinant in liver steatosis and hypertriglyceridemia production, through increased DNL and PNPLA3 expression, and reduced FA catabolism.

Spliced or Unspliced, That Is the Question: The Biological Roles of XBP1 Isoforms in Pathophysiology

X-box binding protein 1 (XBP1) is a member of the CREB/ATF basic region leucine zipper family transcribed as the unspliced isoform (XBP1-u), which, upon exposure to endoplasmic reticulum stress, is spliced into its spliced isoform (XBP1-s). XBP1-s interacts with the cAMP response element of major histocompatibility complex class II gene and plays critical role in unfolded protein response (UPR) by regulating the transcriptional activity of genes involved in UPR. XBP1-s is also involved in other physiological pathways, including lipid metabolism, insulin metabolism, and differentiation of immune cells. Its aberrant expression is closely related to inflammation, neurodegenerative disease, viral infection, and is crucial for promoting tumor progression and drug resistance. Meanwhile, recent studies reported that the function of XBP1-u has been underestimated, as it is not merely a precursor of XBP1-s. Instead, XBP-1u is a critical factor involved in various biological pathways including autophagy and tumorigenesis through post-translational regulation. Herein, we summarize recent research on the biological functions of both XBP1-u and XBP1-s, as well as their relation to diseases.

Dysregulated lipid metabolism in colorectal cancer

PURPOSE OF REVIEW

Lipid metabolism presents a targetable metabolic vulnerability in colorectal cancer (CRC). Lipid signatures and cancer-cell lipid requirements may serve as noninvasive diagnostic and prognostic biomarkers and as a therapeutic target, respectively.

RECENT FINDINGS

A growing body of new studies highlight the complexity of lipid metabolism in CRC. Cancer cells are able to utilize an alternative fatty acid desaturation pathway, underlining the metabolic plasticity of tumors. CRC tissue shows a robust triglyceride-species signature with prognostic value in CRC patients. Lipidomic analyses in germfree and colonized mice identify a unique lipid signature and suggest that bacteria inhibit metabolism of polyunsaturated fatty acids by blocking desaturase and elongase activities. Cellular stress responses, particularly the well characterized unfolded protein response, are involved in regulating lipid synthesis and homeostasis, and contribute to adaptation of the lipid environment. Together, lipid metabolism, the intestinal microbiota and cellular stress responses unarguably play crucial roles in CRC.

SUMMARY

A number of recent advances in our understanding of dysregulated lipid metabolism in CRC underline the importance of this research field. An improved knowledge of the complex interplay between lipid metabolism, cellular stress and the intestinal microbiota in the context of CRC may lead to novel therapeutic strategies.

Scutellarin ameliorates hepatic lipid accumulation by enhancing autophagy and suppressing IRE1α/XBP1 pathway

Nonalcoholic fatty liver disease is the most prevalent liver disease characterized by excessive lipid accumulation in hepatocytes. Endoplasmic reticulum (ER) stress and autophagy play an important role in lipid accumulation. In this study, scutellarin (Scu) was examined in palmitic acid-treated HepG2 cells and C57/BL6 mice fed a high-fat diet (HFD). Scu reduced intracellular lipid content and inhibited sterol regulatory element binding protein-1c (SREBP-1c)-mediated lipid synthesis and fatty acid translocase-mediated lipid uptake in HepG2 cells. Additionally, Scu restored impaired autophagy and inhibited excessive activation of ER stress in vivo and in vitro. Moreover, Scu upregulated forkhead box O transcription factor 1-mediated autophagy by inhibiting inositol-requiring enzyme 1α (IRE1α)/X-box-binding protein 1 (XBP1) branch activation, while XBP1s overexpression exacerbated the lipid accumulation and impaired autophagy in HepG2 cells and also weakened the positive effects of Scu. Furthermore, Scu attenuated ER stress by activating autophagy, ultimately downregulating SREBP-1c-mediated lipid synthesis, and autophagy inhibitors offset these beneficial effects. Scu inhibited the crosstalk between autophagy and ER stress and downregulated saturated fatty acid-induced lipid accumulation in hepatocytes. These findings demonstrate that Scu ameliorates hepatic lipid accumulation by enhancing autophagy and suppressing ER stress via the IRE1α/XBP1 pathway.

Intracellular C3 prevents hepatic steatosis by promoting autophagy and very-low-density lipoprotein secretion

Complement component C3, mainly synthesized by hepatocytes, acts as the convergence point of three different pathways in activating the complement cascade. Besides its well-established roles in the extracellular milieu, C3 performs various intracellular functions such as immunomodulation and pathogen recognition. Although C3 is present at extremely high concentrations in hepatocytes, little is known about its intrahepatic function. In this study, we found that C3 knockout (C3^-/- ) mice displayed accelerated hepatic triglyceride (TG) accumulation compared with C57BL/6J wild type mice. Mechanistically, C3 deficiency impaired lipophagy in hepatocytes, owing to the disrupted interaction between C3 and autophagy-related 16 like 1, which is essential for autolysosome assembly. Furthermore, lipophagy deficiency affected the function of the endoplasmic reticulum in C3^-/- mice, subsequently affecting the expression of protein disulfide isomerase and activity of microsomal TG transfer protein, and ultimately impairing the production of hepatic very-low-density lipoproteins (VLDLs). Rapamycin and thapsigargin treatment accelerated VLDL secretion and alleviated hepatic lipid accumulation in C3^-/- mice. Our study demonstrates that C3 promotes lipophagy to facilitate VLDL secretion in hepatocytes, thus playing an essential role in balancing TG levels in the liver.

Pharmacological Inhibition of Inositol-Requiring Enzyme 1α RNase Activity Protects Pancreatic Beta Cell and Improves Diabetic Condition in Insulin Mutation-Induced Diabetes

β-cell ER stress plays an important role in β-cell dysfunction and death during the pathogenesis of diabetes. Proinsulin misfolding is regarded as one of the primary initiating factors of ER stress and unfolded protein response (UPR) activation in β-cells. Here, we found that the ER stress sensor inositol-requiring enzyme 1α (IRE1α) was activated in the Akita mice, a mouse model of mutant insulin gene-induced diabetes of youth (MIDY), a monogenic diabetes. Normalization of IRE1α RNase hyperactivity by pharmacological inhibitors significantly ameliorated the hyperglycemic conditions and increased serum insulin levels in Akita mice. These benefits were accompanied by a concomitant protection of functional β-cell mass, as shown by the suppression of β-cell apoptosis, increase in mature insulin production and reduction of proinsulin level. At the molecular level, we observed that the expression of genes associated with β-cell identity and function was significantly up-regulated and ER stress and its associated inflammation and oxidative stress were suppressed in islets from Akita mice treated with IRE1α RNase inhibitors. This study provides the evidence of the in vivo efficacy of IRE1α RNase inhibitors in Akita mice, pointing to the possibility of targeting IRE1α RNase as a therapeutic direction for the treatment of diabetes.

Signal Transduction and Molecular Regulation in Fatty Liver Disease

Significance: Fatty liver disease is a major liver disorder in the modern societies. Comprehensive understanding of the pathophysiology and molecular mechanisms is essential for the prevention and treatment of the disease. Recent Advances: Remarkable progress has been made in the recent years in basic and translational research in the field of fatty liver disease. Multiple signaling pathways have been implicated in the development of fatty liver disease, including AMP-activated protein kinase, mechanistic target of rapamycin kinase, endoplasmic reticulum stress, oxidative stress, inflammation, transforming growth factor β, and yes1-associated transcriptional regulator/transcriptional coactivator with PDZ-binding motif (YAP/TAZ). In addition, critical molecular regulations at the transcriptional and epigenetic levels have been linked to the pathogenesis of fatty liver disease. Critical Issues: Some critical issues remain to be solved so that research findings can be translated into clinical applications. Robust and reliable biomarkers are needed for diagnosis of different stages of the fatty liver disease. Effective and safe molecular targets remain to be identified and validated. Prevention strategies require solid scientific evidence and population-wide feasibility. Future Directions: As more data are generated with time, integrative approaches are needed to comprehensively understand the disease pathophysiology and mechanisms at multiple levels from population, organismal system, organ/tissue, to cell. The interactions between genes and environmental factors require deeper investigation for the purposes of prevention and personalized treatment of fatty liver disease. Antioxid. Redox Signal. 35, 689-717.

Dextromethorphan Dampens Neonatal Astrocyte Activation and Endoplasmic Reticulum Stress Induced by Prenatal Exposure to Buprenorphine

Prenatal exposure to buprenorphine renders offspring vulnerable to cerebral impairments. In this study, our data demonstrate, for the first time, that prenatal exposure to buprenorphine escalates astrocyte activation concurrent with indications of endoplasmic reticulum (ER) stress in the hippocampi of neonates, and this can be prevented by the coadministration of dextromethorphan with buprenorphine. Furthermore, dextromethorphan can inhibit the accumulation of GPR37 in the hippocampus of newborns caused by buprenorphine and is accompanied by the proapoptotic ER stress response that involves the procaspase-3/CHOP pathway. Primary astrocyte cultures derived from the neonates of the buprenorphine group also displayed aberrant ER calcium mobilization and elevated basal levels of cyclooxygenase-2 (COX-2) at 14 days in vitro while showing sensitivity to lipopolysaccharide-activated expression of COX-2. Similarly, these long-lasting defects in the hippocampus and astrocytes were abolished by dextromethorphan. Our findings suggest that prenatal exposure to buprenorphine might instigate long-lasting effects on hippocampal and astrocytic functions. The beneficial effects of prenatal coadministration of dextromethorphan might be, at least in part, attributed to its properties in attenuating astrocyte activation and hippocampal ER stress in neonates.

Glycoursodeoxycholic acid ameliorates diet-induced metabolic disorders with inhibiting endoplasmic reticulum stress

Recent studies reveal that bile acid metabolite composition and its metabolism are changed in metabolic disorders, such as obesity, type 2 diabetes and metabolic associated fatty liver disease (MAFLD), yet its role and the mechanism remain largely unknown. In the present study, metabolomic analysis of 163 serum and stool samples of our metabolic disease cohort was performed, and we identified glycoursodeoxycholic acid (GUDCA), glycine-conjugated bile acid produced from intestinal bacteria, was decreased in both serum and stool samples from patients with hyperglycemia. RNA-sequencing and quantitative PCR results indicated that GUDCA alleviated endoplasmic reticulum (ER) stress in livers of high fat diet (HFD)-fed mice without alteration of liver metabolism. In vitro, GUDCA reduced palmitic acid induced-ER stress and -apoptosis, as well as stabilized calcium homeostasis. In vivo, GUDCA exerted effects on amelioration of HFD-induced insulin resistance and hepatic steatosis. In parallel, ER stress and apoptosis were decreased in GUDCA-treated mice as compared with vehicle-treated mice in liver. These findings demonstrate that reduced GUDCA is an indicator of hyperglycemia. Supplementation of GUDCA could be an option for the treatment of diet-induced metabolic disorders, including insulin resistance and hepatic steatosis, with inhibiting ER stress.