Endoplasmic reticulum stress and inflammation

Endoplasmic reticulum (ER) stress due to the presence of misfolded or unfolded proteins in the ER invokes a fundamental biological response, termed the unfolded protein response (UPR). The UPR is orchestrated by three main proximal effectors, of which the IRE1/XBP1 pathway represents the evolutionarily most conserved one. Due to its high secretory burden, the intestinal epithelium is highly susceptible to perturbations in the UPR as has been revealed by functional investigations such as in mice that lack Xbp1 expression, specifically in the intestinal epithelial cells. Genetic studies have revealed several ER stress/UPR-associated genes, including XBP1, ORMDL3, AGR2 and MUC19 as risk factors for IBD, and specific functional, rare variants have been described for XBP1. Xbp1(Δ)(IEC) mice spontaneously develop small intestinal enteritis with crypt abscesses reminiscent of human IBD, while Agr2(-/-) mice develop granulomatous ileocolitis. Mechanistic studies into Xbp1(Δ)(IEC) mice revealed a major effect on Paneth cells associated with alterations in host-microbe interactions in the intestine, and the activation of key proinflammatory pathways in the host directly associated with unresolved ER stress and hypomorphic Xbp1 function. Remarkably, the intestinal epithelium of IBD patients commonly exhibits evidence of marked ER stress, which cannot easily be attributed to these genetic risk factors alone and indicates that the paradigm of ER stress-related inflammation might be both a primary originator as well as a potent perpetuator of intestinal inflammation in IBD.

The ATF6 branch of unfolded protein response and apoptosis are activated to promote African swine fever virus infection

African swine fever virus (ASFV) infection induces apoptosis in the infected cell; however, the consequences of this activation on virus replication have not been defined. In order to identify the role of apoptosis in ASFV infection, we analyzed caspase induction during the infection and the impact of caspase inhibition on viral production. Caspases 3, 9 and 12 were activated from 16 h post-infection, but not caspase 8. Indeed, caspase 3 activation during the early stages of the infection appeared to be crucial for efficient virus exit. In addition, the inhibition of membrane blebbing reduced the release of virus particles from the cell. ASFV uses the endoplasmic reticulum (ER) as a site of replication and this process can trigger ER stress and the unfolded protein response (UPR) of the host cell. In addition to caspase 12 activation, indicators of ER stress include the upregulation of the chaperones calnexin and calreticulin upon virus infection. Moreover, ASFV induces transcription factor 6 signaling pathway of the UPR, but not the protein kinase-like ER kinase or the inositol-requiring enzyme 1 pathways. Thus, the capacity of ASFV to regulate the UPR may prevent early apoptosis and ensure viral replication.

Herpes simplex virus-1 disarms the unfolded protein response in the early stages of infection

Accumulation of mis- and unfolded proteins during viral replication can cause stress in the endoplasmic reticulum (ER) and trigger the unfolded protein response (UPR). If unchecked, this process may induce cellular changes detrimental to viral replication. In the report, we investigated the impact of HSV-1 on the UPR during lytic replication. We found that HSV-1 effectively disarms the UPR in early stages of viral infection. Only ATF6 activation was detected during early infection, but with no upregulation of target chaperone proteins. Activity of the eIF2α/ATF4 signaling arm increased at the final stage of HSV-1 replication, which may indicate completion of virion assembly and egress, thus releasing suppression of the UPR. We also found that the promoter of viral ICP0 was responsive to ER stress, an apparent mimicry of cellular UPR genes. These results suggest that HSV-1 may use ICP0 as a sensor to modulate the cellular stress response.

Reducing endoplasmic reticulum stress does not improve steatohepatitis in mice fed a methionine- and choline-deficient diet

Endoplasmic reticulum (ER) stress has been implicated in the pathogenesis of nonalcoholic steatohepatitis. The ER stress response is activated in the livers of mice fed a methionine- and choline-deficient (MCD) diet, yet the role of ER stress in the pathogenesis of MCD diet-induced steatohepatitis is unknown. Using chemical chaperones on hepatic steatosis and markers of inflammation and fibrosis in mice fed a MCD diet, we aim to determine the effects of reducing ER stress. C57BL/6J mice were fed a MCD diet with or without the ER chemical chaperones 4-phenylbutyric acid (PBA) and tauroursodeoxycholic acid (TUDCA) for 2 wk. TUDCA and PBA effectively attenuated the ER stress response in MCD diet-fed mice, as evidenced by reduced protein levels of phosphorylated eukaryotic initiation factor 2α and phosphorylated JNK and suppression of mRNA levels of CCAAT/enhancer binding protein homologous protein, glucose-regulated protein 78 kDa, and X-box binding protein 1. However, PBA and TUDCA did not decrease MCD diet-induced hepatic steatosis. MCD diet-induced hepatic inflammation, as evidenced by increased plasma alanine aminotransferase and induction of hepatic TNFα expression, was also not reduced by PBA or TUDCA. PBA and TUDCA did not attenuate MCD diet-induced upregulation of the fibrosis-associated genes tissue inhibitor of metalloproteinase-1 and matrix metalloproteinase-9. ER chemical chaperones reduce MCD diet-induced ER stress, yet they do not improve MCD diet-induced hepatic steatosis, inflammation, or activation of genes associated with fibrosis. These data suggest that although the ER stress response is activated by the MCD diet, it does not have a primary role in the pathogenesis of MCD diet-induced steatohepatitis.

Apoptosis, autophagy and ER stress in mevalonate cascade inhibition-induced cell death of human atrial fibroblasts

3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors (statins) are cholesterol-lowering drugs that exert other cellular effects and underlie their beneficial health effects, including those associated with myocardial remodeling. We recently demonstrated that statins induces apoptosis and autophagy in human lung mesenchymal cells. Here, we extend our knowledge showing that statins simultaneously induces activation of the apoptosis, autophagy and the unfolded protein response (UPR) in primary human atrial fibroblasts (hATF). Thus we tested the degree to which coordination exists between signaling from mitochondria, endoplasmic reticulum and lysosomes during response to simvastatin exposure. Pharmacologic blockade of the activation of ER-dependent cysteine-dependent aspartate-directed protease (caspase)-4 and lysosomal cathepsin-B and -L significantly decreased simvastatin-induced cell death. Simvastatin altered total abundance and the mitochondrial fraction of proapoptotic and antiapoptotic proteins, while c-Jun N-terminal kinase/stress-activated protein kinase mediated effects on B-cell lymphoma 2 expression. Chemical inhibition of autophagy flux with bafilomycin-A1 augmented simvastatin-induced caspase activation, UPR and cell death. In mouse embryonic fibroblasts that are deficient in autophagy protein 5 and refractory to autophagy induction, caspase-7 and UPR were hyper-induced upon treatment with simvastatin. These data demonstrate that mevalonate cascade inhibition-induced death of hATF manifests from a complex mechanism involving co-regulation of apoptosis, autophagy and UPR. Furthermore, autophagy has a crucial role in determining the extent of ER stress, UPR and permissiveness of hATF to cell death induced by statins.

Docosahexaenoic acid-induced unfolded protein response, cell cycle arrest, and apoptosis in vascular smooth muscle cells are triggered by Ca²⁺-dependent induction of oxidative stress

Proliferation of vascular smooth muscle cells is a characteristic of pathological vascular remodeling and represents a significant therapeutic challenge in several cardiovascular diseases. Docosahexaenoic acid (DHA), a member of the n-3 polyunsaturated fatty acids, was shown to inhibit proliferation of numerous cell types, implicating several different mechanisms. In this study we examined the molecular events underlying the inhibitory effects of DHA on proliferation of primary human smooth muscle cells isolated from small pulmonary artery (hPASMCs). DHA concentration-dependently inhibited hPASMC proliferation, induced G1 cell cycle arrest, and decreased cyclin D1 protein expression. DHA activated the unfolded protein response (UPR), evidenced by increased mRNA expression of HSPA5, increased phosphorylation of eukaryotic initiation factor 2α, and splicing of X-box binding protein 1. DHA altered cellular lipid composition and led to increased reactive oxygen species (ROS) production. DHA-induced ROS were dependent on both intracellular Ca(2+) release and entry of extracellular Ca(2+). Overall cellular ROS and mitochondrial ROS were decreased by RU360, a specific inhibitor of mitochondrial Ca(2+) uptake. DHA-induced mitochondrial dysfunction was evidenced by decreased mitochondrial membrane potential and decreased cellular ATP content. DHA triggered apoptosis as found by increased numbers of cleaved caspase-3- and TUNEL-positive cells. The free radical scavenger Tempol counteracted DHA-induced ROS, cell cycle arrest, induction of UPR, and apoptosis. We conclude that Ca(2+)-dependent oxidative stress is the central and initial event responsible for induction of UPR, cell cycle arrest, and apoptosis in DHA-treated hPASMCs.

Unraveling the function of Arabidopsis thaliana OS9 in the endoplasmic reticulum-associated degradation of glycoproteins

In the endoplasmic reticulum, immature polypeptides coincide with terminally misfolded proteins. Consequently, cells need a well-balanced quality control system, which decides about the fate of individual proteins and maintains protein homeostasis. Misfolded and unassembled proteins are sent for destruction via the endoplasmic reticulum-associated degradation (ERAD) machinery to prevent the accumulation of potentially toxic protein aggregates. Here, we report the identification of Arabidopsis thaliana OS9 as a component of the plant ERAD pathway. OS9 is an ER-resident glycoprotein containing a mannose-6-phosphate receptor homology domain, which is also found in yeast and mammalian lectins involved in ERAD. OS9 fused to the C-terminal domain of YOS9 can complement the ERAD defect of the corresponding yeast Δyos9 mutant. An A. thaliana OS9 loss-of-function line suppresses the severe growth phenotype of the bri1-5 and bri1-9 mutant plants, which harbour mutated forms of the brassinosteroid receptor BRI1. Co-immunoprecipitation studies demonstrated that OS9 associates with Arabidopsis SEL1L/HRD3, which is part of the plant ERAD complex and with the ERAD substrates BRI1-5 and BRI1-9, but only the binding to BRI1-5 occurs in a glycan-dependent way. OS9-deficiency results in activation of the unfolded protein response and reduces salt tolerance, highlighting the role of OS9 during ER stress. We propose that OS9 is a component of the plant ERAD machinery and may act specifically in the glycoprotein degradation pathway.

The unfolded protein response regulates an angiogenic response by the kidney epithelium during ischemic stress

Ischemic injuries permanently affect kidney tissue and challenge cell viability, promoting inflammation and fibrogenesis. Ischemia results in nutrient deprivation, which triggers endoplasmic reticulum stress, ultimately resulting in the unfolded protein response (UPR). The aim of this study was to test whether the UPR could promote an angiogenic response independently of the HIF-1α pathway during ischemic stress in the human kidney epithelium. Glucose deprivation induced the secretion of vascular endothelial growth factor A (VEGFA), basic fibroblast growth factor (bFGF) and angiogenin (ANG) in human kidney epithelial cells independently of HIF-1α. Glucose deprivation, but not hypoxia, triggered endoplasmic reticulum stress and activated the UPR. RNA interference-mediated inhibition of the gene encoding the kinase PERK decreased VEGFA and bFGF expression, but neither gene was affected by the inhibition of IRE1α or ATF6. Furthermore, we show that the expression of angiogenin, which inhibits protein synthesis, is regulated by both IRE1α and PERK, which could constitute a complementary function of the UPR in the repression of translation. In a rat model of acute ischemic stress, we show that the UPR is activated in parallel with VEGFA, bFGF, and ANG expression and independently of HIF-1α.

Unfolded protein stress in the endoplasmic reticulum and mitochondria: a role in neurodegeneration

Protein-folding occurs in several intracellular locations including the endoplasmic reticulum and mitochondria. In normal conditions there is a balance between the levels of unfolded proteins and protein folding machinery. Disruption of homeostasis and an accumulation of unfolded proteins trigger stress responses, or unfolded protein responses (UPR), in these organelles. These pathways signal to increase the folding capacity, inhibit protein import or expression, increase protein degradation, and potentially trigger cell death. Many aging-related neurodegenerative diseases involve the accumulation of misfolded proteins in both the endoplasmic reticulum and mitochondria. The exact participation of the UPRs in the onset of neurodegeneration is unclear, but there is significant evidence for the alteration of these pathways in the endoplasmic reticulum and mitochondria. Here we will discuss the involvement of endoplasmic reticulum and mitochondrial stress and the possible contributions of the UPR in these organelles to the development of two neurodegenerative diseases, Parkinson's disease (PD) and Alzheimer's disease (AD).

How do trypanosomes change gene expression in response to the environment?

All organisms are able to modulate gene expression in response to internal and external stimuli. Trypanosomes represent a group that diverged early during the radiation of eukaryotes and do not utilise regulated initiation of transcription by RNA polymerase II. Here, the mechanisms present in trypanosomes to alter gene expression in response to stress and change of host environment are discussed and contrasted with those operating in yeast and cultured mammalian cells.

Endoplasmic reticulum stress and type 2 diabetes

Given the functional importance of the endoplasmic reticulum (ER), an organelle that performs folding, modification, and trafficking of secretory and membrane proteins to the Golgi compartment, the maintenance of ER homeostasis in insulin-secreting β-cells is very important. When ER homeostasis is disrupted, the ER generates adaptive signaling pathways, called the unfolded protein response (UPR), to maintain homeostasis of this organelle. However, if homeostasis fails to be restored, the ER initiates death signaling pathways. New observations suggest that both chronic hyperglycemia and hyperlipidemia, known as important causative factors of type 2 diabetes (T2D), disrupt ER homeostasis to induce unresolvable UPR activation and β-cell death. This review examines how the UPR pathways, induced by high glucose and free fatty acids (FFAs), interact to disrupt ER function and cause β-cell dysfunction and death.

Endoplasmic reticulum stress, the unfolded protein response, autophagy, and the integrated regulation of breast cancer cell fate

How breast cancer cells respond to the stress of endocrine therapies determines whether they will acquire a resistant phenotype or execute a cell-death pathway. After a survival signal is successfully executed, a cell must decide whether it should replicate. How these cell-fate decisions are regulated is unclear, but evidence suggests that the signals that determine these outcomes are highly integrated. Central to the final cell-fate decision is signaling from the unfolded protein response, which can be activated following the sensing of stress within the endoplasmic reticulum. The duration of the response to stress is partly mediated by the duration of inositol-requiring enzyme-1 activation following its release from heat shock protein A5. The resulting signals appear to use several B-cell lymphoma-2 family members to both suppress apoptosis and activate autophagy. Changes in metabolism induced by cellular stress are key components of this regulatory system, and further adaptation of the metabolome is affected in response to stress. Here we describe the unfolded protein response, autophagy, and apoptosis, and how the regulation of these processes is integrated. Central topologic features of the signaling network that integrate cell-fate regulation and decision execution are discussed.

Protein phosphatase 1 subunit Ppp1r15a/GADD34 regulates cytokine production in polyinosinic:polycytidylic acid-stimulated dendritic cells

In response to inflammatory stimulation, dendritic cells (DCs) have a remarkable pattern of differentiation that exhibits specific mechanisms to control the immune response. Here we show that in response to polyriboinosinic:polyribocytidylic acid (pI:C), DCs mount a specific integrated stress response during which the transcription factor ATF4 and the growth arrest and DNA damage-inducible protein 34 (GADD34/Ppp1r15a), a phosphatase 1 (PP1) cofactor, are expressed. In agreement with increased GADD34 levels, an extensive dephosphorylation of the translation initiation factor eIF2α was observed during DC activation. Unexpectedly, although DCs display an unusual resistance to protein synthesis inhibition induced in response to cytosolic dsRNA, GADD34 expression did not have a major impact on protein synthesis. GADD34, however, was shown to be required for normal cytokine production both in vitro and in vivo. These observations have important implications in linking further pathogen detection with the integrated stress response pathways.

The endoplasmic reticulum in plant immunity and cell death

The endoplasmic reticulum (ER) is a highly dynamic organelle in eukaryotic cells and a major production site of proteins destined for vacuoles, the plasma membrane, or apoplast in plants. At the ER, these secreted proteins undergo multiple processing steps, which are supervised and conducted by the ER quality control system. Notably, processing of secreted proteins can considerably elevate under stress conditions and exceed ER folding capacities. The resulting accumulation of unfolded proteins is defined as ER stress. The efficiency of cells to re-establish proper ER function is crucial for stress adaptation. Besides delivering proteins directly antagonizing and resolving stress conditions, the ER monitors synthesis of immune receptors. This indicates the significance of the ER for the establishment and function of the plant immune system. Recent studies point out the fragility of the entire system and highlight the ER as initiator of programed cell death (PCD) in plants as was reported for vertebrates. This review summarizes current knowledge on the impact of the ER on immune and PCD signaling. Understanding the integration of stress signals by the ER bears a considerable potential to optimize development and to enhance stress resistance of plants.

Cellular chaperones and folding enzymes are vital contributors to membrane bound replication and movement complexes during plant RNA virus infection

Cellular chaperones and folding enzymes play central roles in the formation of positive-strand and negative-strand RNA virus infection. This article examines the key cellular chaperones and discusses evidence that these factors are diverted from their cellular functions to play alternative roles in virus infection. For most chaperones discussed, their primary role in the cell is to ensure protein quality control. They are system components that drive substrate protein folding, complex assembly or disaggregation. Their activities often depend upon co-chaperones and ATP hydrolysis. During plant virus infection, Hsp70 and Hsp90 proteins play central roles in the formation of membrane-bound replication complexes for certain members of the tombusvirus, tobamovirus, potyvirus, dianthovirus, potexvirus, and carmovirus genus. There are several co-chaperones, including Yjd1, RME-8, and Hsp40 that associate with the bromovirus replication complex, pomovirus TGB2, and tospovirus Nsm movement proteins. There are also examples of plant viruses that rely on chaperone systems in the endoplasmic reticulum (ER) to support cell-to-cell movement. TMV relies on calreticulin to promote virus intercellular transport. Calreticulin also resides in the plasmodesmata and plays a role in calcium sequestration as well as glycoprotein folding. The pomovirus TGB2 interacts with RME-8 in the endosome. The potexvirus TGB3 protein stimulates expression of ER resident chaperones via the bZIP60 transcription factor. Up-regulating factors involved in protein folding may be essential to handling the load of viral proteins translated along the ER. In addition, TGB3 stimulates SKP1 which is a co-factor in proteasomal degradation of cellular proteins. Such chaperones and co-factors are potential targets for antiviral defense.

Virus-induced ER stress and the unfolded protein response

The accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum (ER) results in ER stress that triggers cytoprotective signaling pathways, termed the unfolded protein response (UPR), to restore and maintain homeostasis in the ER or to induce apoptosis if ER stress remains unmitigated. The UPR signaling network encompasses three core elements, i.e., PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring protein-1 (IRE1). Activation of these three branch pathways of the UPR leads to the translation arrest and degradation of misfolded proteins, the expression of ER molecular chaperones, and the expansion of the ER membrane to decrease the load of proteins and increase the protein-folding capacity in the ER. Recently, the essential roles of the UPR have been implicated in a number of mammalian diseases, particularly viral diseases. In virus-infected cells, the cellular translation machinery is hijacked by the infecting virus to produce large amounts of viral proteins, which inevitably perturbs ER homeostasis and causes ER stress. This review summarizes current knowledge about the UPR signaling pathways, highlights two identified UPR pathways in plants, and discuss progress in elucidating the UPR in virus-infected cells and its functional roles in viral infection.

Role of pro-oncogenic protein disulfide isomerase (PDI) family member anterior gradient 2 (AGR2) in the control of endoplasmic reticulum homeostasis

The protein-disulfide isomerase (PDI) family member anterior gradient 2 (AGR2) is reportedly overexpressed in numerous cancers and plays a role in cancer development. However, to date the molecular functions of AGR2 remain to be characterized. Herein we have identified AGR2 as bound to newly synthesized cargo proteins using a proteomics analysis of endoplasmic reticulum (ER) membrane-bound ribosomes. Nascent protein chains that translocate into the ER associate with specific ER luminal proteins, which in turn ensures proper folding and posttranslational modifications. Using both imaging and biochemical approaches, we confirmed that AGR2 localizes to the lumen of the ER and indirectly associates with ER membrane-bound ribosomes through nascent protein chains. We showed that AGR2 expression is controlled by the unfolded protein response and is in turn is involved in the maintenance of ER homeostasis. Remarkably, we have demonstrated that siRNA-mediated knockdown of AGR2 significantly alters the expression of components of the ER-associated degradation machinery and reduces the ability of cells to cope with acute ER stress, properties that might be relevant to the role of AGR2 in cancer development.

Methionine adenosyltransferase 1A gene deletion disrupts hepatic very low-density lipoprotein assembly in mice

Very low-density lipoprotein (VLDL) secretion provides a mechanism to export triglycerides (TG) from the liver to peripheral tissues, maintaining lipid homeostasis. In nonalcoholic fatty liver disease (NAFLD), VLDL secretion disturbances are unclear. Methionine adenosyltransferase (MAT) is responsible for S-adenosylmethionine (SAMe) synthesis and MAT I and III are the products of the MAT1A gene. Deficient MAT I and III activities and SAMe content in the liver have been associated with NAFLD, but whether MAT1A is required for normal VLDL assembly remains unknown. We investigated the role of MAT1A on VLDL assembly in two metabolic contexts: in 3-month-old MAT1A-knockout mice (3-KO), with no signs of liver injury, and in 8-month-old MAT1A-knockout mice (8-KO), harboring nonalcoholic steatohepatitis. In 3-KO mouse liver, there is a potent effect of MAT1A deletion on lipid handling, decreasing mobilization of TG stores, TG secretion in VLDL and phosphatidylcholine synthesis via phosphatidylethanolamine N-methyltransferase. MAT1A deletion also increased VLDL-apolipoprotein B secretion, leading to small, lipid-poor VLDL particles. Administration of SAMe to 3-KO mice for 7 days recovered crucial altered processes in VLDL assembly and features of the secreted lipoproteins. The unfolded protein response was activated in 8-KO mouse liver, in which TG accumulated and the phosphatidylcholine-to-phosphatidylethanolamine ratio was reduced in the endoplasmic reticulum, whereas secretion of TG and apolipoprotein B in VLDL was increased and the VLDL physical characteristics resembled that in 3-KO mice. MAT1A deletion also altered plasma lipid homeostasis, with an increase in lipid transport in low-density lipoprotein subclasses and decrease in high-density lipoprotein subclasses.

CONCLUSION

MAT1A is required for normal VLDL assembly and plasma lipid homeostasis in mice. Impaired VLDL synthesis, mainly due to SAMe deficiency, contributes to NAFLD development in MAT1A-KO mice.

The unfolded protein response and its role in intestinal homeostasis and inflammation

The unfolded protein response (UPR) is a signaling pathway from the endoplasmic reticulum (ER) to the nucleus that protects cells from the stress caused by misfolded or unfolded proteins [1, 2]. As such, ER stress is an ongoing challenge for all cells given the central biologic importance of secretion as part of normal physiologic functions. This is especially the case for cells that are highly dependent upon secretory function as part of their major duties. Within mucosal tissues, the intestinal epithelium is especially dependent upon an intact UPR for its normal activities [3]. This review will discuss the UPR and the special role that it provides in the functioning of the intestinal epithelium and, when dysfunctional, its implications for understanding mucosal homeostasis and intestinal inflammation, as occurs in inflammatory bowel disease (IBD).

Abnormal calcium homeostasis and protein folding stress at the ER: A common factor in familial and infectious prion disorders

Prion-related disorders (PrDs) are caused by the accumulation of a misfolded and protease-resistant form of the cellular prion, leading to neuronal dysfunction and massive neuronal loss. In humans, PrDs have distinct etiologies including sporadic, infectious and familial forms, which present common clinical features; however, the possible existence of common neuropathogenic events are not known. Several studies suggest that alterations in protein folding and quality control mechanisms at the endoplasmic reticulum (ER) are a common factor involved in PrDs. However, the mechanism underlying ER dysfunction in PrDs remains unknown. We have recently reported that alterations in ER calcium homeostasis are common pathological events observed in both infectious and familial PrD models. Perturbation in calcium homeostasis directly correlated with the occurrence of ER stress and higher susceptibility to protein folding stress. We envision a model where alterations in ER function are central and common events underlying prion pathogenesis, leading to general alterations on protein homeostasis networks.

Genetically determined epithelial dysfunction and its consequences for microflora-host interactions

The intestinal epithelium forms a highly active functional interface between the relatively sterile internal body surfaces and the enormously complex and diverse microbiota that are contained within the lumen. Genetic models that allow for manipulation of genes specifically in the intestinal epithelium have provided an avenue to understand the diverse set of pathways whereby intestinal epithelial cells (IECs) direct the immune state of the mucosa associated with homeostasis versus either productive or non-productive inflammation as occurs during enteropathogen invasion or inflammatory bowel disease (IBD), respectively. These pathways include the unfolded protein response (UPR) induced by stress in the endoplasmic reticulum (ER), autophagy, a self-cannibalistic pathway important for intracellular bacterial killing and proper Paneth cell function as well as the interrelated functions of NOD2/NF-κB signaling which also regulate autophagy induction. Multiple genes controlling these IEC pathways have been shown to be genetic risk factors for human IBD. This highlights the importance of these pathways not only for proper IEC function but also suggesting that IECs may be one of the cellular originators of organ-specific and systemic inflammation as in IBD.

Impact of the autophagy machinery on hepatitis C virus infection

Autophagy is a cellular process that catabolizes cytoplasmic components and maintains energy homeostasis. As a stress response, the autophagy machinery interconnects a wide range of cellular pathways, enhancing the spread of certain pathogens while limiting others, and has become a highly active research area over the past several years. Independent laboratories have recently reported that autophagy vesicles accumulate in hepatitis C virus (HCV) infected cells and that autophagy proteins can function as proviral factors required for HCV replication. In this review, we summarize what is currently known about the interplay between autophagy and HCV and the possible mechanisms whereby autophagy proteins might favor HCV propagation.

Lack of de novo phosphatidylinositol synthesis leads to endoplasmic reticulum stress and hepatic steatosis in cdipt-deficient zebrafish

Hepatic steatosis is the initial stage of nonalcoholic fatty liver disease (NAFLD) and may predispose to more severe hepatic disease, including hepatocellular carcinoma. Endoplasmic reticulum (ER) stress has been recently implicated as a novel mechanism that may lead to NAFLD, although the genetic factors invoking ER stress are largely unknown. During a screen for liver defects from a zebrafish insertional mutant library, we isolated the mutant cdipthi559Tg/+ (hi559). CDIPT is known to play an indispensable role in phosphatidylinositol (PtdIns) synthesis. Here we show that cdipt is expressed in the developing liver, and its disruption in hi559 mutants abrogates de novo PtdIns synthesis, resulting in hepatomegaly at 5 days postfertilization. The hi559 hepatocytes display features of NAFLD, including macrovesicular steatosis, ballooning, and necroapoptosis. Gene set enrichment of microarray profiling revealed significant enrichment of endoplasmic reticulum stress response (ERSR) genes in hi559 mutants. ER stress markers, including atf6, hspa5, calr, and xbp1, are selectively up-regulated in the mutant liver. The hi559 expression profile showed significant overlap with that of mammalian hepatic ER stress and NAFLD. Ultrastructurally, the hi559 hepatocytes display marked disruption of ER architecture with hallmarks of chronic unresolved ER stress. Induction of ER stress by tunicamycin in wild-type larvae results in a fatty liver similar to hi559, suggesting that ER stress could be a fundamental mechanism contributing to hepatic steatosis.

CONCLUSION

cdipt-deficient zebrafish exhibit hepatic ER stress and NAFLD pathologies, implicating a novel link between PtdIns, ER stress, and steatosis. The tractability of hi559 mutant provides a valuable tool to dissect ERSR components, their contribution to molecular pathogenesis, and evaluation of novel therapeutics of NAFLD.

Role of unfolded protein response in plant virus infection

A new study of Potato virus X (PVX) revealed that a viral movement protein, named TGBp3, triggers the unfolded protein response (UPR) which monitors accumulation of aberrant proteins the endoplasmic reticulum (ER) and targets them for degradation. The PVX TGBp3 resides in ER and activates bZIP60, a transcription factor involved in the UPR pathway. Knockdown of bZIP60 hampers virus infection in protoplasts and whole plants. Preliminary evidence indicates that UPR regulates cellular cytotoxicity that could otherwise lead to cell death if the PVX TGBp3 reaches high levels in the ER. SKP1 expression appears to be linked to bZIP60 and is a component of the SCF-complex mediating proteasomal degradation of cellular substrates. SKP1 expression is induced by PVX TGBp3 and plays a role in regulating PVX spread in whole plants. We propose that SKP1 might be linked to TGBp1-mediated degradation of AGO1. 

Activating transcription factor 6 plays protective and pathological roles in steatosis due to endoplasmic reticulum stress in zebrafish

Many etiologies of fatty liver disease (FLD) are associated with the hyperactivation of one of the three pathways composing the unfolded protein response (UPR), which is a harbinger of endoplasmic reticulum (ER) stress. The UPR is mediated by pathways initiated by PRKR-like endoplasmic reticulum kinase, inositol-requiring 1A/X box binding protein 1, and activating transcription factor 6 (ATF6), and each of these pathways has been implicated to have a protective or pathological role in FLD. We used zebrafish with FLD and hepatic ER stress to explore the relationship between Atf6 and steatosis. A mutation of the foie gras (foigr) gene caused FLD and hepatic ER stress. The prolonged treatment of wild-type larvae with tunicamycin (TN), which caused chronic ER stress, phenocopied foigr. In contrast, acute exposure to a high dose of TN robustly activated the UPR but was less effective at inducing steatosis. The sterol regulatory element binding protein transcription factors were not required for steatosis in any of these models. Instead, depleting larvae of active Atf6 either through a membrane-bound transcription factor peptidase site 1 mutation or an atf6 morpholino injection protected them against steatosis caused by chronic ER stress, but exacerbated steatosis caused by acute TN treatment.

CONCLUSION

ER stress causes FLD. A loss of Atf6 prevents steatosis caused by chronic ER stress but can also potentiate steatosis caused by acute ER stress. This demonstrates that Atf6 can play both protective and pathological roles in FLD.