UPR pathways combine to prevent hepatic steatosis caused by ER stress-mediated suppression of transcriptional master regulators

Endoplasmic reticulum stress response in yeast and humans

Stress pathways monitor intracellular systems and deploy a range of regulatory mechanisms in response to stress. One of the best-characterized pathways, the UPR (unfolded protein response), is an intracellular signal transduction pathway that monitors ER (endoplasmic reticulum) homoeostasis. Its activation is required to alleviate the effects of ER stress and is highly conserved from yeast to human. Although metazoans have three UPR outputs, yeast cells rely exclusively on the Ire1 (inositol-requiring enzyme-1) pathway, which is conserved in all Eukaryotes. In general, the UPR program activates hundreds of genes to alleviate ER stress but it can lead to apoptosis if the system fails to restore homoeostasis. In this review, we summarize the major advances in understanding the response to ER stress in Sc (Saccharomyces cerevisiae), Sp (Schizosaccharomyces pombe) and humans. The contribution of solved protein structures to a better understanding of the UPR pathway is discussed. Finally, we cover the interplay of ER stress in the development of diseases.

Potential For Cell-mediated Immune Responses In Mouse Models Of Pelizaeus-Merzbacher Disease

Although activation of the innate and adaptive arms of the immune system are undoubtedly involved in the pathophysiology of neurodegenerative diseases, it is unclear whether immune system activation is a primary or secondary event. Increasingly, published studies link primary metabolic stress to secondary inflammatory responses inside and outside of the nervous system. In this study, we show that the metabolic stress pathway known as the unfolded protein response (UPR) leads to secondary activation of the immune system. First, we observe innate immune system activation in autopsy specimens from Pelizaeus-Merzbacher disease (PMD) patients and mouse models stemming from PLP1 gene mutations. Second, missense mutations in mildly- and severely-affected Plp1-mutant mice exhibit immune-associated expression profiles with greater disease severity causing an increasingly proinflammatory environment. Third, and unexpectedly, we find little evidence for dysregulated expression of major antioxidant pathways, suggesting that the unfolded protein and oxidative stress responses are separable. Together, these data show that UPR activation can precede innate and/or adaptive immune system activation and that neuroinflammation can be titrated by metabolic stress in oligodendrocytes. Whether or not such activation leads to autoimmune disease in humans is unclear, but the case report of steroid-mitigated symptoms in a PMD patient initially diagnosed with multiple sclerosis lends support.

Susceptibility of Nrf2-null mice to steatohepatitis and cirrhosis upon consumption of a high-fat diet is associated with oxidative stress, perturbation of the unfolded protein response, and disturbance in the expression of metabolic enzymes but not with insulin resistance

Mice lacking the transcription factor NF-E2 p45-related factor 2 (Nrf2) develop more severe nonalcoholic steatohepatitis (NASH), with cirrhosis, than wild-type (Nrf2^+/+) mice when fed a high-fat (HF) diet for 24 weeks. Although NASH is usually associated with insulin resistance, HF-fed Nrf2^−/− mice exhibited better insulin sensitivity than HF-fed Nrf2^+/+ mice. In livers of HF-fed mice, loss of Nrf2 resulted in greater induction of lipogenic genes, lower expression of β-oxidation genes, greater reduction in AMP-activated protein kinase (AMPK) levels, and diminished acetyl coenzyme A (CoA) carboxylase phosphorylation than in the wild-type livers, which is consistent with greater fatty acid (FA) synthesis in Nrf2^−/− livers. Moreover, primary Nrf2^−/− hepatocytes displayed lower glucose and FA oxidation than Nrf2^+/+ hepatocytes, with FA oxidation partially rescued by treatment with AMPK activators. The unfolded protein response (UPR) was perturbed in control regular-chow (RC)-fed Nrf2^−/− mouse livers, and this was associated with constitutive activation of NF-κB and JNK, along with upregulation of inflammatory genes. The HF diet elicited an antioxidant response in Nrf2^+/+ livers, and as this was compromised in Nrf2^−/− livers, they suffered oxidative stress. Therefore, Nrf2 protects against NASH by suppressing lipogenesis, supporting mitochondrial function, increasing the threshold for the UPR and inflammation, and enabling adaptation to HF-diet-induced oxidative stress.

Hepatoprotective role of Sestrin2 against chronic ER stress

Upon prolonged endoplasmic reticulum (ER) stress, cells attenuate protein translation to prevent accumulation of unfolded proteins. Here we show that Sestrin2 is critical for this process. Sestrin2 expression is induced by an ER stress-activated transcription factor CCAAT-enhancer-binding protein beta (c/EBPβ). Once induced, Sestrin2 halts protein synthesis by inhibiting mammalian target of rapamycin complex 1 (mTORC1). As Sestrin2-deficient cells continue to translate a large amount of proteins during ER stress, they are highly susceptible to ER stress-associated cell death. Accordingly, dietary or genetically-induced obesity, which does not lead to any pathological indication other than simple fat accumulation in liver of WT mice, can provoke Sestrin2-deficient mice to develop severe ER stress-associated liver pathologies such as extensive liver damage, steatohepatitis and fibrosis. These pathologies are suppressed by liver-specific Sestrin2 reconstitution, mTORC1 inhibition or chemical chaperone administration. The Sestrin2-mediated unfolded protein response (UPR) may be a general protective mechanism against ER stress-associated diseases.

Activating transcription factor 6 is necessary and sufficient for alcoholic fatty liver disease in zebrafish

Fatty liver disease (FLD) is characterized by lipid accumulation in hepatocytes and is accompanied by secretory pathway dysfunction, resulting in induction of the unfolded protein response (UPR). Activating transcription factor 6 (ATF6), one of three main UPR sensors, functions to both promote FLD during acute stress and reduce FLD during chronic stress. There is little mechanistic understanding of how ATF6, or any other UPR factor, regulates hepatic lipid metabolism to cause disease. We addressed this using zebrafish genetics and biochemical analyses and demonstrate that Atf6 is necessary and sufficient for FLD. atf6 transcription is significantly upregulated in the liver of zebrafish with alcoholic FLD and morpholino-mediated atf6 depletion significantly reduced steatosis incidence caused by alcohol. Moreover, overexpression of active, nuclear Atf6 (nAtf6) in hepatocytes caused FLD in the absence of stress. mRNA-Seq and qPCR analyses of livers from five day old nAtf6 transgenic larvae revealed upregulation of genes promoting glyceroneogenesis and fatty acid elongation, including fatty acid synthase (fasn), and nAtf6 overexpression in both zebrafish larvae and human hepatoma cells increased the incorporation of ¹⁴C-acetate into lipids. Srebp transcription factors are key regulators of lipogenic enzymes, but reducing Srebp activation by scap morpholino injection neither prevented FLD in nAtf6 transgenics nor synergized with atf6 knockdown to reduce alcohol-induced FLD. In contrast, fasn morpholino injection reduced FLD in nAtf6 transgenic larvae and synergistically interacted with atf6 to reduce alcoholic FLD. Thus, our data demonstrate that Atf6 is required for alcoholic FLD and epistatically interacts with fasn to cause this disease, suggesting triglyceride biogenesis as the mechanism of UPR induced FLD.

Fatty liver disease (steatosis) is the most common liver disease worldwide and is commonly caused by obesity, type 2 diabetes, or alcohol abuse. All of these conditions are associated with impaired hepatocyte protein secretion, resulting in hypoproteinemia that contributes to the systemic complications of these diseases. The unfolded protein response (UPR) is activated in response to stress in the protein secretory pathway and a wealth of data indicates that UPR activation can contribute to steatosis, but the mechanistic basis for this relationship is poorly understood. We identify activating transcription factor 6 (Atf6), one of three UPR sensors, as necessary and sufficient for steatosis and show that Atf6 activation can promote lipogenesis, providing a direct connection between the stress response and lipid metabolism. Blocking Atf6 in zebrafish larvae prevents alcohol-induced steatosis and Atf6 overexpression in zebrafish hepatocytes induces genes that drive lipogenesis, increases lipid production and causes steatosis. Fatty acid synthase (fasn) is a key lipogenic enzyme and we show that fasn is required for fatty liver in response to both ethanol and Atf6 overexpression. Our findings point to Atf6 as a potential therapeutic target for fatty liver disease.

ER stress and hepatic lipid metabolism

The endoplasmic reticulum (ER) is an important player in regulating protein synthesis and lipid metabolism. Perturbation of ER homeostasis, referred as “ER stress,” has been linked to numerous pathological conditions, such as inflammation, cardiovascular diseases, and metabolic disorders. The liver plays a central role in regulating nutrient and lipid metabolism. Accumulating evidence implicates that ER stress disrupts lipid metabolism and induces hepatic lipotoxicity. Here, we review the major ER stress signaling pathways, how ER stress contributes to the dysregulation of hepatic lipid metabolism, and the potential causative mechanisms of ER stress in hepatic lipotoxicity. Understanding the role of ER stress in hepatic metabolism may lead to the identification of new therapeutic targets for metabolic diseases.

Seipin promotes adipose tissue fat storage through the ER Ca²⁺-ATPase SERCA

Adipose tissue is central to the regulation of lipid metabolism. Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2), one of the most severe lipodystrophy diseases, is caused by mutation of the Seipin gene. Seipin plays an important role in adipocyte differentiation and lipid homeostasis, but its exact molecular functions are still unknown. Here, we show that Seipin physically interacts with the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) in both Drosophila and man. SERCA, an endoplasmic reticulum (ER) calcium pump, is solely responsible for transporting cytosolic calcium into the ER lumen. Like dSeipin, dSERCA cell-autonomously promotes lipid storage in Drosophila fat cells. dSeipin affects dSERCA activity and modulates intracellular calcium homeostasis. Adipose tissue-specific knockdown of the ER-to-cytosol calcium release channel ryanodine receptor (RyR) partially restores fat storage in dSeipin mutants. Our results reveal that Seipin promotes adipose tissue fat storage by regulating intracellular calcium homeostasis.

Impaired autophagic flux is associated with increased endoplasmic reticulum stress during the development of NAFLD

The pathogenic mechanisms underlying the progression of non-alcoholic fatty liver disease (NAFLD) are not fully understood. In this study, we aimed to assess the relationship between endoplasmic reticulum (ER) stress and autophagy in human and mouse hepatocytes during NAFLD. ER stress and autophagy markers were analyzed in livers from patients with biopsy-proven non-alcoholic steatosis (NAS) or non-alcoholic steatohepatitis (NASH) compared with livers from subjects with histologically normal liver, in livers from mice fed with chow diet (CHD) compared with mice fed with high fat diet (HFD) or methionine-choline-deficient (MCD) diet and in primary and Huh7 human hepatocytes loaded with palmitic acid (PA). In NASH patients, significant increases in hepatic messenger RNA levels of markers of ER stress (activating transcription factor 4 (ATF4), glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP)) and autophagy (BCN1) were found compared with NAS patients. Likewise, protein levels of GRP78, CHOP and p62/SQSTM1 (p62) autophagic substrate were significantly elevated in NASH compared with NAS patients. In livers from mice fed with HFD or MCD, ER stress-mediated signaling was parallel to the blockade of the autophagic flux assessed by increases in p62, microtubule-associated protein 2 light chain 3 (LC3-II)/LC3-I ratio and accumulation of autophagosomes compared with CHD fed mice. In Huh7 hepatic cells, treatment with PA for 8 h triggered activation of both unfolding protein response and the autophagic flux. Conversely, prolonged treatment with PA (24 h) induced ER stress and cell death together with a blockade of the autophagic flux. Under these conditions, cotreatment with rapamycin or CHOP silencing ameliorated these effects and decreased apoptosis. Our results demonstrated that the autophagic flux is impaired in the liver from both NAFLD patients and murine models of NAFLD, as well as in lipid-overloaded human hepatocytes, and it could be due to elevated ER stress leading to apoptosis. Consequently, therapies aimed to restore the autophagic flux might attenuate or prevent the progression of NAFLD.

Nuclear receptor LRH-1/NR5A2 is required and targetable for liver endoplasmic reticulum stress resolution

Chronic endoplasmic reticulum (ER) stress results in toxicity that contributes to multiple human disorders. We report a stress resolution pathway initiated by the nuclear receptor LRH-1 that is independent of known unfolded protein response (UPR) pathways. Like mice lacking primary UPR components, hepatic Lrh-1-null mice cannot resolve ER stress, despite a functional UPR. In response to ER stress, LRH-1 induces expression of the kinase Plk3, which phosphorylates and activates the transcription factor ATF2. Plk3-null mice also cannot resolve ER stress, and restoring Plk3 expression in Lrh-1-null cells rescues ER stress resolution. Reduced or heightened ATF2 activity also sensitizes or desensitizes cells to ER stress, respectively. LRH-1 agonist treatment increases ER stress resistance and decreases cell death. We conclude that LRH-1 initiates a novel pathway of ER stress resolution that is independent of the UPR, yet equivalently required. Targeting LRH-1 may be beneficial in human disorders associated with chronic ER stress.

DOI:http://dx.doi.org/10.7554/eLife.01694.001

A protein can only work properly if it has been folded into the correct shape. However, it is estimated that about one third of new proteins have the wrong shape. This is a major challenge for cells because misfolded proteins are often toxic, and cause many neurodegenerative and metabolic disorders.

In eukaryotic cells, most protein folding takes place inside a part of the cell called the endoplasmic reticulum (ER). If an incorrectly folded protein is detected, it is prevented from leaving the ER until it is refolded correctly, or destroyed. If too many proteins are misfolded, a process called the unfolded protein response helps the cell to cope with this ‘ER stress’ by expanding the ER and producing more of the molecules that assist protein folding. If this does not relieve the ER stress, the cell self-destructs. Neighboring cells then have to increase protein production to compensate for what would have been produced by the dead cell, thereby increasing the chance that they will also experience ER stress.

Activation of a protein called LRH-1 (short for liver receptor homolog-1) that is produced in the liver, pancreas and intestine can relieve the symptoms of the various metabolic diseases that are associated with chronic ER stress, including type II diabetes and fatty liver disease. However, researchers have been puzzled by the fact that although LRH-1 performs many different roles, its molecular structure provides few clues as to how it can do this.

Mamrosh et al. now confirm the speculated link between LRH-1 and ER stress relief in mice. LRH-1 triggers a previously unknown pathway that can relieve ER stress and is completely independent of the unfolded protein response. Targeting LRH-1 with certain chemical compounds alters its activity, suggesting that drug treatments could be developed to relieve ER stress. As similar targets for drugs have not been found in the unfolded protein response, the discovery of the LRH-1 pathway could lead to new approaches to the treatment of the diseases that result from ER stress.

DOI:http://dx.doi.org/10.7554/eLife.01694.002

Hepatic steatosis exacerbated by endoplasmic reticulum stress-mediated downregulation of FXR in aging mice

BACKGROUND & AIMS

Non-alcoholic fatty liver disease (NAFLD) is characterized by an increase in hepatic triglyceride (TG) contents. The prevalence of NAFLD is increased with aging. However, the molecular mechanism for aging-induced fatty liver remains poorly understood.

METHODS

Hepatic TG contents and gene expression profiles were analyzed in body weight-matched young (2 months), middle (8 months) and old (18 months) C57BL/6 mice. Endoplasmic reticulum (ER) stress and farnesoid X receptor (FXR) expression were examined. The mechanism of ER stress activation in the regulation of FXR expression was further investigated.

RESULTS

In the present study, we found that TG was markedly accumulated and lipogenic genes were up-regulated in the liver of C57BL/6 mice aged 18 months. FXR, a key regulator of hepatic lipid metabolism was down-regulated in these old mice. At molecular levels, ER stress was activated in old mice and repressed FXR expression through inhibition of hepatocyte nuclear factor 1 alpha (HNF1α) transcriptional activity.

CONCLUSIONS

Our findings demonstrate that FXR down-regulation plays a critical role in aging-induced fatty liver.

Phosphorylation of eIF2α is dispensable for differentiation but required at a posttranscriptional level for paneth cell function and intestinal homeostasis in mice

BACKGROUND

Recent studies link endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) to inflammatory bowel disease. Altered eIF2α phosphorylation (eIF2α-P), a regulatory hub of the UPR, was observed in mucosal tissue of patients with inflammatory bowel disease. In this study, we examined the mechanistic role of eIF2α-P in intestinal epithelial cell (IEC) function and intestinal homeostasis in mice.

METHODS

We generated mice with villin-Cre-mediated conditional expression of nonphosphorylatable Ser51Ala mutant eIF2α in IECs (AA mice). We analyzed AA mice under normal conditions and on challenge with oral infection of Salmonella Typhimurium or dextran sulfate sodium-induced colitis.

RESULTS

Loss of eIF2α-P did not affect the normal proliferation or differentiation of IECs. However, AA mice expressed decreased secretory proteins including lysozyme, suggesting eIF2α-P is required for Paneth cell function. The ultrastructure of AA Paneth cells exhibited a reduced number of secretory granules, a fragmented ER, and distended mitochondria under normal conditions. UPR gene expression was defective in AA IECs. Translation of Paneth cell specific messenger RNAs encoding lysozyme and cryptidins was significantly defective leading to the observed granule-deficient phenotype, which was associated with reduced ribosomal recruitment of these messenger RNAs to the ER membrane. Consequently, AA mice were more susceptible to oral Salmonella infection and dextran sulfate sodium-induced colitis.

CONCLUSIONS

We conclude eIF2α phosphorylation is required for the normal function of intestinal Paneth cells and mucosal homeostasis by activating UPR signaling and promoting messenger RNA recruitment to the ER membrane for translation.

How does protein misfolding in the endoplasmic reticulum affect lipid metabolism in the liver?

PURPOSE OF REVIEW

The endoplasmic reticulum (ER) maintains cellular metabolic homeostasis by coordinating protein synthesis, secretion activities, lipid biosynthesis and calcium (Ca²⁺) storage. In this review, we will discuss how altered ER homeostasis contributes to dysregulation of hepatic lipid metabolism and contributes to liver-associated metabolic diseases.

RECENT FINDINGS

Perturbed ER functions or accumulation of unfolded protein in the ER leads to the activation of the unfolded protein response (UPR) to protect the cell from ER stress. Recent findings pinpoint the key regulatory role of the UPR in hepatic lipid metabolism and demonstrate the potential causal mechanism of ER stress in metabolic dysregulation including diabetes and obesity.

SUMMARY

A wide range of factors can alter the protein-folding environment in the ER of hepatocytes and contribute to dysregulation of hepatic lipid metabolism and liver disease. The UPR constitutes a series of adaptive programs that preserve ER protein-folding environment and maintain hepatic lipid homeostasis. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human liver-associated metabolic diseases.

Hepatic IRE1α regulates fasting-induced metabolic adaptive programs through the XBP1s-PPARα axis signalling

Although the mammalian IRE1α-XBP1 branch of the cellular unfolded protein response has been implicated in glucose and lipid metabolism, the exact metabolic role of IRE1α signalling in vivo remains poorly understood. Here we show that hepatic IRE1α functions as a nutrient sensor that regulates the metabolic adaptation to fasting. We find that prolonged deprivation of food or consumption of a ketogenic diet activates the IRE1α-XBP1 pathway in mouse livers. Hepatocyte-specific abrogation of Ire1α results in impairment of fatty acid β-oxidation and ketogenesis in the liver under chronic fasting or ketogenic conditions, leading to hepatosteatosis; liver-specific restoration of XBP1s reverses the defects in IRE1α null mice. XBP1s directly binds to and activates the promoter of PPARα, the master regulator of starvation responses. Hence, our results demonstrate that hepatic IRE1α promotes the adaptive shift of fuel utilization during starvation by stimulating mitochondrial β-oxidation and ketogenesis through the XBP1s-PPARα axis.

Inactivation of C/ebp homologous protein-driven immune-metabolic interactions exacerbate obesity and adipose tissue leukocytosis

Successful adaptation to periods of chronic caloric excess is a highly coordinated event that is critical to the survival and propagation of species. Transcription factor C/ebp homologous protein (Chop) is thought to be an important molecular mediator that integrates nutrient signals to endoplasmic reticulum (ER) stress and innate immune activation. Given that aberrant ER stress response is implicated in inducing metabolic inflammation and insulin resistance, we hypothesized that ER stress target gene Chop integrates immune and metabolic systems to adapt to chronic positive energy balance. Here we report that inactivation of Chop in mice fed a high fat diet led to significant increase in obesity caused by a reduction in energy expenditure without any change in food intake. Importantly, ablation of Chop does not induce metabolically healthy obesity, because Chop-deficient mice fed a high fat diet had increased hepatic steatosis with significantly higher insulin resistance. Quantification of adipose tissue leukocytosis revealed that elimination of Chop during obesity led to substantial increase in number of adipose tissue T and B lymphocytes. In addition, deficiency of Chop led to increase in total number of myeloid subpopulations like neutrophils and F4/80(+) adipose tissue macrophages without any alterations in the frequency of M1- or M2-like adipose tissue macrophages. Further investigation of inflammatory mechanisms revealed that ablation of Chop increases the sensitivity of macrophages to inflammasome-induced activation of IL-β in macrophages. Our findings indicate that regulated expression of Chop during obesity is critical for adaptation to chronic caloric excess and maintenance of energy homeostasis via integration of metabolic and immune systems.

miR-107 orchestrates ER stress induction and lipid accumulation by post-transcriptional regulation of fatty acid synthase in hepatocytes

MicroRNAs, a class of small non-coding RNAs, are believed to regulate several biological pathways and processes and are implicated in several diseases. They mostly regulate the levels of their target genes at the post transcriptional stage by primarily binding to the 3' UTR. Elevated hepatic levels of miR-107 are a consistent feature associated with several obese and diabetic models. Here, we show that miR-107 post-transcriptionally regulates fatty acid synthase (FASN) by binding to its 3' UTR and reduces its protein levels and the 3'UTR luciferase reporter activity, which are blunted by the miR-107 inhibitor and mutation in the miR-107 binding site in the 3' UTR. Knock-down of endogenous miR-107 levels increased FASN levels in a dose-dependent manner. Overexpression of miR-107 led to significant accumulation of malonyl CoA, accompanied by ER stress induction. All these events were prevented in the presence of the miR-107 inhibitor. While overexpression of FASN could attenuate miR-107 mediated ER stress markers' induction; the ER stress inhibitor, 4-phenyl-butyric acid did not rescue miR-107 induced FASN inhibition. This was followed by increased triglyceride formation and lipid accumulation in the presence of miR-107. These indicate that miR-107 inhibits FASN levels by binding to its 3' UTR and this interaction promotes ER stress induction and malonyl CoA and lipid accumulation in HepG2 cells and primary hepatocytes. Our results suggest that increased levels of miR-107 are critical in promoting lipid accumulation in hepatocytes and this might form the basis of diverse etiologies encountered in a fatty liver.

Unfolded protein response signaling and metabolic diseases

The endoplasmic reticulum (ER) is a central organelle for protein biosynthesis, folding, and traffic. Perturbations in ER homeostasis create a condition termed ER stress and lead to activation of the complex signaling cascade called the unfolded protein response (UPR). Recent studies have documented that the UPR coordinates multiple signaling pathways and controls various physiologies in cells and the whole organism. Furthermore, unresolved ER stress has been implicated in a variety of metabolic disorders, such as obesity and type 2 diabetes. Therefore, intervening in ER stress and modulating signaling components of the UPR would provide promising therapeutics for the treatment of human metabolic diseases.

Regulation of the transcriptome by ER stress: non-canonical mechanisms and physiological consequences

The mammalian unfolded protein response (UPR) is propagated by three ER-resident transmembrane proteins, each of which initiates a signaling cascade that ultimately culminates in production of a transcriptional activator. The UPR was originally characterized as a pathway for upregulating ER chaperones, and a comprehensive body of subsequent work has shown that protein synthesis, folding, oxidation, trafficking, and degradation are all transcriptionally enhanced by the UPR. However, the global reach of the UPR extends to genes involved in diverse physiological processes having seemingly little to do with ER protein folding, and this includes a substantial number of mRNAs that are suppressed by stress rather than stimulated. Through multiple non-canonical mechanisms emanating from each of the UPR pathways, the cell dynamically regulates transcription and mRNA degradation. Here we highlight these mechanisms and their increasingly appreciated impact on physiological processes.

Synthetic embryonic lethality upon deletion of the ER cochaperone p58(IPK) and the ER stress sensor ATF6α

The unfolded protein response (UPR) is activated as a consequence of alterations to ER homeostasis. It upregulates a group of ER chaperones and cochaperones, as well as other genes that improve protein processing within the secretory pathway. The UPR effector ATF6α augments-but is not essential for-maximal induction of ER chaperones during stress, yet its role, if any, in protecting cellular function during normal development and physiology is unknown. A systematic analysis of multiple tissues from Atf6α-/- mice revealed that all tissues examined were grossly insensitive to loss of ATF6α. However, combined deletion of ATF6α and the ER cochaperone p58(IPK) resulted in synthetic embryonic lethality. These findings reveal for the first time that an intact UPR can compensate for the genetic impairment of protein folding in the ER in vivo. The also expose a role for p58(IPK) in normal embryonic development.

CEACAM1 loss links inflammation to insulin resistance in obesity and non-alcoholic steatohepatitis (NASH)

Mounting epidemiological evidence points to an association between metabolic syndrome and non-alcoholic steatohepatitis (NASH), an increasingly recognized new epidemic. NASH pathologies include hepatocellular ballooning, lobular inflammation, hepatocellular injury, apoptosis, and hepatic fibrosis. We will review the relationship between insulin resistance and inflammation in visceral obesity and NASH in an attempt to shed more light on the pathogenesis of these major metabolic diseases. Moreover, we will identify loss of the carcinoembryonic antigen-related cell adhesion molecule 1 as a unifying mechanism linking the immunological and metabolic abnormalities in NASH.

Endoplasmic Reticulum Stress and Related Pathological Processes

The endoplasmic reticulum (ER) plays a pivotal role in lipid and protein biosynthesis as well as calcium store regulation, which determines its essential role in cell function. Hypoxia, nutrient deprivation, perturbation of redox status and aberrant calcium regulation can all trigger the ER stress response, which is mediated through three main sensors, namely inositol requiring element-1 (IRE-1), protein kinase-like ER kinase (PERK) and activating transcription factor 6 (ATF6). This review explores the interaction of ER stress and ER stress-associated pathological processes, including inflammation, apoptosis, aberrant autophagy, mitochondrial dysfunction and hypoxic responses. In addition, the correlation of ER stress with lipid and calcium homeostasis and dysregulation, and its role in disease development is also presented. Improved understanding of ER stress and its cofactors in pathological processes may provide new perspective on disease development and control.

SIRT7 represses Myc activity to suppress ER stress and prevent fatty liver disease

Nonalcoholic fatty liver disease is the most common chronic liver disorder in developed countries. Its pathogenesis is poorly understood, and therapeutic options are limited. Here, we show that SIRT7, an NAD(+)-dependent H3K18Ac deacetylase, functions at chromatin to suppress ER stress and prevent the development of fatty liver disease. SIRT7 is induced upon ER stress and is stabilized at the promoters of ribosomal proteins through its interaction with the transcription factor Myc to silence gene expression and to relieve ER stress. SIRT7-deficient mice develop chronic hepatosteatosis resembling human fatty liver disease. Myc inactivation or pharmacological suppression of ER stress alleviates fatty liver caused by SIRT7 deficiency. Importantly, SIRT7 suppresses ER stress and reverts the fatty liver disease in diet-induced obese mice. Our study identifies SIRT7 as a cofactor of Myc for transcriptional repression and delineates a druggable regulatory branch of the ER stress response that prevents and reverts fatty liver disease.

The unfolded protein response in fatty liver disease

The unfolded protein response (UPR) is a protective cellular response activated under conditions of endoplasmic reticulum (ER) stress. The hepatic UPR is activated in several forms of liver disease including nonalcoholic fatty liver disease (NAFLD). Recent data defining the role of the UPR in hepatic lipid metabolism have identified molecular mechanisms that may underlie the association between UPR activation and NAFLD. It has become increasingly evident that the IRE1α/Xbp1 pathway of the UPR is critical for hepatic lipid homeostasis, and dysregulation of this evolutionarily conserved pathway is associated with human nonalcoholic steatohepatitis (NASH). Although increasing evidence has delineated the importance of UPR pathway signaling in fatty liver disorders, the regulation of the hepatic UPR in normal physiology and fatty liver disorders remains incompletely understood. Understanding the role of the UPR in hepatic lipid metabolism may lead to the identification of novel therapeutic targets for the treatment of NAFLD.

Endoplasmic reticulum stress impairs cholesterol efflux and synthesis in hepatic cells

Metabolic disorders such as type 2 diabetes cause hepatic endoplasmic reticulum (ER) stress, which affects neutral lipid metabolism. However, the role of ER stress in cholesterol metabolism is incompletely understood. Here, we show that induction of acute ER stress in human hepatic HepG2 cells reduced ABCA1 expression and caused ABCA1 redistribution to tubular perinuclear compartments. Consequently, cholesterol efflux to apoA-I, a key step in nascent HDL formation, was diminished by 80%. Besides ABCA1, endogenous apoA-I expression was reduced upon ER stress induction, which contributed to reduced cholesterol efflux. Liver X receptor, a key regulator of ABCA1 in peripheral cells, was not involved in this process. Despite reduced cholesterol efflux, cellular cholesterol levels remained unchanged during ER stress. This was due to impaired de novo cholesterol synthesis by reduction of HMG-CoA reductase activity by 70%, although sterol response element-binding protein-2 activity was induced. In mice, ER stress induction led to a marked reduction of hepatic ABCA1 expression. However, HDL cholesterol levels were unaltered, presumably because of scavenger receptor class B, type I downregulation under ER stress. Taken together, our data suggest that ER stress in metabolic disorders reduces HDL biogenesis due to impaired hepatic ABCA1 function.

Eukaryotic initiation factor 2α--a downstream effector of mammalian target of rapamycin--modulates DNA repair and cancer response to treatment

In an effort to circumvent resistance to rapamycin – an mTOR inhibitor - we searched for novel rapamycin-downstream-targets that may be key players in the response of cancer cells to therapy. We found that rapamycin, at nM concentrations, increased phosphorylation of eukaryotic initiation factor (eIF) 2α in rapamycin-sensitive and estrogen-dependent MCF-7 cells, but had only a minimal effect on eIF2α phosphorylation in the rapamycin-insensitive triple-negative MDA-MB-231 cells. Addition of salubrinal – an inhibitor of eIF2α dephosphorylation – decreased expression of a surface marker associated with capacity for self renewal, increased senescence and induced clonogenic cell death, suggesting that excessive phosphorylation of eIF2α is detrimental to the cells' survival. Treating cells with salubrinal enhanced radiation-induced increase in eIF2α phosphorylation and clonogenic death and showed that irradiated cells are more sensitive to increased eIF2α phosphorylation than non-irradiated ones. Similar to salubrinal - the phosphomimetic eIF2α variant - S51D - increased sensitivity to radiation, and both abrogated radiation-induced increase in breast cancer type 1 susceptibility gene, thus implicating enhanced phosphorylation of eIF2α in modulation of DNA repair. Indeed, salubrinal inhibited non-homologous end joining as well as homologous recombination repair of double strand breaks that were induced by I-SceI in green fluorescent protein reporter plasmids. In addition to its effect on radiation, salubrinal enhanced eIF2α phosphorylation and clonogenic death in response to the histone deacetylase inhibitor – vorinostat. Finally, the catalytic competitive inhibitor of mTOR - Ku-0063794 - increased phosphorylation of eIF2α demonstrating further the involvement of mTOR activity in modulating eIF2α phosphorylation. These experiments suggest that excessive phosphorylation of eIF2α decreases survival of cancer cells; making eIF2α a worthy target for drug development, with the potential to enhance the cytotoxic effects of established anti-neoplastic therapies and circumvent resistance to rapalogues and possibly to other drugs that inhibit upstream components of the mTOR pathway.

KLF15 is a molecular link between endoplasmic reticulum stress and insulin resistance

Obesity places major demands on the protein folding capacity of the endoplasmic reticulum (ER), resulting in ER stress, a condition that promotes hepatic insulin resistance and steatosis. Here we identify the transcription factor, Kruppel-like factor 15 (KLF15), as an essential mediator of ER stress-induced insulin resistance in the liver. Mice with a targeted deletion of KLF15 exhibit increased hepatic ER stress, inflammation, and JNK activation compared to WT mice; however, KLF15^-/- mice are protected against hepatic insulin resistance and fatty liver under high-fat feeding conditions and in response to pharmacological induction of ER stress. The mammalian target of rapamycin complex 1 (mTORC1), a key regulator of cellular energy homeostasis, has been shown to cooperate with ER stress signaling pathways to promote hepatic insulin resistance and lipid accumulation. We find that the uncoupling of ER stress and insulin resistance in KLF15^-/- liver is associated with the maintenance of a low energy state characterized by decreased mTORC1 activity, increased AMPK phosphorylation and PGC-1α expression and activation of autophagy, an intracellular degradation process that enhances hepatic insulin sensitivity. Furthermore, in primary hepatocytes, KLF15 deficiency markedly inhibits activation of mTORC1 by amino acids and insulin, suggesting a mechanism by which KLF15 controls mTORC1-mediated insulin resistance. This study establishes KLF15 as an important molecular link between ER stress and insulin action.

Combined effects of multiple endoplasmic reticulum stresses on cytokine secretion in macrophage

Cells show various stress signs when they are challenged with severe physiological problems. Majority of such cellular stresses are conveyed to endoplasmic reticulum (ER) and unfolded protein response (UPR) serves as typical defense mechanism against ER stress. This study investigated an interaction between ER stress agents using macropage cell line Raw 264.7. When activated by lipopolysaccharide (LPS), the cell lines showed typical indicators of ER stress. Along with molecular chaperones, the activation process leads to the production of additional infl ammatory mediators. Following activation, the macrophage cell line was further treated with TUN and characterized in terms of chaperone expression and cytokine secretion. When treated with TUN, the activated macrophage cell leads to increased secretion of IL-6 although expression of ER stress markers, GRP94 and GRP78 increased. The secretion of cytokines continued until the addition of BFA which inhibits protein targeting from ER to Golgi. However, secretion of cytokines was ceased upon dual treatments with BFA and TG. This result strongly implies that cells may differently deal with various polypeptides depending on the urgency in cellular function under ER stress. Considering IL-6 is one of the most important signal molecules in macrophage, the molecule might be able to circumvent ER stress and UPR to reach its targeting site.

Role of oxidative stress in the pathogenesis of alcohol-induced liver disease

Chronic alcohol consumption is a well-known risk factor for liver disease, which represents a major cause of morbidity and mortality worldwide. The pathological process of alcohol-induced liver disease is characterized by a broad spectrum of morphological changes ranging from steatosis with minimal injury to more advanced liver damage, including steato-hepatitis and fibrosis/cirrhosis. Experimental and clinical studies increasingly show that the oxidative damage induced by ethanol contribute in many ways to the pathogenesis of alcohol hepatotoxicity. This article describes the contribution of oxidative mechanisms to liver damage by alcohol.

Temporal clustering of gene expression links the metabolic transcription factor HNF4α to the ER stress-dependent gene regulatory network

The unfolded protein response (UPR) responds to disruption of endoplasmic reticulum (ER) function by initiating signaling cascades that ultimately culminate in extensive transcriptional regulation. Classically, this regulation includes genes encoding ER chaperones, ER-associated degradation factors, and others involved in secretory protein folding and processing, and is carried out by the transcriptional activators that are produced as a consequence of UPR activation. However, up to half of the mRNAs regulated by ER stress are downregulated rather than upregulated, and the mechanisms linking ER stress and UPR activation to mRNA suppression are poorly understood. To begin to address this issue, we used a "bottom-up" approach to study the metabolic gene regulatory network controlled by the UPR in the liver, because ER stress in the liver leads to lipid accumulation, and fatty liver disease is the most common liver disease in the western world. qRT-PCR profiling of mouse liver mRNAs during ER stress revealed that suppression of the transcriptional regulators C/EBPα, PPARα, and PGC-1α preceded lipid accumulation, and was then followed by suppression of mRNAs encoding key enzymes involved in fatty acid oxidation and lipoprotein biogenesis and transport. Mice lacking the ER stress sensor ATF6α, which experience persistent ER stress and profound lipid accumulation during challenge, were then used as the basis for a functional genomics approach that allowed genes to be grouped into distinct expression profiles. This clustering predicted that ER stress would suppress the activity of the metabolic transcriptional regulator HNF4α-a finding subsequently confirmed by chromatin immunopreciptation at the Cebpa and Pgc1a promoters. Our results establish a framework for hepatic gene regulation during ER stress and suggest that HNF4α occupies the apex of that framework. They also provide a unique resource for the community to further explore the temporal regulation of gene expression during ER stress in vivo.

Involvement of endoplasmic reticulum stress in apoptosis of testicular cells induced by low-dose radiation

The study examined the role of endoplasmic reticulum stress (ERS) and signaling pathways of inositol-requiring enzyme-1 (IRE1), RNA-activated protein kinase-like ER kinase (PERK) and activating transcription factor-6 (ATF6) in apoptosis of mouse testicular cells treated with low-dose radiation (LDR). In the dose-dependent experiment, the mice were treated with whole-body X-ray irradiation at different doses (25, 50, 75, 100 or 200 mGy) and sacrificed 12 h later. In the time-dependent experiment, the mice were exposed to 75 mGy X-ray irradiation and killed at different time points (3, 6, 12, 18 or 24 h). Testicular cells were harvested for experiments. H(2)O(2) and NO concentrations, and Ca(2+)-ATPase activity were detected by biochemical assays, the calcium ion concentration ([Ca(2+)]i) by flow cytometry using fluo-3 probe, and GRP78 mRNA and protein expressions by quantitative real-time RT-PCR (qRT-PCR) and Western blotting, respectively. The mRNA expressions of S-XBP1, JNK, caspase-12 and CHOP were measured by qRT-PCR, and the protein expressions of IRE1α, S-XBP1, p-PERK, p-eIF2α, ATF6 p50, p-JNK, pro-caspase-12, cleaved caspase-12 and CHOP by Western blotting. The results showed that the concentrations of H2O2 and NO, the mRNA expressions of GRP78, S-XBP1, JNK, caspase-12 and CHOP, and the protein expressions of GRP78, S-XBP1, IRE1α, p-PERK, p-eIF2α, ATF6 p50, p-JNK, pro-caspase-12, cleaved caspase-12 and CHOP were significantly increased in a time- and dose-dependent manner after LDR. But the [Ca(2+)]i and Ca(2+)-ATPase activities were significantly decreased in a time- and dose-dependent manner. It was concluded that the ERS, regulated by IRE1, PERK and ATF6 pathways, is involved in the apoptosis of testicular cells in LDR mice, which is associated with ERS-apoptotic signaling molecules of JNK, caspase-12 and CHOP.

Hepatic cannabinoid receptor type 1 mediates alcohol-induced regulation of bile acid enzyme genes expression via CREBH

Bile acids concentration in liver is tightly regulated to prevent cell damage. Previous studies have demonstrated that deregulation of bile acid homeostasis can lead to cholestatic liver disease. Recently, we have shown that ER-bound transcription factor Crebh is a downstream effector of hepatic Cb1r signaling pathway. In this study, we have investigated the effect of alcohol exposure on hepatic bile acid homeostasis and elucidated the mediatory roles of Cb1r and Crebh in this process. We found that alcohol exposure or Cb1r-agonist 2-AG treatment increases hepatic bile acid synthesis and serum ALT, AST levels in vivo alongwith significant increase in Crebh gene expression and activation. Alcohol exposure activated Cb1r, Crebh, and perturbed bile acid homeostasis. Overexpression of Crebh increased the expression of key bile acid synthesis enzyme genes via direct binding of Crebh to their promoters, whereas Cb1r knockout and Crebh-knockdown mice were protected against alcohol-induced perturbation of bile acid homeostasis. Interestingly, insulin treatment protected against Cb1r-mediated Crebh-induced disruption of bile acid homeostasis. Furthermore, Crebh expression and activation was found to be markedly increased in insulin resistance conditions and Crebh knockdown in diabetic mice model (db/db) significantly reversed alcohol-induced disruption of bile acid homeostasis. Overall, our study demonstrates a novel regulatory mechanism of hepatic bile acid metabolism by alcohol via Cb1r-mediated activation of Crebh, and suggests that targeting Crebh can be of therapeutic potential in ameliorating alcohol-induced perturbation of bile acid homeostasis.

Endoplasmic reticulum stress impairs IL-4/IL-13 signaling through C/EBPβ-mediated transcriptional suppression

Activation of the unfolded protein response (UPR) by endoplasmic reticulum (ER) stress culminates in extensive gene regulation, with transcriptional upregulation of genes that improve the protein folding capacity of the organelle. However, a substantial number of genes are downregulated by ER stress, and the mechanisms that lead to this downregulation and its consequences on cellular function are poorly understood. We found that ER stress led to coordinated transcriptional suppression of diverse cellular processes, including those involved in cytokine signaling. Using expression of the IL-4/IL-13 receptor subunit Il4ra as a sentinel, we sought to understand the mechanism behind this suppression and its impact on inflammatory signaling. We found that reinitiation of global protein synthesis by GADD34-mediated dephosphorylation of eIF2α resulted in preferential expression of the inhibitory LIP isoform of the transcription factor C/EBPβ. This regulation was in turn required for the suppression of Il4ra and related inflammatory genes. Suppression of Il4ra was lost in Cebpb(-/-) cells but could be induced by LIP overexpression. As a consequence of Il4ra suppression, ER stress impaired IL-4/IL-13 signaling. Strikingly, Cebpb(-/-) cells lacking Il4ra downregulation were protected from this signaling impairment. This work identifies a novel role for C/EBPβ in regulating transcriptional suppression and inflammatory signaling during ER stress.

ILDR2: an endoplasmic reticulum resident molecule mediating hepatic lipid homeostasis

Ildr2, a modifier of diabetes susceptibility in obese mice, is expressed in most organs, including islets and hypothalamus, with reduced levels in livers of diabetes-susceptible B6.DBA mice congenic for a 1.8 Mb interval of Chromosome 1. In hepatoma and neuronal cells, ILDR2 is primarily located in the endoplasmic reticulum membrane. We used adenovirus vectors that express shRNA or are driven by the CMV promoter, respectively, to knockdown or overexpress Ildr2 in livers of wild type and ob/ob mice. Livers in knockdown mice were steatotic, with increased hepatic and circulating triglycerides and total cholesterol. Increased circulating VLDL, without reduction in triglyceride clearance suggests an effect of reduced hepatic ILDR2 on hepatic cholesterol clearance. In animals that overexpress Ildr2, hepatic triglyceride and total cholesterol levels were reduced, and strikingly so in ob/ob mice. There were no significant changes in body weight, energy expenditure or glucose/insulin homeostasis in knockdown or overexpressing mice. Knockdown mice showed reduced expression of genes mediating synthesis and oxidation of hepatic lipids, suggesting secondary suppression in response to increased hepatic lipid content. In Ildr2-overexpressing ob/ob mice, in association with reduced liver fat content, levels of transcripts related to neutral lipid synthesis and cholesterol were increased, suggesting “relief” of the secondary suppression imposed by lipid accumulation. Considering the fixed location of ILDR2 in the endoplasmic reticulum, we investigated the possible participation of ILDR2 in ER stress responses. In general, Ildr2 overexpression was associated with increases, and knockdown with decreases in levels of expression of molecular components of canonical ER stress pathways. We conclude that manipulation of Ildr2 expression in liver affects both lipid homeostasis and ER stress pathways. Given these reciprocal interactions, and the relatively extended time-course over which these studies were conducted, we cannot assign causal primacy to either the effects on hepatic lipid homeostasis or ER stress responses.

Ethanol metabolism and oxidative stress are required for unfolded protein response activation and steatosis in zebrafish with alcoholic liver disease

Secretory pathway dysfunction and lipid accumulation (steatosis) are the two most common responses of hepatocytes to ethanol exposure and are major factors in the pathophysiology of alcoholic liver disease (ALD). However, the mechanisms by which ethanol elicits these cellular responses are not fully understood. Recent data indicates that activation of the unfolded protein response (UPR) in response to secretory pathway dysfunction can cause steatosis. Here, we examined the relationship between alcohol metabolism, oxidative stress, secretory pathway stress and steatosis using zebrafish larvae. We found that ethanol was immediately internalized and metabolized by larvae, such that the internal ethanol concentration in 4-day-old larvae equilibrated to 160 mM after 1 hour of exposure to 350 mM ethanol, with an average ethanol metabolism rate of 56 μmol/larva/hour over 32 hours. Blocking alcohol dehydrogenase 1 (Adh1) and cytochrome P450 2E1 (Cyp2e1), the major enzymes that metabolize ethanol, prevented alcohol-induced steatosis and reduced induction of the UPR in the liver. Thus, we conclude that ethanol metabolism causes ALD in zebrafish. Oxidative stress generated by Cyp2e1-mediated ethanol metabolism is proposed to be a major culprit in ALD pathology. We found that production of reactive oxygen species (ROS) increased in larvae exposed to ethanol, whereas inhibition of the zebrafish CYP2E1 homolog or administration of antioxidants reduced ROS levels. Importantly, these treatments also blocked ethanol-induced steatosis and reduced UPR activation, whereas hydrogen peroxide (H₂O₂) acted as a pro-oxidant that synergized with low doses of ethanol to induce the UPR. Collectively, these data demonstrate that ethanol metabolism and oxidative stress are conserved mechanisms required for the development of steatosis and hepatic dysfunction in ALD, and that these processes contribute to ethanol-induced UPR activation and secretory pathway stress in hepatocytes.

Nuclear factor-erythroid 2-related factor 1 regulates expression of proteasome genes in hepatocytes and protects against endoplasmic reticulum stress and steatosis in mice

The ubiquitin-proteasome system is important in maintaining protein homeostasis. NFE2-related factor 1 (Nrf1), a transcription factor in the cap 'n' collar basic-leucine zipper family, regulates expression of cytoprotective genes. It was previously shown that liver-specific knockout of Nrf1 (Nrf1LKO) leads to hepatic cell death, steatohepatitis and cancer. However, the mechanisms underlying these pathologies are not clear. Here, we report that Nrf1 is critical for proteasome gene expression in the liver. Liver-specific knockout of Nrf1 results in impaired basal and induced expression of proteasome genes, and diminished proteasome activity in hepatocytes. In addition, our findings demonstrated that endoplasmic reticulum stress signaling pathway was also activated in Nrf1LKO livers. Inhibition of proteasome activity leads to endoplasmic reticulum stress in Nrf1-deficient hepatocytes, prompting the development of steatosis in the liver. Our results indicate that Nrf1 plays an integral role in the maintenance of proteasome function in hepatocytes and in the prevention of liver steatosis development. Moreover, these results highlight an association between proteasome dysfunction, endoplasmic reticulum stress and steatosis.

Activation of PPARα ameliorates hepatic insulin resistance and steatosis in high fructose-fed mice despite increased endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is suggested to cause hepatic insulin resistance by increasing de novo lipogenesis (DNL) and directly interfering with insulin signaling through the activation of the c-Jun N-terminal kinase (JNK) and IκB kinase (IKK) pathway. The current study interrogated these two proposed mechanisms in a mouse model of hepatic insulin resistance induced by a high fructose (HFru) diet with the treatment of fenofibrate (FB) 100 mg/kg/day, a peroxisome proliferator-activated receptor α (PPARα) agonist known to reduce lipid accumulation while maintaining elevated DNL in the liver. FB administration completely corrected HFru-induced glucose intolerance, hepatic steatosis, and the impaired hepatic insulin signaling (pAkt and pGSK3β). Of note, both the IRE1/XBP1 and PERK/eIF2α arms of unfolded protein response (UPR) signaling were activated. While retaining the elevated DNL (indicated by the upregulation of SREBP1c, ACC, FAS, and SCD1 and [3H]H2O incorporation into lipids), FB treatment markedly increased fatty acid oxidation (indicated by induction of ACOX1, p-ACC, β-HAD activity, and [14C]palmitate oxidation) and eliminated the accumulation of diacylglycerols (DAGs), which is known to have an impact on insulin signaling. Despite the marked activation of UPR signaling, neither JNK nor IKK appeared to be activated. These findings suggest that lipid accumulation (mainly DAGs), rather than the activation of JNK or IKK, is pivotal for ER stress to cause hepatic insulin resistance. Therefore, by reducing the accumulation of deleterious lipids, activation of PPARα can ameliorate hepatic insulin resistance against increased ER stress.

Transcriptional cross talk between orphan nuclear receptor ERRγ and transmembrane transcription factor ATF6α coordinates endoplasmic reticulum stress response

Orphan nuclear receptor ERRγ is a member of nuclear receptor superfamily that regulates several important cellular processes including hepatic glucose and alcohol metabolism. However, mechanistic understanding of transcriptional regulation of the ERRγ gene remains to be elucidated. Here, we report that activating transcription factor 6α (ATF6α), an endoplasmic reticulum (ER)-membrane–bound basic leucine zipper (bZip) transcription factor, directly regulates ERRγ gene expression in response to ER stress. ATF6α binds to ATF6α responsive element in the ERRγ promoter. The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α) is required for this transactivation. Chromatin immunoprecipitation (ChIP) assay confirmed the binding of both ATF6α and PGC1α on the ERRγ promoter. ChIP assay demonstrated histone H3 and H4 acetylation occurs at the ATF6α and PGC1α binding site. Of interest, ERRγ along with PGC1α induce ATF6α gene transcription upon ER stress. ERRγ binds to an ERRγ responsive element in the ATF6α promoter. ChIP assay confirmed that both ERRγ and PGC1α bind to a site in the ATF6α promoter that exhibits histone H3 and H4 acetylation. Overall, for the first time our data show a novel pathway of cross talk between nuclear receptors and ER-membrane–bound transcription factors and suggest a positive feed-forward loop regulates ERRγ and ATF6α gene transcription.

Defining hepatic dysfunction parameters in two models of fatty liver disease in zebrafish larvae

Fatty liver disease in humans can progress from steatosis to hepatocellular injury, fibrosis, cirrhosis, and liver failure. We developed a series of straightforward assays to determine whether zebrafish larvae with either tunicamycin- or ethanol-induced steatosis develop hepatic dysfunction. We found altered expression of genes involved in acute phase response and hepatic function, and impaired hepatocyte secretion and disruption of canaliculi in both models, but glycogen deficiency in hepatocytes and dilation of hepatic vasculature occurred only in ethanol-treated larvae. Hepatic stellate cells (HSCs) become activated during liver injury and HSC numbers increased in both models. Whether the excess lipids in hepatocytes are a direct cause of hepatocyte dysfunction in fatty liver disease has not been defined. We prevented ethanol-induced steatosis by blocking activation of the sterol response element binding proteins (Srebps) using gonzo(mbtps1) mutants and scap morphants and found that hepatocyte dysfunction persisted even in the absence of lipid accumulation. This suggests that lipotoxicity is not the primary cause of hepatic injury in these models of fatty liver disease. This study provides a panel of parameters to assess liver disease that can be easily applied to zebrafish mutants, transgenics, and for drug screening in which liver function is an important consideration.

Using natural variation in Drosophila to discover previously unknown endoplasmic reticulum stress genes

Natural genetic variation is a rich resource for identifying novel elements of cellular pathways such as endoplasmic reticulum (ER) stress. ER stress occurs when misfolded proteins accumulate in the ER and cells respond with the conserved unfolded protein response (UPR), which includes large-scale gene expression changes. Although ER stress can be a cause or a modifying factor of human disease, little is known of the amount of variation in the response to ER stress and the genes contributing to such variation. To study natural variation in ER stress response in a model system, we measured the survival time in response to tunicamycin-induced ER stress in flies from 114 lines from the sequenced Drosophila Genetic Reference Panel of wild-derived inbred strains. These lines showed high heterogeneity in survival time under ER stress conditions. To identify the genes that may be driving this phenotypic variation, we profiled ER stress-induced gene expression and performed an association study. Microarray analysis identified variation in transcript levels of numerous known and previously unknown ER stress-responsive genes. Survival time was significantly associated with polymorphisms in candidate genes with known (i.e., Xbp1) and unknown roles in ER stress. Functional testing found that 17 of 25 tested candidate genes from the association study have putative roles in ER stress. In both approaches, one-third of ER stress genes had human orthologs that contribute to human disease. This study establishes Drosophila as a useful model for studying variation in ER stress and identifying ER stress genes that may contribute to human disease.

Potential for therapeutic manipulation of the UPR in disease

Increased endoplasmic reticulum (ER) stress and the activated unfolded protein response (UPR) signaling associated with it play key roles in physiological processes as well as under pathological conditions. The UPR normally protects cells and re-establishes cellular homeostasis, but prolonged UPR activation can lead to the development of various pathologies. These features make the UPR signaling pathway an attractive target for the treatment of diseases whose pathogenesis is characterized by chronic activation of this pathway. Here, we focus on the molecular signaling pathways of the UPR and suggest possible ways to target this response for therapeutic purposes.

Lipid biology of breast cancer

Alterations in lipid metabolism have been reported in many types of cancer. Lipids have been implicated in the regulation of proliferation, differentiation, apoptosis, inflammation, autophagy, motility and membrane homeostasis. It is required that their biosynthesis is tightly regulated to ensure homeostasis and to prevent unnecessary energy expenditure. This review focuses on the emerging understanding of the role of lipids and lipogenic pathway regulation in breast cancer, including parallels drawn from the study of metabolic disease models, and suggestions on how these findings can potentially be exploited to promote gains in HER2/neu-positive breast cancer research. This article is part of a Special Issue entitled Lipid Metabolism in Cancer.

Endoplasmic reticulum stress induces hepatic steatosis via increased expression of the hepatic very low-density lipoprotein receptor

Recent evidence suggests that obese animals exhibit increased endoplasmic reticulum (ER) stress in the liver and adipose tissue. Although ER stress is closely associated with lipid homeostasis, it is largely unknown how ER stress contributes to hepatic steatosis. In this study, we demonstrate that the induction of ER stress stimulates hepatic steatosis through increased expression of the hepatic very low-density lipoprotein receptor (VLDLR). Among the unfolded protein response sensors, the protein kinase RNA-like ER kinase-activating transcription factor 4 signaling pathway was required for hepatic VLDLR up-regulation. In primary hepatocytes, ER stress-dependent VLDLR expression induced intracellular triglyceride accumulation in the presence of very low-density lipoprotein. Moreover, ER stress-dependent hepatic steatosis was diminished in the livers of VLDLR-deficient and apolipoprotein E-deficient mice compared with wild-type mice. In addition, the VLDLR-deficient mice exhibited decreased hepatic steatosis upon high-fat diet feeding.

CONCLUSION

These data suggest that ER stress-dependent expression of hepatic VLDLR leads to hepatic steatosis by increasing lipoprotein delivery to the liver, which might be a novel mechanism explaining ER stress-induced hepatic steatosis.

hnRNP A1 mediates the activation of the IRES-dependent SREBP-1a mRNA translation in response to endoplasmic reticulum stress

A growing amount of evidence suggests the involvement of ER (endoplasmic reticulum) stress in lipid metabolism and in the development of some liver diseases such as steatosis. The transcription factor SREBP-1 (sterol-regulatory-element-binding protein 1) modulates the expression of several enzymes involved in lipid synthesis. Previously, we showed that ER stress increased the SREBP-1a protein level in HepG2 cells, by inducing a cap-independent translation of SREBP-1a mRNA, through an IRES (internal ribosome entry site), located in its leader region. In the present paper, we report that the hnRNP A1 (heterogeneous nuclear ribonucleoprotein A1) interacts with 5'-UTR (untranslated region) of SREBP-1a mRNA, as an ITAF (IRES trans-acting factor), regulating SREBP-1a expression in HepG2 cells and in primary rat hepatocytes. Overexpression of hnRNP A1 in HepG2 cells and in rat hepatocytes increased both the SREBP-1a IRES activity and SREBP-1a protein level. Knockdown of hnRNP A1 by small interfering RNA reduced either the SREBP-1a IRES activity or SREBP-1a protein level. hnRNP A1 mediates the increase of SREBP-1a protein level and SREBP-1a IRES activity in Hep G2 cells and in rat hepatocytes upon tunicamycin- and thapsigargin-induced ER stress. The induced ER stress triggered the cytosolic relocation of hnRNP A1 and caused the increase in hnRNP A1 bound to the SREBP-1a 5'-UTR. These data indicate that hnRNP A1 participates in the IRES-dependent translation of SREBP-1a mRNA through RNA-protein interaction. A different content of hnRNP A1 was found in the nuclei from high-fat-diet-fed mice liver compared with standard-diet-fed mice liver, suggesting an involvement of ER stress-mediated hnRNP A1 subcellular redistribution on the onset of metabolic disorders.

Proteasome dysfunction mediates obesity-induced endoplasmic reticulum stress and insulin resistance in the liver

Chronic endoplasmic reticulum (ER) stress is a major contributor to obesity-induced insulin resistance in the liver. However, the molecular link between obesity and ER stress remains to be identified. Proteasomes are important multicatalytic enzyme complexes that degrade misfolded and oxidized proteins. Here, we report that both mouse models of obesity and diabetes and proteasome activator (PA)28-null mice showed 30-40% reduction in proteasome activity and accumulation of polyubiquitinated proteins in the liver. PA28-null mice also showed hepatic steatosis, decreased hepatic insulin signaling, and increased hepatic glucose production. The link between proteasome dysfunction and hepatic insulin resistance involves ER stress leading to hyperactivation of c-Jun NH₂-terminal kinase in the liver. Administration of a chemical chaperone, phenylbutyric acid (PBA), partially rescued the phenotypes of PA28-null mice. To confirm part of the results obtained from in vivo experiments, we pretreated rat hepatoma-derived H4IIEC3 cells with bortezomib, a selective inhibitor of the 26S proteasome. Bortezomib causes ER stress and insulin resistance in vitro--responses that are partly blocked by PBA. Taken together, our data suggest that proteasome dysfunction mediates obesity-induced ER stress, leading to insulin resistance in the liver.

Prevention and reversal of hepatic steatosis with a high-protein diet in mice

The hallmark of NAFLD is steatosis of unknown etiology. We tested the effect of a high-protein (HP)(2) diet on diet-induced steatosis in male C57BL/6 mice with and without pre-existing fatty liver. Mice were fed all combinations of semisynthetic low-fat (LF) or high-fat (HF) and low-protein (LP) or HP diets for 3weeks. To control for reduced energy intake by HF/HP-fed mice, a pair-fed HF/LP group was included. Reversibility of pre-existing steatosis was investigated by sequentially feeding HF/LP and HF/HP diets. HP-containing diets decreased hepatic lipids to ~40% of corresponding LP-containing diets, were more efficient in this respect than reducing energy intake to 80%, and reversed pre-existing diet-induced steatosis. Compared to LP-containing diets, mice fed HP-containing diets showed increased mitochondrial oxidative capacity (elevated Pgc1α, mAco, and Cpt1 mRNAs, complex-V protein, and decreased plasma free and short-chain acyl-carnitines, and [C0]/[C16+C18] carnitine ratio); increased gluconeogenesis and pyruvate cycling (increased PCK1 protein and fed plasma-glucose concentration without increased G6pase mRNA); reduced fatty-acid desaturation (decreased Scd1 expression and [C16:1n-7]/[C16:0] ratio) and increased long-chain PUFA elongation; a selective increase in plasma branched-chain amino acids; a decrease in cell stress (reduced phosphorylated eIF2α, and Fgf21 and Chop expression); and a trend toward less inflammation (lower Mcp1 and Cd11b expression and less phosphorylated NFκB).

CONCLUSION

HP diets prevent and reverse steatosis independently of fat and carbohydrate intake more efficiently than a 20% reduction in energy intake. The effect appears to result from fuel-generated, highly distributed small, synergistic increases in lipid and BCAA catabolism, and a decrease in cell stress.

CHO cell engineering to prevent polypeptide aggregation and improve therapeutic protein secretion

The ability to efficiently produce recombinant proteins in a secreted form is highly desirable and cultured mammalian cells such as CHO cells have become the preferred host as they secrete proteins with human-like post-translational modifications. However, attempts to express high levels of particular proteins in CHO cells may consistently result in low yields, even for non-engineered proteins such as immunoglobulins. In this study, we identified the responsible faulty step at the stage of translational arrest, translocation and early processing for such a "difficult-to-express" immunoglobulin, resulting in improper cleavage of the light chain and its precipitation in an insoluble cellular fraction unable to contribute to immunoglobulin assembly. We further show that proper processing and secretion were restored by over-expressing human signal receptor protein SRP14 and other components of the secretion pathway. This allowed the expression of the difficult-to-express protein to high yields, and it also increased the production of an easy-to-express protein. Our results demonstrate that components of the secretory and processing pathways can be limiting, and that engineering of the secretory pathway may be used to improve the secretion efficiency of therapeutic proteins from CHO cells.

CARD3 deficiency exacerbates diet-induced obesity, hepatosteatosis, and insulin resistance in male mice

Caspase activation and recruitment domain 3 (CARD3) is a 61-kDa protein kinase with an N-terminal serine/threonine kinase domain and a C-terminal CARD. Previous research on the function of CARD3 has focused on its role in the immune response and inflammatory diseases. Obesity is now a worldwide health problem and is generally recognized as an inflammatory disease. Unexpectedly, we found that CARD3 expression was lower during obesity. In this study, we explored the biological and genetic bases of obesity using CARD3-knockout (KO) and wild-type (WT) mice fed a high-fat diet (HFD) for 24 weeks. We demonstrate that KO mice were more obese than their WT littermates, and KO mice exhibited obvious visceral fat accumulation and liver weight gains after 24 weeks of HFD feeding. We also observed more severe hepatosteatosis in KO mice compared with the WT controls. Hepatic steatosis in the HFD-fed KO mice was linked to a significant increase in the expression of key lipogenic and cholesterol synthesis enzymes, whereas the expression of the enzymes involves in β-oxidation was dramatically reduced. Furthermore, we confirmed the repression of AMP-activated protein kinase signaling and activation of the endoplasmic reticulum stress response. Fatty liver impaired the global glucose and lipid metabolism, which further exacerbated the insulin resistance associated with the repression of Akt signaling and up-regulated systemic inflammation through the M1/M2 (pro- and anti-inflammation) type switch and the activation of the nuclear factor-κB pathway. Our studies demonstrate the crucial role of CARD3 in metabolism and indicate that CARD3 deficiency promotes the diet-induced phenotype of type 2 diabetes.

Ultrastructural pathology and interorganelle cross talk in hepatotoxicity

Mitochondria, endoplasmic reticulum (ER), cytoplasmic lipid droplets (CLD), and Golgi vesicles use cross talk to control hepatocyte metabolism, growth, and stress. Interpretation of ultrastructural change requires knowledge of how cross talk pathways function, how differential activation of hepatocellular signals influences organelle structure, and how organelles position themselves to become central hubs for stress responses. Mitochondria, by coupling energy production to pathways for protection, form critical platforms for innate signaling. Mitochondrial outer and inner membranes activate channels and signals to translocate peptides that drive oxidative phosphorylation, β-oxidation of fatty acids, and calcium ion (Ca(2+)) flux. In cell stress, mitochondrial signals initiate fusion and fission, reactive oxygen species (ROS) control, autophagy, apoptosis, and senescence. Specialized tethering proteins tie mitochondria to ER to support translocation of metabolites. For Ca(2+) translocation, ER pores are connected to mitochondrial voltage-dependent anion channels, and for mitochondrial fission, unique membrane proteins pull ER to mitochondria. In toxic injury, cytosolic cytokines translocate to alter metabolism. Toxic effects on ER lipid synthesis lead to Golgi vesicle reduplication and transport of perilipin and other protein cargos into CLDs. How cellular proteostasis, oxidative homeostasis, and ion balance are maintained depend upon the effectiveness of mitochondrial ROS defense responses, unfolded protein responses in mitochondria and ER, and other organelle defenses.

Xenobiotic perturbation of ER stress and the unfolded protein response

The proper folding, assembly, and maintenance of cellular proteins is a highly regulated process and is critical for cellular homeostasis. Multiple cellular compartments have adapted their own systems to ensure proper protein folding, and quality control mechanisms are in place to manage stress due to the accumulation of unfolded proteins. When the accumulation of unfolded proteins exceeds the capacity to restore homeostasis, these systems can result in a cell death response. Unfolded protein accumulation in the endoplasmic reticulum (ER) leads to ER stress with activation of the unfolded protein response (UPR) governed by the activating transcription factor 6 (ATF6), inositol requiring enzyme-1 (IRE1), and PKR-like endoplasmic reticulum kinase (PERK) signaling pathways. Many xenobiotics have been shown to influence ER stress and UPR signaling with either pro-survival or pro-death features. The ultimate outcome is dependent on many factors including the mechanism of action of the xenobiotic, concentration of xenobiotic, duration of exposure (acute vs. chronic), cell type affected, nutrient levels, oxidative stress, state of differentiation, and others. Assessing perturbations in activation or inhibition of ER stress and UPR signaling pathways are likely to be informative parameters to measure when analyzing mechanisms of action of xenobiotic-induced toxicity.