ER Stress Inhibits Liver Fatty Acid Oxidation while Unmitigated Stress Leads to Anorexia-Induced Lipolysis and Both Liver and Kidney Steatosis

Linking Dysregulated AMPK Signaling and ER Stress in Ethanol-Induced Liver Injury in Hepatic Alcohol Dehydrogenase Deficient Deer Mice

Ethanol (EtOH) metabolism itself can be a predisposing factor for initiation of alcoholic liver disease (ALD). Therefore, a dose dependent study to evaluate liver injury was conducted in hepatic alcohol dehydrogenase (ADH) deficient (ADH⁻) and ADH normal (ADH⁺) deer mice fed 1%, 2% or 3.5% EtOH in the liquid diet daily for 2 months. Blood alcohol concentration (BAC), liver injury marker (alanine amino transferase (ALT)), hepatic lipids and cytochrome P450 2E1 (CYP2E1) activity were measured. Liver histology, endoplasmic reticulum (ER) stress, AMP-activated protein kinase (AMPK) signaling and cell death proteins were evaluated. Significantly increased BAC, plasma ALT, hepatic lipids and steatosis were found only in ADH⁻ deer mice fed 3.5% EtOH. Further, a significant ER stress and increased un-spliced X-box binding protein 1 were evident only in ADH⁻ deer mice fed 3.5% EtOH. Both strains fed 3.5% EtOH showed deactivation of AMPK, but increased acetyl Co-A carboxylase 1 and decreased carnitine palmitoyltransferase 1A favoring lipogenesis were found only in ADH⁻ deer mice fed 3.5% EtOH. Therefore, irrespective of CYP2E1 overexpression; EtOH dose and hepatic ADH deficiency contribute to EtOH-induced steatosis and liver injury, suggesting a linkage between ER stress, dysregulated hepatic lipid metabolism and AMPK signaling.

Impact of nutrient overload on metabolic homeostasis

Nutrient overload occurs worldwide as a consequence of the modern diet pattern and the physical inactivity that sometimes accompanies it. Cells initiate multiple protective mechanisms to adapt to elevated intracellular metabolites and restore metabolic homeostasis, but irreversible injury to the cells can occur in the event of prolonged nutrient overload. Many studies have advanced the understanding of the different detrimental effects of nutrient overload; however, few reports have made connections and given the full picture of the impact of nutrient overload on cellular metabolism. In this review, detailed changes in metabolic and energy homeostasis caused by chronic nutrient overload, as well as their associations with the development of metabolic disorders, are discussed. Overnutrition-induced changes in key organelles and sensors rewire cellular bioenergetic pathways and facilitate the shift of the metabolic state toward biosynthesis, thereby leading to the onset of various metabolic disorders, which are essentially the downstream manifestations of a misbalanced metabolic equilibrium. Based on these mechanisms, potential therapeutic targets for metabolic disorders and new research directions are proposed.

Endoplasmic reticulum stress: A master regulator of metabolic syndrome

Endoplasmic reticulum (ER) stress, a change in the ER homeostasis, leads to initiation of the unfolded protein response (UPR). The primary functions of the UPR are to restore the ER's physiological activity and coordinate the apoptotic and adaptive responses. Pathophysiological conditions that augment ER stress include hypoxia, misfolded and/or mutated protein accumulation, and high glucose. Prolonged ER stress is a critical factor in the pathogenesis of metabolic syndrome including type 2 diabetes mellitus, cardiovascular diseases, atherosclerosis, obesity, and fatty liver disease. UPR is a complex homeostatic pathway between newly synthesized proteins and their maturation, although the regulatory mechanisms contributing to the UPR and the possible therapeutic strategies are yet to be clarified. Therefore, a comprehensive understanding of the underlying molecular mechanisms is necessary to develop therapeutic interventions targeting ER stress response. In this review, we discuss the role of ER stress and UPR signaling in the pathogenesis of metabolic syndrome, highlighting the main functions of UPR components. We have emphasized the use of novel small molecular chemical chaperones, considered as modulators of ER stress. The initial studies with these chemical chaperones are promising, but detailed studies are required to define their efficacy and adverse effects during therapeutic use in humans.

Endoplasmic reticulum stress and liver diseases

Endoplasmic reticulum (ER) stress occurs when ER homeostasis is perturbed with accumulation of unfolded/misfolded protein or calcium depletion. The unfolded protein response (UPR), comprising of inositol-requiring enzyme 1α (IRE1α), PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6) signaling pathways, is a protective cellular response activated by ER stress. However, UPR activation can also induce cell death upon persistent ER stress. The liver is susceptible to ER stress given its synthetic and other biological functions. Numerous studies from human liver samples and animal disease models have indicated a crucial role of ER stress and UPR signaling pathways in the pathogenesis of liver diseases, including non-alcoholic fatty liver disease, alcoholic liver disease, alpha-1 antitrypsin deficiency, cholestatic liver disease, drug-induced liver injury, ischemia/reperfusion injury, viral hepatitis and hepatocellular carcinoma. Extensive investigations have demonstrated the potential underlying mechanisms of the induction of ER stress and the contribution of UPR pathways during the development of the diseases. Moreover ER stress and the UPR proteins and genes have become emerging therapeutic targets to treat liver diseases.

Pharmacologic ATF6 activation confers global protection in widespread disease models by reprograming cellular proteostasis

Pharmacologic activation of stress-responsive signaling pathways provides a promising approach for ameliorating imbalances in proteostasis associated with diverse diseases. However, this approach has not been employed in vivo. Here we show, using a mouse model of myocardial ischemia/reperfusion, that selective pharmacologic activation of the ATF6 arm of the unfolded protein response (UPR) during reperfusion, a typical clinical intervention point after myocardial infarction, transcriptionally reprograms proteostasis, ameliorates damage and preserves heart function. These effects were lost upon cardiac myocyte-specific Atf6 deletion in the heart, demonstrating the critical role played by ATF6 in mediating pharmacologically activated proteostasis-based protection of the heart. Pharmacological activation of ATF6 is also protective in renal and cerebral ischemia/reperfusion models, demonstrating its widespread utility. Thus, pharmacologic activation of ATF6 represents a proteostasis-based therapeutic strategy for ameliorating ischemia/reperfusion damage, underscoring its unique translational potential for treating a wide range of pathologies caused by imbalanced proteostasis.

ATF6α downregulation of PPARα promotes lipotoxicity-induced tubulointerstitial fibrosis

Tubulointerstitial fibrosis is a strong predictor of progression in patients with chronic kidney disease, and is often accompanied by lipid accumulation in renal tubules. However, the molecular mechanisms modulating the relationship between lipotoxicity and tubulointerstitial fibrosis remain obscure. ATF6α, a transcription factor of the unfolded protein response, is reported to be an upstream regulator of fatty acid metabolism. Owing to their high energy demand, proximal tubular cells (PTCs) use fatty acids as their main energy source. We therefore hypothesized that ATF6α regulates PTC fatty acid metabolism, contributing to lipotoxicity-induced tubulointerstitial fibrosis. Overexpression of activated ATF6α transcriptionally downregulated peroxisome proliferator-activated receptor-α (PPARα), the master regulator of lipid metabolism, leading to reduced activity of fatty acid β-oxidation and cytosolic accumulation of lipid droplets in a human PTC line (HK-2). ATF6α-induced lipid accumulation caused mitochondrial dysfunction, enhanced apoptosis, and increased expression of connective tissue growth factor (CTGF), as well as reduced cell viability. Atf6α-/- mice had sustained expression of PPARα and less tubular lipid accumulation following unilateral ischemia-reperfusion injury (uIRI), resulting in the amelioration of apoptosis; reduced expression of CTGF, α-smooth muscle actin, and collagen I; and less tubulointerstitial fibrosis. Administration of fenofibrate, a PPARα agonist, reduced lipid accumulation and tubulointerstitial fibrosis in the uIRI model. Taken together, these findings suggest that ATF6α deranges fatty acid metabolism in PTCs, which leads to lipotoxicity-mediated apoptosis and CTGF upregulation, both of which promote tubulointerstitial fibrosis.

Hepatoprotective benefits of vitamin C against perfluorooctane sulfonate-induced liver damage in mice through suppressing inflammatory reaction and ER stress

Our previous studies show that vitamin C (VC) plays promising hepatoprotection in mice. Intrahepatic exposure of perfluorooctane sulfonate (PFOS) can induce dose-dependent cytotoxicity. However, pharmacology-based assessment of VC on PFOS remains uninvestigated. This study aimed to evaluate the therapeutic benefits of VC on inhibiting PFOS-induced liver steatosis in mice, followed by representative biochemical analysis and immunoassay. As results, VC was beneficial for reduced PFOS-induced liver damages, as showed in reductions of serological levels of transaminases (ALT and AST), lipids (TG and TC), fasting glucose and insulin, inflammatory cytokines (TNF-α and IL6), while content of fibroblast growth factor 21 (FGF21) in serum was increased. In addition, VC reduced histiocytic changes of PFOS-lesioned livers, as revealed in reduced TNF-α-labeled cells and increased FGF21-labeled cells in immunofluorescence assay. Further, intrahepatic expressions of endoplasmic reticulum (ER) stress-based ATF6, eIF2α, GRP78, XBP1 proteins were down-regulated by treatments of VC. Taken together, our preliminary findings set forth that VC exerts pharmacological benefits against PFOS-induced liver steatosis in mice, and the underlying biological mechanism may be linked to suppressing hepatocellular inflammatory reaction and ER stress.

Non-alcoholic fatty liver disease in underweight patients with inflammatory bowel disease: A case-control study

Non-alcoholic fatty liver disease (NAFLD) was shown to also occur in lean and underweight patients. So far, the prevalence of NAFLD in underweight individuals with and without inflammatory bowel disease (IBD) is insufficiently enlightened. In this cross-sectional age, gender and disease-matched case-control study, underweight patients (BMI<18.5 kg/m2) with inflammatory bowel disease (IBD), who underwent abdominal MRI at 1.5 T/3 T with fat-saturated fast-spin-echo imaging from 10/2005-07/2018 were analysed (control-to-case-ratio 1:1, n = 130). All patients were additionally investigated for duration, history of surgery, medical treatment, laboratory values, liver and spleen diameters. On MRI, liver fat was quantified by two observers based on the relative signal loss on T2-weighted fast spin-echo MR images with fat saturation compared to images without fat saturation. The prevalence of NAFLD/liver steatosis, defined as a measured intrahepatic fat content of at least 5%, was significantly higher in underweight IBD patients than in normal weight patients (87.6% versus 21.5%, p<0.001). Compared to the cases, the liver fat content of the controls was reduced by -0.19 units on average (-19%; 95%Cl: -0.20; -0.14). Similar results were obtained for the subgroup of non-IBD individuals (n = 12; -0.25 units on average (-25%); 95%Cl: -0.35; -0.14). Patients with extremely low body weight (BMI <17.5 kg/m2) showed the highest liver fat content (+0.15 units on average (+15%) compared to underweight patients with a BMI of 17.5-18.5 kg/m2 (p<0.05)). Furthermore, underweight patients showed slightly increased liver enzymes and liver diameters. There were no indications of significant differences in disease duration, type of medications or surgery between cases and controls and also, there were no significant differences between observers or field strengths (p>0.05). The prevalence of liver steatosis was higher among underweight IBD and non-IBD patients compared to normal weight controls. Also, underweight patients showed slightly increased liver enzymes and liver diameters, hinting at initial metabolic disturbances.

Endoplasmic reticulum stress signalling and the pathogenesis of non-alcoholic fatty liver disease

The global epidemic of obesity has been accompanied by a rising burden of non-alcoholic fatty liver disease (NAFLD), with manifestations ranging from simple steatosis to non-alcoholic steatohepatitis, potentially developing into hepatocellular carcinoma. Although much attention has focused on NAFLD, its pathogenesis remains largely obscure. The hallmark of NAFLD is the hepatic accumulation of lipids, which subsequently leads to cellular stress and hepatic injury, eventually resulting in chronic liver disease. Abnormal lipid accumulation often coincides with insulin resistance in steatotic livers and is associated with perturbed endoplasmic reticulum (ER) proteostasis in hepatocytes. In response to chronic ER stress, an adaptive signalling pathway known as the unfolded protein response is triggered to restore ER proteostasis. However, the unfolded protein response can cause inflammation, inflammasome activation and, in the case of non-resolvable ER stress, the death of hepatocytes. Experimental data suggest that the unfolded protein response influences hepatic tumour development, aggressiveness and response to treatment, offering novel therapeutic avenues. Herein, we provide an overview of the evidence linking ER stress to NAFLD and discuss possible points of intervention.

Induction of Liver Steatosis in BAP31-Deficient Mice Burdened with Tunicamycin-Induced Endoplasmic Reticulum Stress

Endoplasmic reticulum (ER) stress is highly associated with liver steatosis. B-cell receptor-associated protein 31 (BAP31) has been reported to be involved in ER homeostasis, and plays key roles in hepatic lipid metabolism in high-fat diet-induced obese mice. However, whether BAP31 modulates hepatic lipid metabolism via regulating ER stress is still uncertain. In this study, wild-type and liver-specific BAP31-depleted mice were administrated with ER stress activator of Tunicamycin, the markers of ER stress, liver steatosis, and the underlying molecular mechanisms were determined. BAP31 deficiency increased Tunicamycin-induced hepatic lipid accumulation, aggravated liver dysfunction, and increased the mRNA levels of ER stress markers, including glucose-regulated protein 78 (GRP78), X-box binding protein 1 (XBP1), inositol-requiring protein-1α (IRE1α) and C/EBP homologous protein (CHOP), thus promoting ER stress in vivo and in vitro. Hepatic lipid export via very low-density lipoprotein (VLDL) secretion was impaired in BAP31-depleted mice, accompanied by reduced Apolipoprotein B (APOB) and microsomal triglyceride transfer protein (MTTP) expression. Exogenous lipid clearance was also inhibited, along with impaired gene expression related to fatty acid transportation and fatty acid β-oxidation. Finally, BAP31 deficiency increased Tunicamycin-induced hepatic inflammatory response. These results demonstrate that BAP31 deficiency increased Tunicamycin-induced ER stress, impaired VLDL secretion and exogenous lipid clearance, and reduced fatty acid β-oxidation, which eventually resulted in liver steatosis.

De-silencing Grb10 contributes to acute ER stress-induced steatosis in mouse liver

The growth factor receptor bound protein GRB10 is an imprinted gene product and a key negative regulator of the insulin, IGF1 and mTORC1 signaling pathways. GRB10 is highly expressed in mouse fetal liver but almost completely silenced in adult mice, suggesting a potential detrimental role of this protein in adult liver function. Here we show that the Grb10 gene could be reactivated in adult mouse liver by acute endoplasmic reticulum stress (ER stress) such as tunicamycin or a short-term high-fat diet (HFD) challenge, concurrently with increased unfolded protein response (UPR) and hepatosteatosis. Lipogenic gene expression and acute ER stress-induced hepatosteatosis were significantly suppressed in the liver of the liver-specific GRB10 knockout mice, uncovering a key role of Grb10 reactivation in acute ER stress-induced hepatic lipid dysregulation. Mechanically, acute ER stress induces Grb10 reactivation via an ATF4-mediated increase in Grb10 gene transcription. Our study demonstrates for the first time that the silenced Grb10 gene can be reactivated by acute ER stress and its reactivation plays an important role in the early development of hepatic steatosis.

Liver function and dysfunction - a unique window into the physiological reach of ER stress and the unfolded protein response

The unfolded protein response (UPR) improves endoplasmic reticulum (ER) protein folding in order to alleviate stress. Yet it is becoming increasingly clear that the UPR regulates processes well beyond those directly involved in protein folding, in some cases by mechanisms that fall outside the realm of canonical UPR signaling. These pathways are highly specific from one cell type to another, implying that ER stress signaling affects each tissue in a unique way. Perhaps nowhere is this more evident than in the liver, which-beyond being a highly secretory tissue-is a key regulator of peripheral metabolism and a uniquely proliferative organ upon damage. The liver provides a powerful model system for exploring how and why the UPR extends its reach into physiological processes that occur outside the ER, and how ER stress contributes to the many systemic diseases that involve liver dysfunction. This review will highlight the ways in which the study of ER stress in the liver has expanded the view of the UPR to a response that is a key guardian of cellular homeostasis outside of just the narrow realm of ER protein folding.