The unfolded protein response transducer IRE1α prevents ER stress-induced hepatic steatosis

Microglial ER stress response via IRE1α regulates diet-induced metabolic imbalance and obesity in mice

BACKGROUND

Chronic high-fat diet (HFD) feeding triggers hypothalamic inflammation and systemic metabolic dysfunction associated with endoplasmic reticulum (ER) stress. Glial cells, specifically microglia and astrocytes, are central mediators of hypothalamic inflammation. However, the role of Inositol-Requiring Enzyme 1α (IRE1α), a primary ER stress sensor, in glial cells and its contributions to metabolic dysfunction remains elusive.

OBJECTIVES

To investigate the role of IRE1α in microglia in mediating HFD-induced metabolic dysfunction.

METHODS

Using novel conditional knockout mouse models (CX3CR1GFPΔIRE1 and TMEM119ERΔIRE1), we deleted IRE1α in immune cells or exclusively in microglia and studied its impact on metabolic health and hypothalamic transcriptional changes in mice fed with HFD for 16 weeks.

RESULTS

Deleting IRE1α in microglia significantly reduced LPS-induced pro-inflammatory cytokine gene expression in vitro. IRE1α deletion in microglia protected male mice from HFD-induced obesity, glucose intolerance, and hypothalamic inflammation, with no metabolic benefits observed in female mice. RNA-sequencing revealed significant transcriptional reprogramming of the hypothalamus, including upregulation of genes related to mitochondrial fatty acid oxidation, metabolic adaptability, and anti-inflammatory responses.

CONCLUSIONS

Our findings reveal that IRE1α-mediated ER stress response in microglia significantly contributes to hypothalamic inflammation and systemic metabolic dysfunction in response to HFD, particularly in males, demonstrating an important role of microglial ER stress response in diet-induced obesity and metabolic diseases.

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.

Treating metabolic dysfunction-associated steatohepatitis: The fat-trimming FGF21 approach

Metabolic dysfunction-associated steatohepatitis (MASH) is a condition characterized by hepatosteatosis, inflammation, and tissue damage, with steatosis as the initial stage, which involves chronic, excess deposition of lipids in hepatic lipid droplets. Despite the growing prevalence and serious risks it poses, including liver decompensation, the need for transplantation, and increased patient mortality, MASH currently faces no approved pharmacotherapy. Several promising treatment candidates have emerged from recent clinical trials, including analogs of FGF21 and agonists of the associated FGFR1-KLB complex. These agents were well-tolerated in trials and have demonstrated significant improvements in both histological and biochemical markers of liver fat content, inflammation, injury, and fibrosis in patients with MASH. Endocrine FGF21 plays a vital role in maintaining homeostasis of lipid, glucose, and energy metabolism. It achieves this through pathways that target lipids or lipid droplets in adipocytes and hepatocytes. Mechanistically, pharmacological FGF21 acts as a potent catabolic factor to promote lipid or lipid droplet lipolysis, fatty acid oxidation, mitochondrial catabolic flux, and heat-dissipating energy expenditure, leading to effective clearance of hepatic and systemic gluco-lipotoxicity and inflammatory stress, thereby preventing obesity, diabetes, and MASH pathologies. In this review, we aim to provide an update on the outcomes of clinical trials for several FGF21 mimetics. We compare these outcomes with preclinical studies and offer a lipid-centric perspective on the mechanisms underlying the clinical benefits of these agents for MASH.

Effects of Heparan Sulfate Trisaccharide Containing Oleanolic Acid in Attenuating Hyperphosphorylated Tau-Induced Cell Dysfunction Associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, marked by progressive brain degeneration and cognitive decline. A major pathological feature of AD is the accumulation of hyperphosphorylated tau (p-tau) in the form of neurofibrillary tangles (NFTs), which leads to neuronal death and neurodegeneration. P-tau also induces endoplasmic reticulum (ER) stress and activates the unfolded protein response, causing inflammation and apoptosis. Additionally, p-tau spreads in the brain through interactions with heparan sulfate (HS) proteoglycans, promoting aggregation and internalization. Targeting the tau-HS interaction offers a potential therapeutic strategy for AD. We present a novel HS mimetic with a lipophilic oleanolic acid linker and a sulfated trisaccharide, which shows strong cytoprotective effects against p-tau. Moreover, this compound alleviates p-tau-induced ER stress and inflammation. Molecular docking studies indicate that the conjugation of oleanolic acid enhances binding between the ligand and tau protofilament cores, facilitating protective interactions. These findings provide a foundation for the development of novel HS mimetics, enabling further investigation of tau-HS interactions in AD and other tauopathies.

The role of the ER stress sensor IRE1 in cardiovascular diseases

Despite enormous advances in the treatment of cardiovascular diseases, including I/R injury and heart failure, heart diseases remain a leading cause of mortality worldwide. Inositol-requiring enzyme 1 (IRE1) is an evolutionarily conserved sensor endoplasmic reticulum (ER) transmembrane protein that senses ER stress. It manages ER stress induced by the accumulation of unfolded/misfolded proteins via the unfolded protein response (UPR). However, if the stress still persists, the UPR pathways are activated and induce cell death. Emerging evidence shows that, beyond the UPR, IRE1 participates in the progression of cardiovascular diseases by regulating inflammation levels, immunity, and lipid metabolism. Here, we summarize the recent findings and discuss the potential therapeutic effects of IRE1 in the treatment of cardiovascular diseases.

Inhalation exposure to airborne PM2.5 attenuates hepatic metabolic pathways through S-nitrosylation of the primary ER stress sensor

Inhalation exposure to airborne fine particulate matter (aerodynamic diameter: <2.5 µm, PM2.5) is known to cause metabolic dysfunction-associated steatohepatitis (MASH) and the associated metabolic syndrome. Hepatic lipid accumulation and inflammation are the key characteristics of MASH. However, the mechanism by which PM2.5 exposure induces lipid accumulation and inflammation in the liver remains to be further elucidated. In this study, we revealed that inhalation exposure to PM2.5 induces nitrosative stress in mouse livers by suppressing hepatic S-nitrosoglutathione reductase activities, which leads to S-nitrosylation modification of the primary unfolded protein response (UPR) transducer inositol-requiring 1 α (IRE1α), an endoplasmic reticulum-resident protein kinase and endoribonuclease (RNase). S-nitrosylation suppresses the RNase activity of IRE1α and subsequently decreases IRE1α-mediated splicing of the mRNA encoding X-box binding protein 1 (XBP1) and IRE1α-dependent degradation of select microRNAs (miRNAs), including miR-200 family members, miR-34, miR-223, miR-155, and miR-146, in the livers of the mice exposed to PM2.5. Elevation of IRE1α-target miRNAs, due to impaired IRE1α RNase activity by PM2.5-triggered S-nitrosylation, leads to decreased expression of the major regulators of fatty acid oxidation, lipolysis, and anti-inflammatory response, including XBP1, sirtuin 1, peroxisome proliferator-activated receptor α, and peroxisome proliferator-activated receptor γ, in the liver, which account at least partially for hepatic lipid accumulation and inflammation in mice exposed to airborne PM2.5. In summary, our study revealed a novel pathway by which PM2.5 causes cytotoxicity and promotes MASH-like phenotypes through inducing hepatic nitrosative stress and S-nitrosylation of the primary UPR transducer and subsequent elevation of select miRNAs involved in metabolism and inflammation in the liver.NEW & NOTEWORTHY Exposure to fine airborne particulate matter PM2.5 causes metabolic dysfunction-associated steatohepatitis characterized by hepatic steatosis, inflammation, and fibrosis. Here, we discovered that inhalation exposure to environmental PM2.5 induces nitrosative stress in livers by suppressing hepatic S-nitrosoglutathione reductase activities, which leads to S-nitrosylation of the unfolded protein response transducer IRE1α. S-nitrosylation decreases IRE1α-dependent degradation of miRNAs in the livers of mice exposed to PM2.5, leading to downregulation of major regulators of energy metabolism and anti-inflammatory response.

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.

6-Gingerol, a Bioactive Compound of Zingiber officinale, Ameliorates High-Fat High-Fructose Diet-Induced Non-Alcoholic Related Fatty Liver Disease in Rats

Purpose

Endoplasmic reticulum (ER) stress has a prominent role in the pathogenesis of high-fat diet-induced non-alcohol related fatty liver disease (NAFLD). The aim of this study is to investigate the effects of 6-G on the reduction of ER stress-induced NAFLD in metabolic syndrome (MetS) rats.

Methods

Twenty-five male Sprague-Dawley rats were fed with a high-fat high-fructose (HFHF) diet for 16 weeks. The rats were treated orally with 6-G (50,100, and 200 mg/kgBW) once daily for eight weeks. At Week 16, all animals were sacrificed, and serum and liver tissue were harvested for biochemical and structural analysis.

Results

NAFLD liver rats were shown to have elevated protein expression of GRP78, and ER-associated apoptotic protein, such as IRE1, TRAF2, p-JNK, and p-NF-κB, which were considerably reduced by the 6-G at three doses treatment. Furthermore, a significant increase in liver apoptosis and non-alcoholic steatohepatitis (NAS) score were observed in the NAFLD rat liver and which were also attenuated by the 6-G treatment at three doses. 6-G treatment also reduced ALT, AST, and ALP serum levels.

Conclusion

Considering all the findings, it is suggested that the 6-G treatment could be a potential candidate therapy in treating ER stress-induced NAFLD in rats.

Basic research and opportunities for translational advancement in the field of mammalian ∼12-hour ultradian chronobiology

Repetitive variations, such as oscillation, are ubiquitous in biology. In this mini review, we present a general summary of the ∼24 h circadian clock and provide a fundamental overview of another biological timekeeper that maintains ∼12 h oscillations. This ∼12 h oscillator is proposed to function independently of the circadian clock to regulate ultradian biological rhythms relevant to both protein homeostasis and liver health. Recent studies exploring these ∼12 h rhythms in humans are discussed, followed by our proposal that mammary gland physiology represents a promising area for further research. We conclude by highlighting potential translational applications in ∼12 h ultradian chronobiology.

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.

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.

IRE1α Mediates the Hypertrophic Growth of Cardiomyocytes Through Facilitating the Formation of Initiation Complex to Promote the Translation of TOP-Motif Transcripts

BACKGROUND

Cardiomyocyte growth is coupled with active protein synthesis, which is one of the basic biological processes in living cells. However, it is unclear whether the unfolded protein response transducers and effectors directly take part in the control of protein synthesis. The connection between critical functions of the unfolded protein response in cellular physiology and requirements of multiple processes for cell growth prompted us to investigate the role of the unfolded protein response in cell growth and underlying molecular mechanisms.

METHODS

Cardiomyocyte-specific inositol-requiring enzyme 1α (IRE1α) knockout and overexpression mouse models were generated to explore its function in vivo. Neonatal rat ventricular myocytes were isolated and cultured to evaluate the role of IRE1α in cardiomyocyte growth in vitro. Mass spectrometry was conducted to identify novel interacting proteins of IRE1α. Ribosome sequencing and polysome profiling were performed to determine the molecular basis for the function of IRE1α in translational control.

RESULTS

We show that IRE1α is required for cell growth in neonatal rat ventricular myocytes under prohypertrophy treatment and in HEK293 cells in response to serum stimulation. At the molecular level, IRE1α directly interacts with eIF4G and eIF3, 2 critical components of the translation initiation complex. We demonstrate that IRE1α facilitates the formation of the translation initiation complex around the endoplasmic reticulum and preferentially initiates the translation of transcripts with 5' terminal oligopyrimidine motifs. We then reveal that IRE1α plays an important role in determining the selectivity and translation of these transcripts. We next show that IRE1α stimulates the translation of epidermal growth factor receptor through an unannotated terminal oligopyrimidine motif in its 5' untranslated region. We further demonstrate a physiological role of IRE1α-governed protein translation by showing that IRE1α is essential for cardiomyocyte growth and cardiac functional maintenance under hemodynamic stress in vivo.

CONCLUSIONS

These studies suggest a noncanonical, essential role of IRE1α in orchestrating protein synthesis, which may have important implications in cardiac hypertrophy in response to pressure overload and general cell growth under other physiological and pathological conditions.

The Role of Endoplasmic Reticulum Stress in Metabolic Diseases

The endoplasmic reticulum (ER), the center of protein folding, also controls the cell's life-and-death signaling mechanisms. ER stress caused by unfolded or misfolded proteins leads to the activation of the unfolded protein response (UPR) in the cell. The UPR utilizes three main signaling pathways to restore disrupted ER homeostasis. These signaling pathways are protein kinase R-like endoplasmic reticulum kinase, inositol-requiring enzyme 1, and activating transcription factor 6. Studies have reported that ER stress (ERS) plays a role in the pathogenesis of metabolic disorders such as diabetes, obesity, atherosclerosis, and nonalcoholic liver disease. This review will briefly discuss the ERS response in these metabolic diseases.

The PPIase Activity of CypB Is Essential for the Activation of Both AKT/mTOR and XBP1s Signaling Pathways during the Differentiation of 3T3-L1 Preadipocytes

In this study, we undertook an extensive investigation to determine how CypB PPIase activity affects preadipocyte differentiation and lipid metabolism. Our findings revealed that inhibition of CypB's PPIase activity suppressed the expression of crucial proteins involved in adipocyte differentiation and induced changes in proteins regulating the cell cycle. Furthermore, we clarified the impact of CypB's PPIase activity on lipid metabolism via the AKT/mTOR signaling pathway. Additionally, we demonstrated the involvement of CypB's PPIase activity in lipid metabolism through the XBP1s pathway. These discoveries offer invaluable insights for devising innovative therapeutic strategies aimed at treating and averting obesity and its related health complications. Targeting CypB's PPIase activity may emerge as a promising avenue for addressing obesity-related conditions. Furthermore, our research opens up opportunities for creating new therapeutic strategies by enhancing our comprehension of the processes involved in cellular endoplasmic reticulum stress.

The IRE1α/XBP1 pathway sustains cytokine responses of group 3 innate lymphoid cells in inflammatory bowel disease

Group 3 innate lymphoid cells (ILC3s) are key players in intestinal homeostasis. ER stress is linked to inflammatory bowel disease (IBD). Here, we used cell culture, mouse models, and human specimens to determine whether ER stress in ILC3s affects IBD pathophysiology. We show that mouse intestinal ILC3s exhibited a 24-hour rhythmic expression pattern of the master ER stress response regulator inositol-requiring kinase 1α/X-box-binding protein 1 (IRE1α/XBP1). Proinflammatory cytokine IL-23 selectively stimulated IRE1α/XBP1 in mouse ILC3s through mitochondrial ROS (mtROS). IRE1α/XBP1 was activated in ILC3s from mice exposed to experimental colitis and in inflamed human IBD specimens. Mice with Ire1α deletion in ILC3s (Ire1αΔRorc) showed reduced expression of the ER stress response and cytokine genes including Il22 in ILC3s and were highly vulnerable to infections and colitis. Administration of IL-22 counteracted their colitis susceptibility. In human ILC3s, IRE1 inhibitors suppressed cytokine production, which was upregulated by an IRE1 activator. Moreover, the frequencies of intestinal XBP1s+ ILC3s in patients with Crohn's disease before administration of ustekinumab, an anti-IL-12/IL-23 antibody, positively correlated with the response to treatment. We demonstrate that a noncanonical mtROS-IRE1α/XBP1 pathway augmented cytokine production by ILC3s and identify XBP1s+ ILC3s as a potential biomarker for predicting the response to anti-IL-23 therapies in IBD.

Endoplasmic Reticulum Stress Activates Hepatic Macrophages through PERK-hnRNPA1 Signaling

Endoplasmic reticulum (ER) stress plays a crucial role in liver diseases, affecting various types of hepatic cells. While studies have focused on the link between ER stress and hepatocytes as well as hepatic stellate cells (HSCs), the precise involvement of hepatic macrophages in ER stress-induced liver injury remains poorly understood. Here, we examined the effects of ER stress on hepatic macrophages and their role in liver injury. Acute ER stress led to the accumulation and activation of hepatic macrophages, which preceded hepatocyte apoptosis. Notably, macrophage depletion mitigated liver injury induced by ER stress, underscoring their detrimental role. Mechanistic studies revealed that ER stress stimulates macrophages predominantly via the PERK signaling pathway, regardless of its canonical substrate ATF4. hnRNPA1 has been identified as a crucial mediator of PERK-driven macrophage activation, as the overexpression of hnRNPA1 effectively reduced ER stress and suppressed pro-inflammatory activation. We observed that hnRNPA1 interacts with mRNAs that encode UPR-related proteins, indicating its role in the regulation of ER stress response in macrophages. These findings illuminate the cell type-specific responses to ER stress and the significance of hepatic macrophages in ER stress-induced liver injury. Collectively, the PERK-hnRNPA1 axis has been discovered as a molecular mechanism for macrophage activation, presenting prospective therapeutic targets for inflammatory hepatic diseases such as acute liver injury.

ER stress signaling at the interphase between MASH and HCC

HCC is the most frequent primary liver cancer with an extremely poor prognosis and often develops on preset of chronic liver diseases. Major risk factors for HCC include metabolic dysfunction-associated steatohepatitis, a complex multifactorial condition associated with abnormal endoplasmic reticulum (ER) proteostasis. To cope with ER stress, the unfolded protein response engages adaptive reactions to restore the secretory capacity of the cell. Recent advances revealed that ER stress signaling plays a critical role in HCC progression. Here, we propose that chronic ER stress is a common transversal factor contributing to the transition from liver disease (risk factor) to HCC. Interventional strategies to target the unfolded protein response in HCC, such as cancer therapy, are also discussed.

The IRE1/Xbp1 axis restores ER and tissue homeostasis perturbed by excess Notch in Drosophila

Notch signaling controls numerous key cellular processes including cell fate determination and cell proliferation. Its malfunction has been linked to many developmental abnormalities and human disorders. Overactivation of Notch signaling is shown to be oncogenic. Retention of excess Notch protein in the endoplasmic reticulum (ER) can lead to altered Notch signaling and cell fate, but the mechanism is not well understood. In this study, we show that V5-tagged or untagged exogenous Notch is retained in the ER when overexpressed in fly tissues. Furthermore, we show that Notch retention in the ER leads to robust ER enlargement and elicits a rough eye phenotype. Gain-of-function of unfolded protein response (UPR) factors IRE1 or spliced Xbp1 (Xbp1-s) alleviates Notch accumulation in the ER, restores ER morphology and ameliorates the rough eye phenotype. Our results uncover a pivotal role of the IRE1/Xbp1 axis in regulating the detrimental effect of ER-localized excess Notch protein during development and tissue homeostasis.

XBP1-mediated transcriptional regulation of SLC5A1 in human epithelial cells in disease conditions

BACKGROUND

Sodium-Glucose cotransporter 1 and 2 (SGLT1/2) belong to the family of glucose transporters, encoded by SLC5A1 and SLC5A2, respectively. SGLT2 is almost exclusively expressed in the renal proximal convoluted tubule cells. SGLT1 is expressed in the kidneys but also in other organs throughout the body. Many SGLT inhibitor drugs have been developed based on the mechanism of blocking glucose (re)absorption mediated by SGLT1/2, and several have gained major regulatory agencies' approval for treating diabetes. Intriguingly these drugs are also effective in treating diseases beyond diabetes, for example heart failure and chronic kidney disease. We recently discovered that SGLT1 is upregulated in the airway epithelial cells derived from patients of cystic fibrosis (CF), a devastating genetic disease affecting greater than 70,000 worldwide.

RESULTS

In the present work, we show that the SGLT1 upregulation is coupled with elevated endoplasmic reticulum (ER) stress response, indicated by activation of the primary ER stress senor inositol-requiring protein 1α (IRE1α) and the ER stress-induced transcription factor X-box binding protein 1 (XBP1), in CF epithelial cells, and in epithelial cells of other stress conditions. Through biochemistry experiments, we demonstrated that the spliced form of XBP1 (XBP1s) acts as a transcription factor for SLC5A1 by directly binding to its promoter region. Targeting this ER stress → SLC5A1 axis by either the ER stress inhibitor Rapamycin or the SGLT1 inhibitor Sotagliflozin was effective in attenuating the ER stress response and reducing the SGLT1 level in these cellular model systems.

CONCLUSIONS

The present work establishes a causal relationship between ER stress and SGLT1 upregulation and provides a mechanistic explanation why SGLT inhibitor drugs benefit diseases beyond diabetes.

Sotagliflozin attenuates liver-associated disorders in cystic fibrosis rabbits

Mutations in the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) gene lead to CF, a life-threating autosomal recessive genetic disease. While recently approved Trikafta dramatically ameliorates CF lung diseases, there is still a lack of effective medicine to treat CF-associated liver disease (CFLD). To address this medical need, we used a recently established CF rabbit model to test whether sotagliflozin, a sodium-glucose cotransporter 1 and 2 (SGLT1/2) inhibitor drug that is approved to treat diabetes, can be repurposed to treat CFLD. Sotagliflozin treatment led to systemic benefits to CF rabbits, evidenced by increased appetite and weight gain as well as prolonged lifespan. For CF liver-related phenotypes, the animals benefited from normalized blood chemistry and bile acid parameters. Furthermore, sotagliflozin alleviated nonalcoholic steatohepatitis-like phenotypes, including liver fibrosis. Intriguingly, sotagliflozin treatment markedly reduced the otherwise elevated endoplasmic reticulum stress responses in the liver and other affected organs of CF rabbits. In summary, our work demonstrates that sotagliflozin attenuates liver disorders in CF rabbits and suggests sotagliflozin as a potential drug to treat CFLD.

Partial limitation of cellular functions and compensatory modulation of unfolded protein response pathways caused by double-knockout of ATF6α and ATF6β

Mammalian cells have three types of endoplasmic reticulum (ER) stress-sensing molecules: ATF6, IRE1, and PERK. Among these, ATF6 is unique in that it is processed in an ER-stress-specific manner and functions as a transcription factor for the activation of anti-ER stress genes (such as BiP). ATF6 is known to have two homologues, ATF6α and ATF6β, and a greater understanding of their functions has been achieved through analyses using cultured cells. Physiological functions are also gradually being investigated in mice lacking ATF6α or ATF6β. However, little is known about the effects on mouse organisms of the deletion of both the ATF6α and ATF6β genes, since such double-knockout (DKO) mice suffer embryonic lethality at an early developmental stage. In this study, we generated and analyzed ATF6 DKO mice in which embryonic lethality was evaded by using Cre/loxP technology. Pancreatic β cell-specific ATF6 DKO mice were born normally and lived without dysregulation of blood-glucose levels but had a reduced tolerance to glucose. Islets isolated from ATF6 DKO mice also showed low production and secretion of insulin and mild enhancement of IRE1 and PERK activity. We further examined the developmental abnormalities of systemic ATF6 DKO mice. The phenotypes of ATF6α-/-; ATF6β-/- mice were similar to those previously reported, but ATF6α+/-; ATF6β-/- and ATF6α-/-; ATF6β+/- mice showed embryonic lethality at middle developmental stages, unlike those reported. Analysis of embryonic fibroblasts derived from these mice revealed that ATF6α and ATF6β have a gene-dose-dependent functional redundancy and display distinct differences in their ability to induce BiP expression. (250 words).

Inhibiting the endoplasmic reticulum stress response enhances the effect of doxorubicin by altering the lipid metabolism of liver cancer cells

Hepatocellular carcinoma (HCC) is characterized by a low and variable response to chemotherapeutic treatments. One contributing factor to the overall pharmacodynamics is the activation of endoplasmic reticulum (ER) stress pathways. This is a cellular stress mechanism that becomes activated when the cell's need for protein synthesis surpasses the ER's capacity to maintain accurate protein folding, and has been implicated in creating drug-resistance in several solid tumors.

OBJECTIVE

To identify the role of ER-stress and lipid metabolism in mediating drug response in HCC.

METHODS

By using a chemically-induced mouse model for HCC, we administered the ER-stress inhibitor 4μ8C and/or doxorubicin (DOX) twice weekly for three weeks post-tumor initiation. Histological analyses were performed alongside comprehensive molecular biology and lipidomics assessments of isolated liver samples. In vitro models, including HCC cells, spheroids, and patient-derived liver organoids were subjected to 4μ8C and/or DOX, enabling us to assess their synergistic effects on cellular viability, lipid metabolism, and oxygen consumption rate.

RESULTS

We reveal a pivotal synergy between ER-stress modulation and drug response in HCC. The inhibition of ER-stress using 4μ8C not only enhances the cytotoxic effect of DOX, but also significantly reduces cellular lipid metabolism. This intricate interplay culminates in the deprivation of energy reserves essential for the sustenance of tumor cells.

CONCLUSIONS

This study elucidates the interplay between lipid metabolism and ER-stress modulation in enhancing doxorubicin efficacy in HCC. This novel approach not only deepens our understanding of the disease, but also uncovers a promising avenue for therapeutic innovation. The long-term impact of our study could open the possibility of ER-stress inhibitors and/or lipase inhibitors as adjuvant treatments for HCC-patients.

Insulin Resistance, Obesity, and Lipotoxicity

Lipotoxicity, originally used to describe the destructive effects of excess fat accumulation on glucose metabolism, causes functional impairments in several metabolic pathways, both in adipose tissue and peripheral organs, like liver, heart, pancreas, and muscle. Ectopic lipid accumulation in the kidneys, liver, and heart has important clinical counterparts like diabetic nephropathy in type 2 diabetes mellitus, obesity-related glomerulopathy, nonalcoholic fatty liver disease, and cardiomyopathy. Insulin resistance due to lipotoxicity indirectly lead to reproductive system disorders, like polycystic ovary syndrome. Lipotoxicity has roles in insulin resistance and pancreatic beta-cell dysfunction. Increased circulating levels of lipids and the metabolic alterations in fatty acid utilization and intracellular signaling have been related to insulin resistance in muscle and liver. Different pathways, like novel protein kinase c pathways and the JNK-1 pathway, are involved as the mechanisms of how lipotoxicity leads to insulin resistance in nonadipose tissue organs, such as liver and muscle. Mitochondrial dysfunction plays a role in the pathogenesis of insulin resistance. Endoplasmic reticulum stress, through mainly increased oxidative stress, also plays an important role in the etiology of insulin resistance, especially seen in non-alcoholic fatty liver disease. Visceral adiposity and insulin resistance both increase the cardiometabolic risk, and lipotoxicity seems to play a crucial role in the pathophysiology of these associations.

PCSK7: A novel regulator of apolipoprotein B and a potential target against non-alcoholic fatty liver disease

BACKGROUND

Epidemiological evidence links the proprotein convertase subtilisin/kexin 7 (PCSK7) to triglyceride (TG) metabolism. We associated the known PCSK7 gain-of-function non-coding SNP rs236918 with higher levels of plasma apolipoprotein B (apoB) and the loss-of-function coding variant p.Pro777Leu (SNP rs201598301) with lower apoB and TG. Herein, we aimed to unravel the in vivo role of liver PCSK7.

METHODS

We biochemically defined the functional role of PCSK7 in lipid metabolism using hepatic cell lines and Pcsk7-/- mice. Our findings were validated following subcutaneous administration of hepatocyte-targeted N-acetylgalactosamine (GalNAc)-antisense oligonucleotides (ASOs) against Pcsk7.

RESULTS

Independent of its proteolytic activity, membrane-bound PCSK7 binds apoB100 in the endoplasmic reticulum and enhances its secretion. Mechanistically, the loss of PCSK7/Pcsk7 leads to apoB100 degradation, triggering an unfolded protein response, autophagy, and β-oxidation, eventually reducing lipid accumulation in hepatocytes. Non-alcoholic fatty liver disease (NAFLD) was induced by a 12-week high fat/fructose/cholesterol diet in wild type (WT) and Pcsk7-/- mice that were then allowed to recover on a 4-week control diet. Pcsk7-/- mice recovered more effectively than WT mice from all NAFLD-related liver phenotypes. Finally, subcutaneous administration of GalNAc-ASOs targeting hepatic Pcsk7 to WT mice validated the above results.

CONCLUSIONS

Our data reveal hepatic PCSK7 as one of the major regulators of apoB, and its absence reduces apoB secretion from hepatocytes favoring its ubiquitination and degradation by the proteasome. This results in a cascade of events, eventually reducing hepatic lipid accumulation, thus supporting the notion of silencing PCSK7 mRNA in hepatocytes for targeting NAFLD.

Lipid Metabolism Regulators Are the Possible Determinant for Characteristics of Myopic Human Scleral Stroma Fibroblasts (HSSFs)

The purpose of the current investigation was to elucidate what kinds of responsible mechanisms induce elongation of the sclera in myopic eyes. To do this, two-dimensional (2D) cultures of human scleral stromal fibroblasts (HSSFs) obtained from eyes with two different axial length (AL) groups, <26 mm (low AL group, n = 2) and >27 mm (high AL group, n = 3), were subjected to (1) measurements of Seahorse mitochondrial and glycolytic indices to evaluate biological aspects and (2) analysis by RNA sequencing. Extracellular flux analysis revealed that metabolic indices related to mitochondrial and glycolytic functions were higher in the low AL group than in the high AL group, suggesting that metabolic activities of HSSF cells are different depending the degree of AL. Based upon RNA sequencing of these low and high AL groups, the bioinformatic analyses using gene ontology (GO) enrichment analysis and ingenuity pathway analysis (IPA) of differentially expressed genes (DEGs) identified that sterol regulatory element-binding transcription factor 2 (SREBF2) is both a possible upstream regulator and a causal network regulator. Furthermore, SREBF1, insulin-induced gene 1 (INSIG1), and insulin-like growth factor 1 (IGF1) were detected as upstream regulators, and protein tyrosine phosphatase receptor type O (PTPRO) was detected as a causal network regulator. Since those possible regulators were all pivotally involved in lipid metabolisms including fatty acid (FA), triglyceride (TG) and cholesterol (Chol) biosynthesis, the findings reported here indicate that FA, TG and Chol biosynthesis regulation may be responsible mechanisms inducing AL elongation via HSSF.

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 in the pathogenesis of alcoholic liver disease

The endoplasmic reticulum (ER) plays a pivotal role in protein synthesis, folding, and modification. Under stress conditions such as oxidative stress and inflammation, the ER can become overwhelmed, leading to an accumulation of misfolded proteins and ensuing ER stress. This triggers the unfolded protein response (UPR) designed to restore ER homeostasis. Alcoholic liver disease (ALD), a spectrum disorder resulting from chronic alcohol consumption, encompasses conditions from fatty liver and alcoholic hepatitis to cirrhosis. Metabolites of alcohol can incite oxidative stress and inflammation in hepatic cells, instigating ER stress. Prolonged alcohol exposure further disrupts protein homeostasis, exacerbating ER stress which can lead to irreversible hepatocellular damage and ALD progression. Elucidating the contribution of ER stress to ALD pathogenesis may pave the way for innovative therapeutic interventions. This review delves into ER stress, its basic signaling pathways, and its role in the alcoholic liver injury.

Galectin-12 Regulates Immune Responses in the Skin through Sebaceous Glands

Sebaceous glands (SGs) are holocrine glands that produce sebum, which primarily contains lipids that help to maintain the barrier function of the skin. Dysregulated lipid production contributes to the progression of some diseases characterized by dry skin, including atopic dermatitis. Although the lipid production of SGs has been well-studied, few studies have assessed their role in skin immune responses. We found that SGs and sebocytes expressed IL-4 receptor and produced high levels of T helper 2-associated inflammatory mediators after IL-4 treatment, suggesting immunomodulatory effects. Galectin-12 is a lipogenic factor expressed in sebocytes that affects their differentiation and proliferation. Using galectin-12-knockdown sebocytes, we showed that galectin-12 regulated the immune response in cells exposed to IL-4 and promoted CCL26 expression by upregulating peroxisome proliferator-activated receptor-γ. Moreover, galectin-12 suppressed the expression of endoplasmic reticulum stress-response molecules, and CCL26 upregulation by IL-4 was reversed after sebocyte treatment with inducers of endoplasmic reticulum stress, suggesting that galectin-12 controls IL-4 signaling by suppressing endoplasmic reticulum stress. Using galectin-12-knockout mice, we showed that galectin-12 positively regulated the IL-4-induced enlargement of SGs and the development of an atopic dermatitis-like phenotype. Thus, galectin-12 regulates the skin immune response by promoting peroxisome proliferator-activated receptor-γ expression and suppressing endoplasmic reticulum stress in SGs.

Interference with the HNF4-dependent gene regulatory network diminishes endoplasmic reticulum stress in hepatocytes

BACKGROUND

In all eukaryotic cell types, the unfolded protein response (UPR) upregulates factors that promote protein folding and misfolded protein clearance to help alleviate endoplasmic reticulum (ER) stress. Yet, ER stress in the liver is uniquely accompanied by the suppression of metabolic genes, the coordination and purpose of which are largely unknown.

METHODS

Here, we combined in silico machine learning, in vivo liver-specific deletion of the master regulator of hepatocyte differentiation HNF4α, and in vitro manipulation of hepatocyte differentiation state to determine how the UPR regulates hepatocyte identity and toward what end.

RESULTS

Machine learning identified a cluster of correlated genes that were profoundly suppressed by persistent ER stress in the liver. These genes, which encode diverse functions including metabolism, coagulation, drug detoxification, and bile synthesis, are likely targets of the master regulator of hepatocyte differentiation HNF4α. The response of these genes to ER stress was phenocopied by liver-specific deletion of HNF4α. Strikingly, while deletion of HNF4α exacerbated liver injury in response to an ER stress challenge, it also diminished UPR activation and partially preserved ER ultrastructure, suggesting attenuated ER stress. Conversely, pharmacological maintenance of hepatocyte identity in vitro enhanced sensitivity to stress.

CONCLUSIONS

Together, our findings suggest that the UPR regulates hepatocyte identity through HNF4α to protect ER homeostasis even at the expense of liver function.

Tauroursodeoxycholic Acid Supplementation in In Vitro Culture of Indicine Bovine Embryos: Molecular and Cellular Effects on the In Vitro Cryotolerance

During embryo development, the endoplasmic reticulum (ER) acts as an important site for protein biosynthesis; however, in vitro culture (IVC) can negatively affect ER homeostasis. Therefore, the aim of our study was to evaluate the effects of the supplementation of tauroursodeoxycholic acid (TUDCA), an ER stress inhibitor, in the IVC of bovine embryos. Two experiments were carried out: Exp. 1: an evaluation of blastocyst rate, hatching kinetics, and gene expression of hatched embryos after being treated with different concentrations of TUDCA (50, 200, or 1000 μM) in the IVC; Exp. 2: an evaluation of the re-expansion, hatching, and gene expression of hatched embryos previously treated with 200 µM of TUDCA at IVC and submitted to vitrification. There was no increase in the blastocyst and hatched blastocyst rates treated with TUDCA in the IVC. However, embryos submitted to vitrification after treatment with 200 µM of TUDCA underwent an increased hatching rate post-warming together with a down-regulation in the expression of ER stress-related genes and the accumulation of lipids. In conclusion, this work showed that the addition of TUDCA during in vitro culture can improve the cryotolerance of the bovine blastocyst through the putative modulation of ER and oxidative stress.

Loss of DDRGK1 impairs IRE1α UFMylation in spondyloepiphyseal dysplasia

Spondyloepiphyseal dysplasia (SEMD) is a rare disease in which cartilage growth is disrupted, and the DDRGK1 mutation is one of the causative genes. In our study, we established Ddrgk1fl/fl, Col2a1-ERT Cre mice, which showed a thickened hypertrophic zone (HZ) in the growth plate, simulating the previous reported SEMD pathology in vivo. Instead of the classical modulation mechanism towards SOX9, our further mechanism study found that DDRGK1 stabilizes the stress sensor endoplasmic reticulum-to-nucleus signaling 1 (IRE1α) to maintain endoplasmic reticulum (ER) homoeostasis. The loss of DDRGK1 decreased the UFMylation and subsequently led to increased ubiquitylation-mediated IRE1α degradation, causing ER dysfunction and activating the PERK/CHOP/Caspase3 apoptosis pathway. Further DDRGK1 K268R-mutant mice revealed the importance of K268 UFMylation site in IRE1α degradation and subsequent ER dysfunction. In conclusion, DDRGK1 stabilizes IRE1α to ameliorate ER stress and following apoptosis in chondrocytes, which finally promote the normal chondrogenesis.

Changes in the liver of Tinca tinca under successive domestication using an integrated multi-omics approach

Domestication is the process of modifying the phenotype of a population through anthropic selection from human perspectives. Successive generations of domestication have influenced the physiological characteristics of tench Tinca tinca. In current study, we investigated gene and protein expression alterations in the liver of fifth-generation (F5). A total of 420 genes were found to be upregulated and 351 genes were downregulated, while 410 proteins were upregulated and 279 proteins were downregulated in domesticated T. tinca (DT). The integrated analysis of omics data revealed a total of 55 genes/proteins exhibiting consistent upregulation and 12 genes/proteins displaying consistent downregulation in DT. The upregulated genes/proteins in DT, such as SSR1, DERLIN2, OS9, DNAJB11, and HYOU1, exhibit enrichment in the protein processing in the endoplasmic reticulum pathway. Additionally, upregulated genes/proteins such as IL2RB, F13B, and IRF3 are associated with immune response. Conversely, downregulated genes/proteins in DT, including HSD11B1, CYP24A1, and COMT, play roles in hormone metabolism. These findings indicate that domestication can have a substantial impact on the physiological modifications related to protein processing, immune response, and hormone metabolism in DT. These adaptations potentially enhance their ability to thrive in artificial aquaculture environments, leading to improved growth and development. The exploration of genetic changes in DT will not only improve aquaculture practices but also provide significant insights into the broader process of domestication and its effects on physiological functions.

PI3K/AKT signaling pathway as a critical regulator of epithelial-mesenchymal transition in colorectal tumor cells

Colorectal cancer (CRC) is one of the most frequent gastrointestinal malignancies that are considered as a global health challenge. Despite many progresses in therapeutic methods, there is still a high rate of mortality rate among CRC patients that is associated with poor prognosis and distant metastasis. Therefore, investigating the molecular mechanisms involved in CRC metastasis can improve the prognosis. Epithelial-mesenchymal transition (EMT) process is considered as one of the main molecular mechanisms involved in CRC metastasis, which can be regulated by various signaling pathways. PI3K/AKT signaling pathway has a key role in CRC cell proliferation and migration. In the present review, we discussed the role of PI3K/AKT pathway CRC metastasis through the regulation of the EMT process. It has been shown that PI3K/AKT pathway can induce the EMT process by down regulation of epithelial markers, while up regulation of mesenchymal markers and EMT-specific transcription factors that promote CRC metastasis. This review can be an effective step toward introducing the PI3K/AKT/EMT axis to predict prognosis as well as a therapeutic target among CRC patients. Video Abstract.

ATF4 suppresses hepatocarcinogenesis by inducing SLC7A11 (xCT) to block stress-related ferroptosis

BACKGROUND & AIMS

Hepatocellular carcinoma (HCC), a leading cause of cancer-related death, is associated with viral hepatitis, non-alcoholic steatohepatitis (NASH), and alcohol-related steatohepatitis, all of which trigger endoplasmic reticulum (ER) stress, hepatocyte death, inflammation, and compensatory proliferation. Using ER stress-prone MUP-uPA mice, we established that ER stress and hypernutrition cooperate to cause NASH and HCC, but the contribution of individual stress effectors, such as activating transcription factor 4 (ATF4), to HCC and their underlying mechanisms of action remained unknown.

METHODS

Hepatocyte-specific ATF4-deficient MUP-uPA mice (MUP-uPA/Atf4Δhep) and control MUP-uPA/Atf4F/F mice were fed a high-fat diet to induce NASH-related HCC, and Atf4F/F and Atf4Δhep mice were injected with diethylnitrosamine to model carcinogen-induced HCC. Histological, biochemical, and RNA-sequencing analyses were performed to identify and define the role of ATF4-induced solute carrier family 7a member 11 (SLC7A11) expression in hepatocarcinogenesis. Reconstitution of SLC7A11 in ATF4-deficient primary hepatocytes and mouse livers was used to study its effects on ferroptosis and HCC development.

RESULTS

Hepatocyte ATF4 ablation inhibited hepatic steatosis, but increased susceptibility to ferroptosis, resulting in accelerated HCC development. Although ATF4 activates numerous genes, ferroptosis susceptibility and hepatocarcinogenesis were reversed by ectopic expression of a single ATF4 target, Slc7a11, coding for a subunit of the cystine/glutamate antiporter xCT, which is needed for glutathione synthesis. A ferroptosis inhibitor also reduced liver damage and inflammation. ATF4 and SLC7A11 amounts were positively correlated in human HCC and livers of patients with NASH.

CONCLUSIONS

Despite ATF4 being upregulated in established HCC, it serves an important protective function in normal hepatocytes. By maintaining glutathione production, ATF4 inhibits ferroptosis-dependent inflammatory cell death, which is known to promote compensatory proliferation and hepatocarcinogenesis. Ferroptosis inhibitors or ATF4 activators may also blunt HCC onset.

IMPACT AND IMPLICATIONS

Liver cancer or hepatocellular carcinoma (HCC) is associated with multiple aetiologies. Most HCC aetiologies cause hepatocyte stress and death, as well as subsequent inflammation, and compensatory proliferation, thereby accelerating HCCdevelopment. The contribution of individual stress effectors to HCC and their underlying mechanisms of action were heretofore unknown. This study shows that the stress-responsive transcription factor ATF4 blunts liver damage and cancer development by suppressing iron-dependent cell death (ferroptosis). Although ATF4 ablation prevents hepatic steatosis, it also increases susceptibility to ferroptosis, due to decreased expression of the cystine/glutamate antiporter SLC7A11, whose expression in human HCC and NASH correlates with ATF4. These findings reinforce the notion that benign steatosis may be protective and does not increase cancer risk unless accompanied by stress-induced liver damage. These results have important implications for prevention of liver damage and cancer.

Unraveling the roles of endoplasmic reticulum-associated degradation in metabolic disorders

Misfolded proteins retained in the endoplasmic reticulum cause many human diseases. ER-associated degradation (ERAD) is one of the protein quality and quantity control system located at ER, which is responsible for translocating the misfolded proteins or properly folded but excess proteins out of the ER for proteasomal degradation. Recent studies have revealed that mice with ERAD deficiency in specific cell types exhibit impaired metabolism homeostasis and metabolic diseases. Here, we highlight the ERAD physiological functions in metabolic disorders in a substrate-dependent and cell type-specific manner.

miR-27b targets MAIP1 to mediate lipid accumulation in cultured human and mouse hepatic cells

Non-alcoholic liver disease (NAFLD) is a condition caused by excessive fat accumulation in the liver and developed via multiple pathways. miR-27b has been suggested to play crucial roles in the development of NAFLD, assuming via targeting genes involved in lipid catabolism and anabolism. However, other pathways regulated by miR-27b are largely unknown. Here we show that lipid accumulation was induced in miR-27b-transfected human and mouse hepatic cells and that knockdowns of three miR-27b-target genes, β-1,4-galactosyltransferase 3 (B4GALT3), matrix AAA peptidase interacting protein 1 (MAIP1) and PH domain and leucine rich repeat protein phosphatase 2 (PHLPP2), induced lipid accumulation. We also show that B4GALT3 and MAIP1 were direct targets of miR-27b and overexpression of MAIP1 ameliorated miR-27b-induced lipid accumulation. In addition, we show that hepatic Maip1 expression declined in mice fed a high-fat diet, suggesting the involvement of decreased Maip1 expression in the condition of fatty liver. Overall, we identified MAIP1/miR-27b axis as a mediator of hepatic lipid accumulation, a potential therapeutic target for NAFLD.

Non-Alcoholic Fatty Liver Disease: Translating Disease Mechanisms into Therapeutics Using Animal Models

Nonalcoholic fatty liver disease (NAFLD) is a range of pathologies arising from fat accumulation in the liver in the absence of excess alcohol use or other causes of liver disease. Its complications include cirrhosis and liver failure, hepatocellular carcinoma, and eventual death. NAFLD is the most common cause of liver disease globally and is estimated to affect nearly one-third of individuals in the United States. Despite knowledge that the incidence and prevalence of NAFLD are increasing, the pathophysiology of the disease and its progression to cirrhosis remain insufficiently understood. The molecular pathogenesis of NAFLD involves insulin resistance, inflammation, oxidative stress, and endoplasmic reticulum stress. Better insight into these molecular pathways would allow for therapies that target specific stages of NAFLD. Preclinical animal models have aided in defining these mechanisms and have served as platforms for screening and testing of potential therapeutic approaches. In this review, we will discuss the cellular and molecular mechanisms thought to contribute to NAFLD, with a focus on the role of animal models in elucidating these mechanisms and in developing therapies.

FKBP11 upregulation promotes proliferation and migration in hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer related deaths world over. Early diagnosis and effective treatment monitoring significantly improves patients' outcomes. FKBP11 gene is highly expressed in HCC and could play a role in its development, early diagnosis and treatment.

OBJECTIVE

This study aimed to evaluate the expression of FKBP11 in HCC, its correlation with patients' clinical characteristics and potential role in HCC development.

METHODS

Expression was determined by bioinformatics analysis, quantitative real-time PCR, western blot, and immunohistochemistry. CCK-8, Transwell and wound healing assays were used to investigate involvement in HCC development.

RESULTS

FKBP11 was significantly upregulated in HCC cells, tissues and blood (all p< 0.001). Its receiver operator characteristic (ROC) curve had an AUC of 0.864 (95% CI: 0.823-0.904), at a sensitivity of 0.86 and specificity of 0.78 indicating a good diagnostic potential in HCC. Its expression was markedly reduced after surgery (p< 0.0001), indicating a potential application in HCC treatment follow-up. Knockdown of FKBP11 in HCC cells attenuated proliferation and migration, suggesting a possible role in HCC pathogenesis.

CONCLUSION

This study thus found that FKBP11 is upregulated in HCC, and the upregulation promotes HCC development. FKBP11 levels are significantly reduced post-surgery and could be a potential diagnostic and prognostic marker for HCC.

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.

Beyond circadian rhythms: emerging roles of ultradian rhythms in control of liver functions

The mammalian liver must cope with various metabolic and physiological changes that normally recur every day and primarily stem from daily cycles of rest-activity and fasting-feeding. Although a large body of evidence supports the reciprocal regulation of circadian rhythms and liver function, the research on the hepatic ultradian rhythms have largely been lagging behind. However, with the advent of more cost-effective high-throughput omics technologies, high-resolution time-lapse imaging, and more robust and powerful mathematical tools, several recent studies have shed new light on the presence and functions of hepatic ultradian rhythms. In this review, we will first very briefly discuss the basic principles of circadian rhythms, and then cover in greater details the recent literature related to ultradian rhythms. Specifically, we will highlight the prevalence and mechanisms of hepatic 12-h rhythms, and 8-h rhythms, which cycle at the second and third harmonics of circadian frequency. Finally, we also refer to ultradian rhythms with other frequencies and examine the limitations of the current approaches as well as the challenges related to identifying ultradian rhythm and addressing their molecular underpinnings.

Boosting UPR transcriptional activator XBP1 accelerates acute wound healing

Patients' suffering from large or deep wounds caused by traumatic and/or thermal injuries have significantly lower chances of recapitulating lost skin function through natural healing. We tested whether enhanced unfolded protein response (UPR) by expression of a UPR transcriptional activator, X-box-binding protein 1 (XBP1) can significantly promote wound repair through stimulating growth factor production and promoting angiogenesis. In mouse models of a second-degree thermal wound, a full-thickness traumatic wound, and a full-thickness diabetic wound, the topical gene transfer of the activated form of XBP1 (spliced XBP1, XBP1s) can significantly enhance re-epithelialization and increase angiogenesis, leading to rapid, nearly complete wound closure with intact regenerated epidermis and dermis. Overexpression of XBP1s stimulated the transcription of growth factors in fibroblasts critical to proliferation and remodeling during wound repair, including platelet-derived growth factor BB, basic fibroblast growth factor, and transforming growth factor beta 3. Meanwhile, the overexpression of XBP1s boosted the migration and tube formation of dermal microvascular endothelial cells in vitro. Our functional and mechanistic investigations of XBP1-mediated regulation of wound healing processes provide novel insights into the previously undermined physiological role of the UPR in skin injuries. The finding opens an avenue to developing potential XBP1-based therapeutic strategies in clinical wound care protocols.

Phenobarbital, a hepatic metabolic enzyme inducer, inhibits preneoplastic hepatic lesions with expression of selective autophagy receptor p62 and ER-phagy receptor FAM134B in high-fat diet-fed rats through the inhibition of ER stress

We investigated the role of endoplasmic reticulum (ER)-phagy in NAFLD-related hepatocarcinogenesis in high-fat diet (HFD)-fed and/or phenobarbital (PB)-treated rats by clustering the expression levels of the selective autophagy receptor p62 and the ER-phagy-specific receptor FAM134B in preneoplastic hepatic lesions. We obtained four clusters with variable expression levels of p62 and FAM134B in preneoplastic lesions, and a variable population of clusters in each group. PB administration increased the clusters with high expression levels of p62 while HFD feeding increased the clusters with high expression levels of both p62 and FAM134B. The areas of preneoplastic lesions of these clusters were significantly increased than those of other clusters with low expression levels of p62 and FAM134B. The combination of HFD feeding with PB counteracted the effects of each other, and the cluster composition was similar to that in the control group. The results were associated with decreased gene expression of ER stress, inflammatory cytokine, autophagy, and increased expression of antioxidant enzyme. The present study demonstrated that clustering analysis is useful for understanding the role of autophagy in each preneoplastic lesion, and that HFD feeding increased preneoplastic lesions through the inhibition of ER-phagy, which was cancelled with PB administration through the induction of ER-phagy.

CHIP Haploinsufficiency Exacerbates Hepatic Steatosis via Enhanced TXNIP Expression and Endoplasmic Reticulum Stress Responses

TXNIP is a critical regulator of glucose homeostasis, fatty acid synthesis, and cholesterol accumulation in the liver, and it has been reported that metabolic diseases, such as obesity, atherosclerosis, hyperlipidemia, type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD), are associated with endoplasmic reticulum (ER) stress. Because CHIP, an E3 ligase, was known to be involved in regulating tissue injury and inflammation in liver, its role in regulating ER stress-induced NAFLD was investigated in two experimental NAFLD models, a tunicamycin (TM)-induced and other diet-induced NAFLD mice models. In the TM-induced NAFLD model, intraperitoneal injection of TM induced liver steatosis in both CHIP^+/+ and CHIP^+/- mice, but it was severely exacerbated in CHIP^+/- mice compared to CHIP^+/+ mice. Key regulators of ER stress and de novo lipogenesis were also enhanced in the livers of TM-inoculated CHIP^+/- mice. Furthermore, in the diet-induced NAFLD models, CHIP^+/- mice developed severely impaired glucose tolerance, insulin resistance and hepatic steatosis compared to CHIP^+/+ mice. Interestingly, CHIP promoted ubiquitin-dependent degradation of TXNIP in vitro, and inhibition of TXNIP was further found to alleviate the inflammation and ER stress responses increased by CHIP inhibition. In addition, the expression of TXNIP was increased in mice deficient in CHIP in the TM- and diet-induced models. These findings suggest that CHIP modulates ER stress and inflammatory responses by inhibiting TXNIP, and that CHIP protects against TM- or HF-HS diet-induced NAFLD and serves as a potential therapeutic means for treating liver diseases.

Interference with the HNF4-dependent gene regulatory network diminishes ER stress in hepatocytes

In all eukaryotic cell types, the unfolded protein response (UPR) upregulates factors that promote protein folding and misfolded protein clearance to help alleviate endoplasmic reticulum (ER) stress. Yet ER stress in the liver is uniquely accompanied by the suppression of metabolic genes, the coordination and purpose of which is largely unknown. Here, we used unsupervised machine learning to identify a cluster of correlated genes that were profoundly suppressed by persistent ER stress in the liver. These genes, which encode diverse functions including metabolism, coagulation, drug detoxification, and bile synthesis, are likely targets of the master regulator of hepatocyte differentiation HNF4α. The response of these genes to ER stress was phenocopied by liver-specific deletion of HNF4 α. Strikingly, while deletion of HNF4α exacerbated liver injury in response to an ER stress challenge, it also diminished UPR activation and partially preserved ER ultrastructure, suggesting attenuated ER stress. Conversely, pharmacological maintenance of hepatocyte identity in vitro enhanced sensitivity to stress. Several pathways potentially link HNF4α to ER stress sensitivity, including control of expression of the tunicamycin transporter MFSD2A; modulation of IRE1/XBP1 signaling; and regulation of Pyruvate Dehydrogenase. Together, these findings suggest that HNF4α activity is linked to hepatic ER homeostasis through multiple mechanisms.

Endoplasmic reticulum stress and lipids in health and diseases

The endoplasmic reticulum (ER) is a complex and dynamic organelle that regulates many cellular pathways, including protein synthesis, protein quality control, and lipid synthesis. When one or multiple ER roles are dysregulated and saturated, the ER enters a stress state, which, in turn, activates the highly conserved unfolded protein response (UPR). By sensing the accumulation of unfolded proteins or lipid bilayer stress (LBS) at the ER, the UPR triggers pathways to restore ER homeostasis and eventually induces apoptosis if the stress remains unresolved. In recent years, it has emerged that the UPR works intimately with other cellular pathways to maintain lipid homeostasis at the ER, and so does at cellular levels. Lipid distribution, along with lipid anabolism and catabolism, are tightly regulated, in part, by the ER. Dysfunctional and overwhelmed lipid-related pathways, independently or in combination with ER stress, can have reciprocal effects on other cellular functions, contributing to the development of diseases. In this review, we summarize the current understanding of the UPR in response to proteotoxic stress and LBS and the breadth of the functions mitigated by the UPR in different tissues and in the context of diseases.

Pancreatic beta cell ER export in health and diabetes

In the secretory pathway of the pancreatic beta cell, proinsulin and other secretory granule proteins are first produced in the endoplasmic reticulum (ER). Beta cell ER homeostasis is vital for normal beta cell functions and is maintained by the delicate balance between protein synthesis, folding, export and degradation. Disruption of ER homeostasis leads to beta cell death and diabetes. Among the four components to maintain ER homeostasis, the role of ER export in insulin biogenesis or beta cell survival was not well-understood. COPII (coat protein complex II) dependent transport is a conserved mechanism for most cargo proteins to exit ER and transport to Golgi apparatus. Emerging evidence began to reveal a critical role of COPII-dependent ER export in beta cells. In this review, we will first discuss the basic components of the COPII transport machinery, the regulation of cargo entry and COPII coat assembly in mammalian cells, and the general concept of receptor-mediated cargo sorting in COPII vesicles. On the basis of these general discussions, the current knowledge and recent developments specific to the beta cell COPII dependent ER export are summarized under normal and diabetic conditions.

Current Understanding on the Role of Lipids in Macrophages and Associated Diseases

Lipid metabolism is the major intracellular mechanism driving a variety of cellular functions such as energy storage, hormone regulation and cell division. Lipids, being a primary component of the cell membrane, play a pivotal role in the survival of macrophages. Lipids are crucial for a variety of macrophage functions including phagocytosis, energy balance and ageing. However, functions of lipids in macrophages vary based on the site the macrophages are residing at. Lipid-loaded macrophages have recently been emerging as a hallmark for several diseases. This review discusses the significance of lipids in adipose tissue macrophages, tumor-associated macrophages, microglia and peritoneal macrophages. Accumulation of macrophages with impaired lipid metabolism is often characteristically observed in several metabolic disorders. Stress signals differentially regulate lipid metabolism. While conditions such as hypoxia result in accumulation of lipids in macrophages, stress signals such as nutrient deprivation initiate lipolysis and clearance of lipids. Understanding the biology of lipid accumulation in macrophages requires the development of potentially active modulators of lipid metabolism.

Luteolin ameliorates palmitate-induced lipotoxicity in hepatocytes by mediating endoplasmic reticulum stress and autophagy

Abnormal accumulation of lipids in liver leads to uncontrolled endoplasmic reticulum (ER) stress and autophagy. Luteolin is known to have antioxidant, anti-inflammatory, and anti-cancer properties, but whether it protects against lipotoxicity in liver remains unclear. In this study, we challenged AML12 liver cells and mouse primary hepatocytes with palmitic acid (PA) with or without luteolin pretreatment. In the presence of PA, reactive oxygen species (ROS) production was increased at 3 h, followed by enhancement of expression of p-PERK, ATF4, p-eIF2α, CHOP, and TXNIP (ER stress markers) and p-p62 and LC3II/LC3I ratio (autophagy markers), in both primary hepatocytes and AML12 cells. When PA treatment was extended up to 24 h, apoptosis was induced as evidenced by an increase in caspase-3 activation. RFP-GFP-LC3B transfection further revealed that the fusion of autophagosomes with lysosomes was damaged by PA. With luteolin treatment, the expression of antioxidant enzymes, i.e., heme oxygenase-1 and glutathione peroxidase, was upregulated, and PA-induced ROS production, ER stress, and cell death were dose-dependently ameliorated. Luteolin could also reverse the damage caused to autophagic flux. These results indicate that luteolin protects hepatocytes against PA assault by enhancing antioxidant defense, which can attenuate ER stress and autophagy as well as promote autophagic flux.