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

Soluble epoxide hydrolase deficiency or inhibition attenuates diet-induced endoplasmic reticulum stress in liver and adipose tissue

Soluble epoxide hydrolase (sEH) is a cytosolic enzyme whose inhibition has beneficial effects in cardiovascular, inflammatory, and metabolic diseases in murine models. Mice with targeted deletion or pharmacological inhibition of sEH exhibit improved insulin signaling in liver and adipose tissue. Herein, we assessed the role of sEH in regulating endoplasmic reticulum (ER) stress in liver and adipose tissue. We report that sEH expression was increased in the livers and adipose tissue of mice fed a high fat diet, the adipose tissue of overweight humans, and palmitate-treated cells. Importantly, sEH deficiency or inhibition in mice attenuated chronic high fat diet-induced ER stress in liver and adipose tissue. Similarly, pharmacological inhibition of sEH in HepG2 cells and 3T3-L1 adipocytes mitigated chemical-induced ER stress and activation of JNK, p38, and cell death. In addition, insulin signaling was enhanced in HepG2 cells treated with sEH substrates and attenuated in cells treated with sEH products. In summary, these findings demonstrate that sEH is a physiological modulator of ER stress and a potential target for mitigating complications associated with obesity.

ER stress signalling through eIF2α and CHOP, but not IRE1α, attenuates adipogenesis in mice

AIMS/HYPOTHESIS

Although obesity is associated with endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR) in adipose tissue, it is not known how UPR signalling affects adipogenesis. To test whether signalling through protein kinase RNA-like ER kinase/eukaryotic initiation factor 2 alpha (PERK/eIF2α) or inositol-requiring enzyme 1 alpha/X-box binding protein 1 (IRE1α/XBP1) is required for adipogenesis, we studied the role of UPR signalling in adipocyte differentiation in vitro and in vivo in mice.

METHODS

The role of UPR signalling in adipogenesis was investigated using 3T3-L1 cells and primary mouse embryonic fibroblasts (MEFs) by activation or inhibition of PERK-mediated phosphorylation of the eIF2α- and IRE1α-mediated splicing of Xbp1 mRNA. Body weight change, fat mass composition and adipocyte number and size were measured in wild-type and genetically engineered mice fed a control or high-fat diet (HFD).

RESULTS

ER stress repressed adipocyte differentiation in 3T3-L1 cells. Impaired eIF2α phosphorylation enhanced adipocyte differentiation in MEFs, as well as in mice. In contrast, increased eIF2α phosphorylation reduced adipocyte differentiation in 3T3-L1 cells. Forced production of CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), a downstream target of eIF2α phosphorylation, inhibited adipogenesis in 3T3-L1 cells. Mice with deletion of Chop (also known as Ddit3) (Chop (-/-)) gained more fat mass than wild-type mice on HFD. In addition, Chop deletion in genetically obese Lepr (db/db) mice increased body fat mass without altering adipocyte size. In contrast to the eIF2α-CHOP pathway, activation or deletion of Ire1a (also known as Ern1) did not alter adipocyte differentiation in 3T3-L1 cells.

CONCLUSIONS/INTERPRETATION

These results demonstrate that eIF2α-CHOP suppresses adipogenesis and limits expansion of fat mass in vivo in mice, rendering this pathway a potential therapeutic target.

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.

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.

Minireview: endoplasmic reticulum stress: control in protein, lipid, and signal homeostasis

Endoplasmic reticulum (ER) stress is a regulatory mechanism that allows cells to adapt to a series of metabolic, redox, and other environmental changes. The role of ER stress was first identified in the maintenance of proteostasis. It has since been shown that ER stress is also critical to the regulation of lipid homeostasis, membrane turnover, and autophagy. ER stress initiates an intrinsic signaling network, the unfolded protein response, one component of the multifold and complex cellular signaling process system, which leads to major changes in the profiles of transcription factors. The unfolded protein response affects several other signaling routes through direct connections and also by indirect means. It directly influences hormone formation and life/death decisions at a cellular level; this relationship also involves connections to nutrient and environmental sensing-biotransformation processes. In conclusion, ER stress represents an integrated complex organelle response that makes an essential contribution to the maintenance of intracellular homeostasis.

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.

Nordihydroguaiaretic acid improves metabolic dysregulation and aberrant hepatic lipid metabolism in mice by both PPARα-dependent and -independent pathways

Creosote bush-derived nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, possesses antioxidant properties and functions as a potent antihyperlipidemic agent in rodent models. Here, we examined the effect of chronic NDGA treatment of ob/ob mice on plasma dyslipidemia, hepatic steatosis, and changes in hepatic gene expression. Feeding ob/ob mice a chow diet supplemented with either low (0.83 g/kg diet) or high-dose (2.5 g/kg diet) NDGA for 16 wk significantly improved plasma triglyceride (TG), inflammatory chemokine levels, hyperinsulinemia, insulin sensitivity, and glucose intolerance. NDGA treatment caused a marked reduction in liver weight and TG content, while enhancing rates of fatty acid oxidation. Microarray analysis of hepatic gene expression demonstrated that NDGA treatment altered genes for lipid metabolism, with genes involved in fatty acid catabolism most significantly increased. NDGA upregulated the mRNA and nuclear protein levels of peroxisome proliferator-activated receptor α (PPARα), and the activated (phosphorylated) form of AMP-activated kinase. NDGA increased PPARα promoter activity in AML12 hepatocytes and also prevented the fatty acid suppression of PPARα expression. In contrast, PPARα siRNA abrogated the stimulatory effect of NDGA on fatty acid catabolism. Likewise, no stimulatory effect of NDGA on hepatic fatty acid oxidation was observed in the livers of PPARα-deficient mice, but the ability of NDGA to reverse fatty liver conditions was unaffected. In conclusion, the beneficial actions of NDGA on dyslipidemia and hepatic steatosis in ob/ob mice are exerted primarily through enhanced fatty acid oxidation via PPARα-dependent pathways. However, PPARα-independent pathways also contribute to NDGA's action to ameliorate hepatic steatosis.

Endoplasmic reticulum and the unfolded protein response: dynamics and metabolic integration

The endoplasmic reticulum (ER) is a dynamic intracellular organelle with multiple functions essential for cellular homeostasis, development, and stress responsiveness. In response to cellular stress, a well-established signaling cascade, the unfolded protein response (UPR), is activated. This intricate mechanism is an important means of re-establishing cellular homeostasis and alleviating the inciting stress. Now, emerging evidence has demonstrated that the UPR influences cellular metabolism through diverse mechanisms, including calcium and lipid transfer, raising the prospect of involvement of these processes in the pathogenesis of disease, including neurodegeneration, cancer, diabetes mellitus and cardiovascular disease. Here, we review the distinct functions of the ER and UPR from a metabolic point of view, highlighting their association with prevalent pathologies.

Exposure to fine airborne particulate matter induces macrophage infiltration, unfolded protein response, and lipid deposition in white adipose tissue

Recent epidemiological studies have suggested a link between exposure to ambient air-pollution and susceptibility to metabolic disorders such as Type II diabetes mellitus. Previously, we provided evidence that both short- and long-term exposure to concentrated ambient particulate matter with aerodynamic diameter <2.5 μm (PM2.5) induces multiple abnormalities associated with the pathogenesis of Type II diabetes mellitus, including insulin resistance, visceral adipose inflammation, brown adipose mitochondrial adipose changes, and hepatic endoplasmic reticulum (ER) stress. In this report, we show that chronic inhalation exposure to PM2.5 (10 months exposure) induces macrophage infiltration and Unfolded Protein Response (UPR), an intracellular stress signaling that regulates cell metabolism and survival, in mouse white adipose tissue in vivo. Gene expression studies suggested that PM2.5 exposure induces two distinct UPR signaling pathways mediated through the UPR transducer inositol-requiring 1α (IRE1α): 1) ER-associated Degradation (ERAD) of unfolded or misfolded proteins, and 2) Regulated IRE1-dependent Decay (RIDD) of mRNAs. Along with the induction of the UPR pathways and macrophage infiltration, expression of genes involved in lipogenesis, adipocyte differentiation, and lipid droplet formation was increased in the adipose tissue of the mice exposed to PM2.5. In vitro study confirmed that PM2.5 can trigger phosphorylation of the UPR transducer IRE1α and activation of macrophages. These results provide novel insights into PM2.5-triggered cell stress response in adipose tissue and increase our understanding of pathophysiological effects of particulate air pollution on the development of metabolic disorders.

C/EBP homologous protein (CHOP) contributes to suppression of metabolic genes during endoplasmic reticulum stress in the liver

The unfolded protein response (UPR) senses stress in the endoplasmic reticulum (ER) and initiates signal transduction cascades that culminate in changes to gene regulation. Long recognized as a means for improving ER protein folding through up-regulation of ER chaperones, the UPR is increasingly recognized to play a role in the regulation of metabolic pathways. ER stress is clearly connected to altered metabolism in tissues such as the liver, but the mechanisms underlying this connection are only beginning to be elucidated. Here, working exclusively in vivo, we tested the hypothesis that the UPR-regulated CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) participates in the transcriptional regulation of metabolism during hepatic ER stress. We found that metabolic dysregulation was associated with induction of eIF2α signaling and CHOP up-regulation during challenge with tunicamycin or Velcade. CHOP was necessary for suppression of genes encoding the transcriptional master regulators of lipid metabolism: Cebpa, Ppara, and Srebf1. This action of CHOP required a contemporaneous CHOP-independent stress signal. CHOP bound directly to C/EBP-binding regions in the promoters of target genes, whereas binding of C/EBPα and C/EBPβ to the same regions was diminished during ER stress. Our results thus highlight a role for CHOP in the transcriptional regulation of metabolism.

Impairment of endoplasmic reticulum in liver as an early consequence of the systemic inflammatory response in rats

It is well known that systemic inflammatory response (SIR) often causes liver dysfunction. The aim of this study was to identify the intracellular compartment in the liver most susceptible to SIR. We analyzed morphology, ultrastructure, proteome, and expression of relevant genes in livers of rats subjected to endotoxic shock. Histological examination revealed that focal necrosis in liver was insignificant to explain liver dysfunction. Electron microscopy revealed no morphological changes in the mitochondrial structure and in the cytosol, but dilated endoplasmic reticulum (ER) cisterns were frequently observed. Apoptosis was found in white blood cells within liver tissue but not in hepatocytes. Mitochondrial, ER, and cytosolic fractions were subjected to proteome analysis by difference gel electrophoresis, and the protein spots with the highest degree of differential regulation were identified with mass spectrometry. The most pronounced proteome changes appeared in the ER, manifested as a remarkable downregulation of several proteins essential for ER functions, such as protein synthesis and transport, whereas the changes in mitochondrial and cytosolic fractions suggested a compensatory response. ER stress, as an underlying mechanism for ER impairment, was confirmed by analysis of upstream (splicing X-box-binding protein 1 mRNA) and downstream (e.g., 78-kDa glucose-regulated protein mRNA) markers, suggesting ongoing unresolved ER stress as a cause for ER dilation. Because ER is the intracellular compartment responsible for the major liver functions, our data suggest that inflammatory mediators induce unresolved ER stress, resulting in the biochemical, functional, and morphological impairment of ER that in turn causes liver dysfunction. The pathway activating ER stress in response to SIR is not known yet.

Endoplasmic reticulum stress in nonalcoholic fatty liver disease

The underlying causes of nonalcoholic fatty liver disease are unclear, although recent evidence has implicated the endoplasmic reticulum in both the development of steatosis and progression to nonalcoholic steatohepatitis. Disruption of endoplasmic reticulum homeostasis, often termed ER stress, has been observed in liver and adipose tissue of humans with nonalcoholic fatty liver disease and/or obesity. Importantly, the signaling pathway activated by disruption of endoplasmic reticulum homeostasis, the unfolded protein response, has been linked to lipid and membrane biosynthesis, insulin action, inflammation, and apoptosis. Therefore, understanding the mechanisms that disrupt endoplasmic reticulum homeostasis in nonalcoholic fatty liver disease and the role of the unfolded protein response in the broader context of chronic, metabolic diseases have become topics of intense investigation. The present review examines the endoplasmic reticulum and the unfolded protein response in the context of nonalcoholic fatty liver disease.

Stress management: How the unfolded protein response impacts fatty liver disease

Induction of the unfolded protein response (UPR) is recognized as central to fatty liver disease (FLD) pathophysiology. This pathway may be a potential therapeutic target for FLD, as well as other diseases. However, fundamental questions as to how UPR contributes to FLD remain unanswered. Conflicting data suggest that this pathway can both protect against and augment this disease. Here, we review the relationship between protein secretion, endoplasmic reticulum function (ER), and UPR activation. The UPR serves to maintain secretory pathway homeostasis by enhancing the protein folding environment in the ER, and we review data investigating the role for individual UPR players in fatty liver (steatosis). We explore a novel concept in the field that all cases of UPR activation do not equal "ER stress". Rather, different types of UPRs that can either protect against or cause FLD are discussed. Refining our current understanding of this complex pathway is particularly important, as drugs that affect the protein folding environment in the ER and affect UPR activation are being successful in clinical trials for FLD.

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

The unfolded protein response (UPR) is a signaling pathway required to maintain endoplasmic reticulum (ER) homeostasis and hepatic lipid metabolism. Here, we identify an essential role for the inositol-requiring transmembrane kinase/endoribonuclease 1α (IRE1α)-X box binding protein 1 (XBP1) arm of the UPR in regulation of hepatic very low-density lipoprotein (VLDL) assembly and secretion. Hepatocyte-specific deletion of Ire1α reduces lipid partitioning into the ER lumen and impairs the assembly of triglyceride (TG)-rich VLDL but does not affect TG synthesis, de novo lipogenesis, or the synthesis or secretion of apolipoprotein B (apoB). The defect in VLDL assembly is, at least in part, due to decreased microsomal triglyceride-transfer protein (MTP) activity resulting from reduced protein disulfide isomerase (PDI) expression. Collectively, our findings reveal a key role for the IRE1α-XBP1s-PDI axis in linking ER homeostasis with regulation of VLDL production and hepatic lipid homeostasis that may provide a therapeutic target for disorders of lipid metabolism.

Silencing of lipid metabolism genes through IRE1α-mediated mRNA decay lowers plasma lipids in mice

XBP1 is a key regulator of the unfolded protein response (UPR), which is involved in a wide range of physiological and pathological processes. XBP1 ablation in liver causes profound hypolipidemia in mice, highlighting its critical role in lipid metabolism. XBP1 deficiency triggers feedback activation of its upstream enzyme IRE1α, instigating regulated IRE1-dependent decay (RIDD) of cytosolic mRNAs. Here, we identify RIDD as a crucial control mechanism of lipid homeostasis. Suppression of RIDD by RNA interference or genetic ablation of IRE1α reversed hypolipidemia in XBP1-deficient mice. Comprehensive microarray analysis of XBP1 and/or IRE1α-deficient liver identified genes involved in lipogenesis and lipoprotein metabolism as RIDD substrates, which might contribute to the suppression of plasma lipid levels by activated IRE1α. Ablation of XBP1 ameliorated hepatosteatosis, liver damage, and hypercholesterolemia in dyslipidemic animal models, suggesting that direct targeting of either IRE1α or XBP1 might be a feasible strategy to treat dyslipidemias.

The hyperglycemia stimulated myocardial endoplasmic reticulum (ER) stress contributes to diabetic cardiomyopathy in the transgenic non-obese type 2 diabetic rats: a differential role of unfolded protein response (UPR) signaling proteins

It has been well demonstrated that excessive blood glucose level could be detrimental to the myocardial function through the variety of mechanisms, of which endoplasmic reticulum stress (ERS) could play an unprecedented role through the activation of unfolded protein response (UPR). Recently, reports are coming out with the evidences that UPR signaling proteins are regulated differentially depend on the experimental conditions and cell types. In addition, ERS has been proposed to be closely associated with the regulation of lipogenesis. Therefore, in this study we tried to find out the expressions of myocardial UPR signaling proteins as well as proteins involved in lipid and glucose metabolism in non-obese type 2 diabetic mellitus (DM) condition using Spontaneous Diabetic Torii (SDT) rat. We have found the significant up-regulation of oxidative, nitrosative and ERS marker proteins in the myocardium of the SDT rats, in comparison to its normal (Sprague-Dawley - SD) rats. In addition, the sub-arm of UPR signaling proteins, such as p-PERK, p-eIF2α, ATF6, CHOP/GADD153, TRAF2, apoptotic signaling proteins, such as BAD, cytochrome C, cleaved caspase-7 and -12, were significantly up-regulated in the SDT rats, in comparison to the SD rats. Interestingly, there were no significant changes in the phosphorylation of IRE-1α, and XBP-1 protein expression. In addition, the proteins involved in lipid and glucose metabolisms, such as PPARα, PPARγ, CPT1, PGC-1α except GLUT4, and the proteins involved in insulin signaling, such as p-Akt and p-PI3K were shown significant attenuation in its expressions in the SDT rats, when compared with the SD rats. Taken together, it is suggested that the activation of PERK and ATF6 pathway are the major determinant rather than the IRE-1α-XBP1 pathway for the ERS-mediated metabolic dysfunction, which might eventually leads to diabetic cardiomyopathy in non-obese type 2 DM.

The impact of the unfolded protein response on human disease

A central function of the endoplasmic reticulum (ER) is to coordinate protein biosynthetic and secretory activities in the cell. Alterations in ER homeostasis cause accumulation of misfolded/unfolded proteins in the ER. To maintain ER homeostasis, eukaryotic cells have evolved the unfolded protein response (UPR), an essential adaptive intracellular signaling pathway that responds to metabolic, oxidative stress, and inflammatory response pathways. The UPR has been implicated in a variety of diseases including metabolic disease, neurodegenerative disease, inflammatory disease, and cancer. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human disease.

The unfolded protein response in secretory cell function

The endoplasmic reticulum (ER) controls many important aspects of cellular function, including processing of secreted and membrane proteins, synthesis of membranes, and calcium storage. Maintenance of ER function is controlled through a network of signaling pathways collectively known as the unfolded protein response (UPR). The UPR balances the load of incoming proteins with the folding capacity of the ER and allows cells to adapt to situations that disrupt this balance. This disruption is referred to as ER stress. Although ER stress often arises in pathological situations, the UPR plays a central role in the normal development and function of cells specializing in secretion. Many aspects of this response are conserved broadly across eukaryotes; most organisms use some subset of a group of ER transmembrane proteins to signal to the nucleus and induce a broad transcriptional upregulation of genes involved in ER function. However, new developments in metazoans, plants, and fungi illustrate interesting variations on this theme. Here, we summarize mechanisms for detecting and counteracting ER stress, the role of the UPR in normal secretory cell function, and how these pathways vary across organisms and among different tissues and cell types.

Do hepatitis B virus and hepatitis C virus co-infections increase hepatocellular carcinoma occurrence through synergistically modulating lipogenic gene expression?

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infections cause a wide range of liver diseases including hepatocellular carcinoma (HCC). Because of the similar modes of transmission, HBV HCV co-infections are found in approximately 7-20 million people globally. Compared with HBV or HCV mono-infections, co-infections are associated with more severe liver diseases and higher risk of HCC. Abnormal lipid biosynthesis and metabolism has been increasingly recognized as a cause for cancer. While HBV infection does not seem to significantly increase the risk of developing hepatic steatosis, steatosis is a prominent feature of chronic hepatitis C (CHC). In addition, steatosis in HBV or HCV mono-infections is a significant and independent risk factor for HCC. However, whether and how HBV HCV co-infections synergistically increase the risk of HCC development through modulating lipid metabolism is not well understood. Possible mechanisms by which steatosis causes HCC include: activation of sterol regulatory element-binding protein-mediated lipogenesis through the PI3K-Akt pathway, abnormal activation of peroxisome proliferator-activated receptors and endoplasmic reticulum stress. Here, we review the potential mechanisms by which HBV HCV co-infections may increase HCC risk through modulation of lipogenic gene expression. We begin with reviewing the impact of HBV and HCV on host lipogenic gene expression and carcinogenesis. We then discuss the potential mechanisms by which HBV and HCV can increase carcinogenesis through synergistically activating lipid biosynthesis and metabolism. We end by sharing our thoughts on future research directions in this emerging paradigm with an ultimate goal of developing effective therapeutics.

New insights into ER stress-induced insulin resistance

Insulin resistance is a major characteristic of obesity and type 2 diabetes (T2DM). During the last decade, endoplasmic reticulum (ER) stress has emerged as a new player in this field and a considerable number of recent studies have pointed out its role in the onset of insulin resistance (IR). ER stress appears to act directly as a negative modulator of the insulin signaling pathway but also indirectly by promoting lipid accumulation. This review aims to summarize and decipher the abundant new literature concerning the emerging and multifaceted involvement of ER stress in the development of metabolic dysfunctions in insulin target tissues.

ERLIN2 promotes breast cancer cell survival by modulating endoplasmic reticulum stress pathways

Background

Amplification of the 8p11-12 region has been found in approximately 15% of human breast cancer and is associated with poor prognosis. Previous genomic analysis has led us to identify the endoplasmic reticulum (ER) lipid raft-associated 2 (ERLIN2) gene as one of the candidate oncogenes within the 8p11-12 amplicon in human breast cancer, particularly in the luminal subtype. ERLIN2, an ER membrane protein, has recently been identified as a novel mediator of ER-associated degradation. Yet, the biological roles of ERLIN2 and molecular mechanisms by which ERLIN2 coordinates ER pathways in breast carcinogenesis remain unclear.

Methods

We established the MCF10A-ERLIN2 cell line, which stably over expresses ERLIN2 in human nontransformed mammary epithelial cells (MCF10A) using the pLenti6/V5-ERLIN2 construct. ERLIN2 over expressing cells and their respective parental cell lines were assayed for in vitro transforming phenotypes. Next, we knocked down the ERLIN2 as well as the ER stress sensor IRE1α activity in the breast cancer cell lines to characterize the biological roles and molecular basis of the ERLIN2 in carcinogenesis. Finally, immunohistochemical staining was performed to detect ERLIN2 expression in normal and cancerous human breast tissues

Results

We found that amplification of the ERLIN2 gene and over expression of the ERLIN2 protein occurs in both luminal and Her2 subtypes of breast cancer. Gain- and loss-of-function approaches demonstrated that ERLIN2 is a novel oncogenic factor associated with the ER stress response pathway. The IRE1α/XBP1 axis in the ER stress pathway modulated expression of ERLIN2 protein levels in breast cancer cells. We also showed that over expression of ERLIN2 facilitated the adaptation of breast epithelial cells to ER stress by supporting cell growth and protecting the cells from ER stress-induced cell death.

Conclusions

ERLIN2 may confer a selective growth advantage for breast cancer cells by facilitating a cytoprotective response to various cellular stresses associated with oncogenesis. The information provided here sheds new light on the mechanism of breast cancer malignancy

Mechanisms for insulin resistance: common threads and missing links

Insulin resistance is a complex metabolic disorder that defies explanation by a single etiological pathway. Accumulation of ectopic lipid metabolites, activation of the unfolded protein response (UPR) pathway, and innate immune pathways have all been implicated in the pathogenesis of insulin resistance. However, these pathways are also closely linked to changes in fatty acid uptake, lipogenesis, and energy expenditure that can impact ectopic lipid deposition. Ultimately, these cellular changes may converge to promote the accumulation of specific lipid metabolites (diacylglycerols and/or ceramides) in liver and skeletal muscle, a common final pathway leading to impaired insulin signaling and insulin resistance.

Endoplasmic reticulum-tethered transcription factor cAMP responsive element-binding protein, hepatocyte specific, regulates hepatic lipogenesis, fatty acid oxidation, and lipolysis upon metabolic stress in mice

cAMP responsive element-binding protein, hepatocyte specific (CREBH), is a liver-specific transcription factor localized in the endoplasmic reticulum (ER) membrane. Our previous work demonstrated that CREBH is activated by ER stress or inflammatory stimuli to induce an acute-phase hepatic inflammation. Here, we demonstrate that CREBH is a key metabolic regulator of hepatic lipogenesis, fatty acid (FA) oxidation, and lipolysis under metabolic stress. Saturated FA, insulin signals, or an atherogenic high-fat diet can induce CREBH activation in the liver. Under the normal chow diet, CrebH knockout mice display a modest decrease in hepatic lipid contents, but an increase in plasma triglycerides (TGs). After having been fed an atherogenic high-fat (AHF) diet, massive accumulation of hepatic lipid metabolites and significant increase in plasma TG levels were observed in the CrebH knockout mice. Along with the hypertriglyceridemia phenotype, the CrebH null mice displayed significantly reduced body-weight gain, diminished abdominal fat, and increased nonalcoholic steatohepatitis activities under the AHF diet. Gene-expression analysis and chromatin-immunoprecipitation assay indicated that CREBH is required to activate the expression of the genes encoding functions involved in de novo lipogenesis, TG and cholesterol biosynthesis, FA elongation and oxidation, lipolysis, and lipid transport. Supporting the role of CREBH in lipogenesis and lipolysis, forced expression of an activated form of CREBH protein in the liver significantly increases accumulation of hepatic lipids, but reduces plasma TG levels in mice.

CONCLUSION

All together, our study shows that CREBH plays a key role in maintaining lipid homeostasis by regulating the expression of the genes involved in hepatic lipogenesis, FA oxidation, and lipolysis under metabolic stress. The identification of CREBH as a stress-inducible metabolic regulator has important implications in the understanding and treatment of metabolic disease.

Pharmacologic ER stress induces non-alcoholic steatohepatitis in an animal model

Endoplasmic reticulum (ER) stress refers to a condition of accumulation of unfolded or misfolded proteins in the ER lumen, which is known to activate an intracellular stress signaling termed Unfolded Protein Response (UPR). A number of pharmacologic reagents or pathophysiologic stimuli can induce ER stress and activation of the UPR signaling, leading to alteration of cell physiology that is associated with the initiation and progression of a variety of diseases. Non-alcoholic steatohepatitis (NASH), characterized by hepatic steatosis and inflammation, has been considered the precursor or the hepatic manifestation of metabolic disease. In this study, we delineated the toxic effect and molecular basis by which pharmacologic ER stress, induced by a bacterial nucleoside antibiotic tunicamycin (TM), promotes NASH in an animal model. Mice of C57BL/6J strain background were challenged with pharmacologic ER stress by intraperitoneal injection of TM. Upon TM injection, mice exhibited a quick NASH state characterized by hepatic steatosis and inflammation. An increase in hepatic triglycerides (TG) and a decrease in plasma lipids, including plasma TG, plasma cholesterol, high-density lipoprotein (HDL), and low-density lipoprotein (LDL), were observed in the TM-treated mice. In response to TM challenge, cleavage of sterol responsive binding protein (SREBP)-1a and SREBP-1c, the key trans-activators for lipid and sterol biosynthesis, was dramatically increased in the liver. Consistent with the hepatic steatosis phenotype, expression of some key regulators and enzymes in de novo lipogenesis and lipid droplet formation was up-regulated, while expression of those involved in lipolysis and fatty acid oxidation was down-regulated in the liver of mice challenged with TM. Moreover, TM treatment significantly increased phosphorylation of NF-κB inhibitors (IκB), leading to the activation of NF-κB-mediated inflammatory pathway in the liver. Our study not only confirmed that pharmacologic ER stress is a strong "hit" that triggers NASH, but also demonstrated crucial molecular links between ER stress, lipid metabolism, and inflammation in the liver in vivo.

IRE1 directs proteasomal and lysosomal degradation of misfolded rhodopsin

Endoplasmic reticulum (ER) is responsible for folding of secreted and membrane proteins in eukaryotic cells. Disruption of ER protein folding leads to ER stress. Chronic ER stress can cause cell death and is proposed to underlie the pathogenesis of many human diseases. Inositol-requiring enzyme 1 (IRE1) directs a key unfolded protein response signaling pathway that controls the fidelity of ER protein folding. IRE1 signaling may be particularly helpful in preventing chronic ER stress and cell injury by alleviating protein misfolding in the ER. To examine this, we used a chemical-genetic approach to selectively activate IRE1 in mammalian cells and tested how artificial IRE1 signaling affected the fate of misfolded P23H rhodopsin linked to photoreceptor cell death. We found that IRE1 signaling robustly promoted the degradation of misfolded P23H rhodopsin without affecting its wild-type counterpart. We also found that IRE1 used both proteasomal and lysosomal degradation pathways to remove P23H rhodopsin. Surprisingly, when one degradation pathway was compromised, IRE1 signaling could still promote misfolded rhodopsin degradation using the remaining pathway. Last, we showed that IRE1 signaling also reduced levels of several other misfolded rhodopsins with lesser effects on misfolded cystic fibrosis transmembrane conductance regulator. Our findings reveal the diversity of proteolytic mechanisms used by IRE1 to eliminate misfolded rhodopsin.

Endoplasmic Reticulum Stress-Associated Lipid Droplet Formation and Type II Diabetes

Diabetes mellitus (DM), a metabolic disorder characterized by hyperglycemia, is caused by insufficient insulin production due to excessive loss of pancreatic β cells (type I diabetes) or impaired insulin signaling due to peripheral insulin resistance (type II diabetes). Pancreatic β cell is the only insulin-secreting cell type that has highly developed endoplasmic reticulum (ER) to cope with high demands of insulin synthesis and secretion. Therefore, ER homeostasis is crucial to the proper function of insulin signaling. Accumulating evidence suggests that deleterious ER stress and excessive intracellular lipids in nonadipose tissues, such as myocyte, cardiomyocyte, and hepatocyte, cause pancreatic β-cell dysfunction and peripheral insulin resistance, leading to type II diabetes. The excessive deposition of lipid droplets (LDs) in specialized cell types, such as adipocytes, hepatocytes, and macrophages, has been found as a hallmark in ER stress-associated metabolic diseases, including obesity, diabetes, fatty liver disease, and atherosclerosis. However, much work remains to be done in understanding the mechanism by which ER stress response regulates LD formation and the pathophysiologic role of ER stress-associated LD in metabolic disease. This paper briefly summarizes the recent advances in ER stress-associated LD formation and its involvement in type II diabetes.

IRE1α activation protects mice against acetaminophen-induced hepatotoxicity

The mammalian stress sensor IRE1α plays a central role in the unfolded protein, or endoplasmic reticulum (ER), stress response by activating its downstream transcription factor XBP1 via an unconventional splicing mechanism. IRE1α can also induce the degradation of a subset of mRNAs in a process termed regulated IRE1-dependent decay (RIDD). Although diverse mRNA species can be degraded by IRE1α in vitro, the pathophysiological functions of RIDD are only beginning to be explored. Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure in young adults in the United States and is primarily caused by CYP1A2-, CYP2E1-, and CYP3A4-driven conversion of APAP into hepatotoxic metabolites. We demonstrate here that genetic ablation of XBP1 results in constitutive IRE1α activation in the liver, leading to RIDD of Cyp1a2 and Cyp2e1 mRNAs, reduced JNK activation, and protection of mice from APAP-induced hepatotoxicity. A pharmacological ER stress inducer that activated IRE1α suppressed the expression of Cyp1a2 and Cyp2e1 in WT, but not IRE1α-deficient mouse liver, indicating the essential role of IRE1α in the down-regulation of these mRNAs upon ER stress. Our study reveals an unexpected function of RIDD in drug metabolism.

Pentoxifylline attenuates methionine- and choline-deficient-diet-induced steatohepatitis by suppressing TNF-α expression and endoplasmic reticulum stress

BACKGROUND

Pentoxifylline (PTX) anti-TNF properties are known to exert hepatoprotective effects in various liver injury models. The aim of this study was to investigate whether PTX has beneficial roles in the development of methionine- and choline-deficient-(MCD-) diet-induced NAFLD SD rats in vivo and TNF-α-induced Hep3B cells in vitro.

METHODS

SD Rats were classified according to diet (chow or MCD diet) and treatment (normal saline or PTX injection) over a period of 4 weeks: group I (chow + saline, n = 4), group II (chow + PTX), group III (MCD + saline), and group IV (MCD + PTX). Hep3B cells were treated with 100 ng/ml TNF-α (24 h) in the absence or presence of PTX (1 mM).

RESULTS

PTX attenuated MCD-diet-induced serum ALT levels and hepatic steatosis. In real-time PCR and western blotting analysis, PTX decreased MCD-diet-induced TNF-alpha mRNA expression and proapoptotic unfolded protein response by ER stress (GRP78, p-eIF2, ATF4, IRE1α, CHOP, and p-JNK activation) in vivo. PTX (1 mM) reduced TNF-α-induced activation of GRP78, p-eIF2, ATF4, IRE1α, and CHOP in vitro.

CONCLUSION

PTX has beneficial roles in the development of MCD-diet-induced steatohepatitis through partial suppression of TNF-α and ER stress.

The unfolded protein response: controlling cell fate decisions under ER stress and beyond

Protein-folding stress at the endoplasmic reticulum (ER) is a salient feature of specialized secretory cells and is also involved in the pathogenesis of many human diseases. ER stress is buffered by the activation of the unfolded protein response (UPR), a homeostatic signalling network that orchestrates the recovery of ER function, and failure to adapt to ER stress results in apoptosis. Progress in the field has provided insight into the regulatory mechanisms and signalling crosstalk of the three branches of the UPR, which are initiated by the stress sensors protein kinase RNA-like ER kinase (PERK), inositol-requiring protein 1α (IRE1α) and activating transcription factor 6 (ATF6). In addition, novel physiological outcomes of the UPR that are not directly related to protein-folding stress, such as innate immunity, metabolism and cell differentiation, have been revealed.

Pharmacological ER stress promotes hepatic lipogenesis and lipid droplet formation

Endoplasmic Reticulum (ER) stress refers to a condition of accumulation of unfolded or misfolded proteins in the ER lumen. A variety of biochemical stimuli or pathophysiologic conditions can directly or indirectly induce ER stress, leading to activation of an ER-originated adaptive signaling response called Unfolded Protein Response (UPR). Recent studies demonstrated that ER stress and UPR signaling are critically involved in the initiation and progression of many diseases, such as metabolic disease, cardiovascular disease, neurodegenerative disease, and cancer. In this study, we show that ER stress induced by pharmacologic reagents, including tunicamycin (TM) and thapsigargin (Tg), promotes hepatic lipogenesis and lipid droplet formation. Using quantitative gene expression analysis, we identified 3 groups of key lipogenic regulators or enzymes that are inducible by pharmacological ER stress in a human hepatoma cell line Huh-7. These ER stress-inducible lipogenic factors include: 1) lipogenic trans-activators including CCAAT/ enhancer binding protein alpha (C/EBPα), peroxisome proliferator-activated receptor gamma (PPARγ), PPARγ coacti-vator 1-alpha (PGC1α), and Liver X receptor alpha (LXRα); 2) components of lipid droplets including fat-specific protein 27 (FSP27), adipose differentiation related protein (ADRP), fat-inducing transcript 2 (FIT2), and adipocyte lipid-binding protein (AP2); 3) key enzymes involved in de novo lipogenesis including acetyl-CoA carboxylase 1 (ACC1) and stearoyl-CoA desaturase-1 (SCD1). Supporting the role of pharmacologic ER stress in up-regulating de novo lipogenesis, TM or Tg treatment significantly increased accumulation of cytosolic lipid droplet formation in the hepatocytes. Moreover, we showed that forced expression of an activated form of X-box binding protein 1 (XBP1), a potent UPR trans-activator, can dramatically increase expression of PPARγ and C/EBPα in Huh-7 cells. The identification of ER stress-inducible lipogenic regulators provides important insights into the molecular basis by which acute ER stress promotes de novo lipogenesis. In summary, the findings from this study have important implication in understanding the link between ER stress and metabolic disease.

Activation of the unfolded protein response pathway causes ceramide accumulation in yeast and INS-1E insulinoma cells

Sphingolipids are not only important components of membranes but also have functions in protein trafficking and intracellular signaling. The LCB1 gene encodes a subunit of the serine palmitoyltransferase, which is responsible for the first step of sphingolipid synthesis. Here, we show that activation of the unfolded protein response (UPR) can restore normal ceramide levels and viability in yeast cells with a conditional defect in LCB1. Dependence on UPR was demonstrated by showing the HAC1-dependence of the suppression. A similar induction of ceramides by UPR seems to take place in mammalian cells. In rat pancreatic INS-1E cells, UPR activation induces the transcription of the CerS6 gene, which encodes a ceramide synthase. This correlates with the specific accumulation of ceramide with a C16 fatty acyl chain upon UPR activation. Therefore, our study reveals a novel connection between UPR induction and ceramide synthesis that seems to be conserved between yeast and mammalian cells.

Endoplasmic reticulum stress and lipid metabolism: mechanisms and therapeutic potential

The endoplasmic reticulum (ER) plays a crucial role in protein folding, assembly, and secretion. Disruption of ER homeostasis may lead to accumulation of misfolded or unfolded proteins in the ER lumen, a condition referred to as ER stress. In response to ER stress, a signal transduction pathway known as the unfolded protein response (UPR) is activated. UPR activation allows the cell to cope with an increased protein-folding demand on the ER. Recent studies have shown that ER stress/UPR activation plays a critical role in lipid metabolism and homeostasis. ER-stress-dependent dysregulation of lipid metabolism may lead to dyslipidemia, insulin resistance, cardiovascular disease, type 2 diabetes, and obesity. In this paper, we examine recent findings illustrating the important role ER stress/UPR signalling pathways play in regulation of lipid metabolism, and how they may lead to dysregulation of lipid homeostasis.

The unfolded protein response: integrating stress signals through the stress sensor IRE1α

Stress induced by accumulation of unfolded proteins at the endoplasmic reticulum (ER) is a classic feature of secretory cells and is observed in many tissues in human diseases including cancer, diabetes, obesity, and neurodegeneration. Cellular adaptation to ER stress is achieved by the activation of the unfolded protein response (UPR), an integrated signal transduction pathway that transmits information about the protein folding status at the ER to the nucleus and cytosol to restore ER homeostasis. Inositol-requiring transmembrane kinase/endonuclease-1 (IRE1α), the most conserved UPR stress sensor, functions as an endoribonuclease that processes the mRNA of the transcription factor X-box binding protein-1 (XBP1). IRE1α signaling is a highly regulated process, controlled by the formation of a dynamic scaffold onto which many regulatory components assemble, here referred to as the UPRosome. Here we provide an overview of the signaling and regulatory mechanisms underlying IRE1α function and discuss the emerging role of the UPR in adaptation to protein folding stress in specialized secretory cells and in pathological conditions associated with alterations in ER homeostasis.

PKA phosphorylation couples hepatic inositol-requiring enzyme 1alpha to glucagon signaling in glucose metabolism

The endoplasmic reticulum (ER)-resident protein kinase/endoribonuclease inositol-requiring enzyme 1 (IRE1) is activated through transautophosphorylation in response to protein folding overload in the ER lumen and maintains ER homeostasis by triggering a key branch of the unfolded protein response. Here we show that mammalian IRE1α in liver cells is also phosphorylated by a kinase other than itself in response to metabolic stimuli. Glucagon-stimulated protein kinase PKA, which in turn phosphorylated IRE1α at Ser(724), a highly conserved site within the kinase activation domain. Blocking Ser(724) phosphorylation impaired the ability of IRE1α to augment the up-regulation by glucagon signaling of the expression of gluconeogenic genes. Moreover, hepatic IRE1α was highly phosphorylated at Ser(724) by PKA in mice with obesity, and silencing hepatic IRE1α markedly reduced hyperglycemia and glucose intolerance. Hence, these results suggest that IRE1α integrates signals from both the ER lumen and the cytoplasm in the liver and is coupled to the glucagon signaling in the regulation of glucose metabolism.

Endoplasmic reticulum stress, obesity and diabetes

The endoplasmic reticulum (ER) stress response, also commonly known as the unfolded protein response (UPR), is an adaptive response used to align ER functional capacity with demand. It is activated in various tissues under conditions related to obesity and type 2 diabetes. Hypothalamic ER stress contributes to inflammation and leptin/insulin resistance. Hepatic ER stress contributes to the development of steatosis and insulin resistance, and components of the UPR regulate liver lipid metabolism. ER stress in enlarged fat tissues induces inflammation and modifies adipokine secretion, and saturated fats cause ER stress in muscle. Finally, prolonged ER stress impairs insulin synthesis and causes pancreatic β cell apoptosis. In this review, we discuss ways in which ER stress operates as a common molecular pathway in the pathogenesis of obesity and diabetes.