ATF6 activated by proteolysis binds in the presence of NF-Y (CBF) directly to the cis-acting element responsible for the mammalian unfolded protein response

Transcriptional and translational control in the Mammalian unfolded protein response

Cells monitor the physiological load placed on their endoplasmic reticulum (ER) and respond to perturbations in ER function by a process known as the unfolded protein response (UPR). In metazoans the UPR has a transcriptional component that up-regulates expression of genes that enhance the capacity of the organelle to deal with the load of client proteins and a translational component that insures tight coupling between protein biosynthesis on the cytoplasmic side and folding in the ER lumen. Together, these two components adapt the secretory apparatus to physiological load and protect cells from the consequences of protein malfolding.

The cargo receptor ERGIC-53 is a target of the unfolded protein response

The accumulation of unfolded proteins in the ER triggers a signaling response known as unfolded protein response (UPR). In yeast the UPR affects several hundred genes that encode ER chaperones and proteins operating at later stages of secretion. In mammalian cells the UPR appears to be more limited to chaperones of the ER and genes assumed to be important after cell recovery from ER stress that are not important for secretion. Here, we report that the mRNA of lectin ERGIC-53, a cargo receptor for the transport of glycoproteins from ER to ERGIC, and of its related protein VIP36 is induced by the known inducers of ER stress, tunicamycin and thapsigargin. In parallel, the rate of synthesis of the ERGIC-53 protein was induced by these agents. The response was due to the UPR since it was also triggered by castanospermine, a specific inducer of UPR, and inhibited by genistein. Thapsigargin-induced upregulation of ERGIC-53 could be fully accounted for by the ATF6 pathway of UPR. The results suggest that in mammalian cells the UPR also affects traffic from and beyond the ER.

P58IPK, a novel endoplasmic reticulum stress-inducible protein and potential negative regulator of eIF2alpha signaling

The unfolded protein response, which is activated in response to the loss of endoplasmic reticulum (ER) Ca(2+) homeostasis and/or the accumulation of misfolded, unassembled, or aggregated proteins in the ER lumen, involves both transcriptional and translational regulation. In the current studies we sought to identify novel ER stress-induced genes by conducting microarray analysis on tunicamycin-treated cells. We identified P58(IPK), an inhibitor of the interferon-induced double-stranded RNA-activated protein kinase, as induced during ER stress. Additional studies suggested that p58(IPK) induction was mediated via ATF6 and that P58(IPK) played a role in down-regulating the activity of the pancreatic eIF2 kinase/eukaryotic initiation factor 2alpha (eIF2alpha)-like ER kinase/activation transcription factor (ATF) 4 pathway. Modulation of P58(IPK) levels altered the phosphorylation status of eIF2alpha, and thereby affected expression of its downstream targets, ATF4 and Gadd153. Overexpression of P58(IPK) inhibited eIF2alpha phosphorylation and reduced ATF4 and Gadd153 protein accumulation, whereas silencing of P58(IPK) expression enhanced pancreatic eIF2alpha-like ER kinase and eIF2alpha phosphorylation and increased ATF4 and Gadd153 accumulation. These findings implicate P58(IPK) as an important component of a negative feedback loop used by the cell to inhibit eIF2alpha signaling, and thus attenuate the unfolded protein response.

Activation of the ATF6, XBP1 and grp78 genes in human hepatocellular carcinoma: a possible involvement of the ER stress pathway in hepatocarcinogenesis

BACKGROUND/AIMS

We identified the glucose-regulated protein (grp) 78 as a transformation-associated gene in hepatocellular carcinoma (HCC). Grp78 is a molecular chaperone involved in the unfolded protein response, the expression of which can be regulated by the transcription factors ATF6 and XBP1. Thus, we investigated the regulatory mechanisms of the grp78 gene in liver malignancy.

METHODS

Expression of grp78, ATF6 and XBP1 was examined by Northern blot, RT-PCR, immunoblot and immunohistochemical analyses. A reporter assay of the grp78 promoter was also performed.

RESULTS

Elevation of grp78 and ATF6 mRNAs and the splicing of XBP1 mRNA, resulting in the activation of XBP1 product, occurred in HCC tissues with increased histological grading. Higher accumulation of the grp78 product in the cytoplasm, concomitantly with marked nuclear localization of the activated ATF6 product (p50ATF6), was observed in moderately to poorly differentiated HCC tissues. Cooperation between the distal DNA segment and the proximal endoplasmic reticulum stress response elements was essential for maximum transcription of the grp78 promoter in HCC cells.

CONCLUSIONS

The endoplasmic reticulum stress pathway mediated by ATF6 and by IRE1-XBP1 systems seems essential for the transformation-associated expression of the grp78 gene in HCCs.

Role of the connecting peptide in insulin biosynthesis

In single-chain insulins (SCIs), the C terminus of the insulin B-chain is contiguous with the N terminus of the A-chain, connected by a short bioengineered linker sequence. SCIs have been proposed to offer potential benefit for gene therapy of diabetes (Lee, H. C., Kim, S. J., Kim, K. S., Shin, H. C., and Yoon, J. W. (2000) Nature 408, 483-488) yet relatively little is known about their folding, intracellular transport, or secretion from mammalian cells. Because SCIs can be considered as mutant proinsulin (with selective shortening of the 35-amino acid connecting peptide that normally includes two sets of flanking dibasic residues), they offer insights into understanding the role of the connecting peptide in insulin biosynthesis. Herein we have explored the relationship of the linker sequence to SCI biosynthesis, folding, and intracellular transport in transiently transfected HEK293 or Chinese hamster ovary cells or in stably transfected AtT20 cells. Despite previous reports that direct linkage of B- and A-chains produces a structure isomorphous with authentic two-chain insulin, we find that constructs with short linkers tend to be synthesized at lower levels, with a significant fraction of molecules exhibiting improper disulfide bonding. Nevertheless, disulfide-mispaired isoforms from a number of different SCI constructs are secreted. While this suggests that a novel folded state goes unrecognized by secretory pathway quality control, we find that misfolded SCIs are detected at higher levels in Chinese hamster ovary cells with artificially activated unfolded protein response mediated by inducible overexpression of active ATF-6. Such a maneuver allows analysis of more seriously misfolded mutants with further foreshortening of the linker sequence or loss (by mutation) of the insulin interchain disulfide bonds.

Transient cerebral ischemia activates processing of xbp1 messenger RNA indicative of endoplasmic reticulum stress

Cells respond to conditions associated with endoplasmic reticulum (ER) dysfunction with activation of the unfolded protein response, characterized by a shutdown of translation and induction of the expression of genes coding for ER stress proteins. The genetic response is based on IRE1-induced processing of xbp1 messenger RNA (mRNA), resulting in synthesis of new XBP1proc protein that functions as a potent transcription factor for ER stress genes. xbp1 processing in models of transient global and focal cerebral ischemia was studied. A marked increase in processed xbp1 mRNA levels during reperfusion was observed, most pronounced (about 35-fold) after 1-h occlusion of the right middle cerebral artery. The rise in processed xbp1 mRNA was not paralleled by a similar increase in XBP1proc protein levels because transient ischemia induces severe suppression of translation. As a result, mRNA levels of genes coding for ER stress proteins were only slightly increased, whereas mRNA levels of heat-shock protein 70 rose about 550-fold. Under conditions associated with ER dysfunction, cells require activation of the entire ER stress-induced signal transduction pathway, to cope with this severe form of stress. After transient cerebral ischemia, however, the block of translation may prevent synthesis of new XBP1proc protein and thus hinder recovery from ischemia-induced ER dysfunction.

Dysfunction of the unfolded protein response during global brain ischemia and reperfusion

A variety of endoplasmic reticulum (ER) stresses trigger the unfolded protein response (UPR), a compensatory response whose most proximal sensors are the ER membrane-bound proteins ATF6, IRE1alpha, and PERK. The authors simultaneously examined the activation of ATF6, IRE1alpha, and PERK, as well as components of downstream UPR pathways, in the rat brain after reperfusion after a 10-minute cardiac arrest. Although ATF6 was not activated, PERK was maximally activated at 10-minute reperfusion, which correlated with maximal eIF2alpha phosphorylation and protein synthesis inhibition. By 4-h reperfusion, there was 80% loss of PERK immunostaining in cortex and 50% loss in brain stem and hippocampus. PERK was degraded in vitro by mu-calpain. Although inactive IRE1alpha was maximally decreased by 90-minute reperfusion, there was no evidence that its substrate xbp-1 messenger RNA had been processed by removal of a 26-nt sequence. Similarly, there was no expression of the UPR effector proteins 55-kd XBP-1, CHOP, or ATF4. These data indicate that there is dysfunction in several key components of the UPR that abrogate the effects of ER stress. In other systems, failure to mount the UPR results in increased cell death. As other studies have shown evidence for ER stress after brain ischemia and reperfusion, the failure of the UPR may play a significant role in reperfusion neuronal death.

The mammalian endoplasmic reticulum as a sensor for cellular stress

The recent elucidation of the mammalian unfolded protein response pathway has revealed a unique and transcriptionally complex signal transduction pathway that protects cells from a variety of physical and biochemical stresses that can occur during normal development and in disease states. Although the stress conditions are monitored in the endoplasmic reticulum, the beneficial effects of this pathway are extended to other cellular organelles and to the organism itself.

MDG1/ERdj4, an ER-resident DnaJ family member, suppresses cell death induced by ER stress

BACKGROUND

Alterations in homeostasis after various cellular stresses, which prevent protein folding and cause an accumulation of misfolding or malfolding proteins in the endoplasmic reticulum (ER), have the potential to induce cellular damage, and are therefore a type of 'ER stress.' To understand the molecular events or cascades underlying the ER stress response regulated by gene transcription and mediated by stress transducers, it is crucial to identify the molecules induced during ER stress and to analyse the roles of these genes.

RESULTS

We identified MDG1/ERdj4, a member of the DnaJ protein family, as an inducible gene during ER stress. MDG1/ERdj4 contains the J domain, which is essential for interacting with Hsp70s, at the N-terminal portion and just at the back of the transmembrane domain. Its trypsin digestion and glycosylation of a chimeric protein composed of MDG1/ERdj4 fused with the extracellular domain of the amyloid precursor protein at its C-terminus, showed that its C-terminal portion containing the J domain could be orientated to the ER lumen. Over-expression of it inhibited the cell death induced by ER stress. In contrast, its mutants with the J domain deleted showed no protective effects against cell death.

CONCLUSIONS

MDG1/ERdj4 may play roles in stabilizing GRP78/BiP binding to unfolded substrate proteins in a J domain-dependent manner and prevent the accumulation of unfolded proteins in the ER, consequently protecting cells from ER stress.

A time-dependent phase shift in the mammalian unfolded protein response

Unfolded or misfolded proteins in the endoplasmic reticulum (ER) must be refolded or degraded to maintain homeostasis of the ER. The ATF6 and IRE1-XBP1 pathways are important for the refolding process in mammalian cells; activation of these transcriptional programs culminates in induction of ER-localized molecular chaperones and folding enzymes. We show here that degradation of misfolded glycoprotein substrates requires transcriptional induction of EDEM (ER degradation-enhancing alpha-mannosidase-like protein), and that this is mediated specifically by IRE1-XBP1 and not by ATF6. As XBP1 is produced after ATF6 activation, our results reveal a time-dependent transition in the mammalian unfolded protein response: an ATF6-mediated unidirectional phase (refolding only) is followed by an XBP1-mediated bidirectional phase (refolding plus degradation) as the response progresses.

Endoplasmic reticulum signaling as a determinant of recombinant protein expression

Generation of functional recombinant proteins requires efficient and undisturbed functioning of the ER-Golgi secretory pathway in host cells. In large-scale production, where target proteins are highly overexpressed, this pathway can be easily congested with unfolded or misfolded proteins. Accumulating evidence suggests that, in addition to responsibility for protein processing, ER is also an important signaling compartment and a sensor of cellular stress. Two ER responses have been described to arise from the overaccumulation of proteins: unfolded protein response (UPR) and ER overload response (EOR). UPR and EOR employ various mechanisms at the transcriptional and the translational levels to deal efficiently and appropriately with encountered stress. This review will outline the molecular bases of ER functioning and stress response, highlight the relevance of ER signaling to the large-scale cell culture productivity and discuss possible approaches to the improvement of the secretion capacities of recombinant cells.

Lead-induced endoplasmic reticulum (ER) stress responses in the nervous system

Lead (Pb) poisoning continues to be a significant health risk because of its pervasiveness in the environment, its known neurotoxic effects in children, and potential endogenous exposure from Pb deposited in bone. New information about mechanisms by which Pb enters cells and its organelle targets within cells are briefly reviewed. Toxic effects of Pb on the endoplasmic reticulum (ER) are considered in detail, based on recent evidence that Pb induces the expression of the gene for 78-kD glucose-regulated protein (GRP78) and other ER stress genes. GRP78 is a molecular chaperone that binds transiently to proteins traversing through the ER and facilitates their folding, assembly, and transport. Models are presented for the induction of ER stress by Pb in astrocytes, the major cell type of the central nervous system, in which Pb accumulates. A key feature of the models is disruption of GRP78 function by direct Pb binding. Possible pathways by which Pb-bound GRP78 stimulates the unfolded protein response (UPR) in the ER are discussed, specifically transduction by IRE1/ATF6 and/or IRE1/JNK. The effect of Pb binding to GRP78 in the ER is expected to be a key component for understanding mechanisms of Pb-induced ER stress gene expression.

Analysis of the role of GADD153 in the control of apoptosis in NS0 myeloma cells

Apoptosis can limit the maximum production of recombinant protein expression from cultured mammalian cells. This article focuses on the links between nutrient deprivation, ER perturbation, the regulation of (growth arrest and DNA damage inducible gene 153) GADD153 expression and apoptosis. During batch culture, decreases in glucose and glutamine correlated with an increase in apoptotic cells. This event was paralleled by a simultaneous increase in GADD153 expression. The expression of GADD153 in batch culture was suppressed by the addition of nutrients and with fed-batch culture the onset of apoptosis was delayed but not completely prevented. In defined stress conditions, glucose deprivation had the greatest effect on cell death when compared to glutamine deprivation or the addition of tunicamycin (an inhibitor of glycosylation), added to generate endoplasmic reticulum stress. However, the contribution of apoptosis to overall cell death (as judged by morphology) was smaller in conditions of glucose deprivation than in glutamine deprivation or tunicamycin treatment. Transient activation of GADD153 expression was found to occur in response to all stresses and occurred prior to detection of the onset of cell death. These results imply that GADD153 expression is either a trigger for apoptosis or offers a valid indicator of the likelihood of cell death arising from stresses of relevance to the bioreactor environment.

Activation of an unfolded protein response during differentiation of antibody-secreting B cells

The unfolded protein response pathway (UPR) is believed to detect and compensate for excessive protein accumulation in the endoplasmic reticulum (ER). The UPR can be induced by pharmacological agents that perturb ER functions, but may also occur during cellular developmental processes such as the transition of B-lymphocytes into antibody-secreting plasma cells. Here we show that major UPR components are activated in B cells stimulated to secrete antibody. Increased expression of UPR targets including the ER chaperones BiP and GRP94 and the transcription factor XBP-1 initiates early in the differentiation program prior to up-regulated synthesis of Ig chains. Furthermore, these same kinetics are observed during differentiation for cleavage of the ER-localized ATF6alpha protein and splicing of XBP-1 mRNA to generate p50ATF6alpha and p54XBP-1, the two known UPR transcriptional activators. All of these UPR events reach maximal levels once Ig synthesis and secretion are markedly induced. Interestingly, these events are not accompanied by expression of CHOP, a transcription factor induced by ER stress agents commonly used to investigate the UPR. These results suggest that a physiological UPR elicited during differentiation of B-lymphocytes into high-rate secretory cells may be distinct from the UPR defined by agents that disrupt protein maturation in the ER.

Differences in the molecular mechanisms involved in the transcriptional activation of the CHOP and asparagine synthetase genes in response to amino acid deprivation or activation of the unfolded protein response

A promoter element called the amino acid response element (AARE), which is essential for the induction of CHOP (a CCAAT/enhancer-binding protein-related gene) transcription by amino acid depletion, has been previously characterized. Conversely, the human asparagine synthetase (AS) promoter contains two cis-acting elements termed nutrient-sensing response elements (NSRE-1 and NSRE-2) that are required to activate the gene by either amino acid deprivation or the endoplasmic reticulum stress response. The results reported here document the comparison between CHOP and AS transcriptional control elements used by the amino acid pathway. We first establish that the AS NSRE-1 sequence shares nucleotide sequence and functional similarities with the CHOP AARE. However, we demonstrate that the CHOP AARE can function independently, whereas AS NSRE-1 is functionally weak by itself and instead requires the presence of NSRE-2. Furthermore, AS NSRE-2 can confer endoplasmic reticulum stress responsiveness to the CHOP AARE. Using activating transcription factor-2-deficient mouse embryonic fibroblasts, we also show that lack of this transcription factor does not abolish the amino acid inducibility of AS transcription, but this transcription factor is necessary to obtain the full AS response to amino acid starvation. Collectively, these results document that there are significant differences in the molecular mechanisms involved in the transcriptional activation of CHOP and AS by amino acid limitation.

Human HRD1 protects against ER stress-induced apoptosis through ER-associated degradation

Stresses that impair the function of the endoplasmic reticulum (ER) lead to an accumulation of unfolded protein in the ER. Under these conditions, the expression of a variety of genes involved in preventing the accumulation of the unfolded proteins is induced. Yeast Hrd1p is an ER stress-inducible ER membrane protein that acts as a ubiquitin ligase (E3) with a RING finger motif and plays a role in the ubiquitination of proteins in the ER. We report here the identification and characterization of a human homolog to yeast Hrd1p. The predicted structures are highly conserved from yeast to humans. Indeed, human HRD1 was localized to the ER and ubiquitinated its substrates. Furthermore, it was found that human HRD1 was up-regulated by ER stress via IRE1 and ATF6, which are ER stress transducers. Interestingly, 293 cells stably expressing wild-type HRD1, but not the C329S mutant, afforded resistance to ER stress-induced apoptosis. These results suggest that the production of HRD1 is up-regulated to protect against ER stress-induced apoptosis by degrading unfolded proteins accumulated in the ER.

Endoplasmic reticulum stress and the development of diabetes: a review

The early steps of insulin biosynthesis occur in the endoplasmic reticulum (ER), and the beta-cell has a highly developed and active ER. All cells regulate the capacity of their ER to fold and process client proteins and they adapt to an imbalance between client protein load and folding capacity (so-called ER stress). Mutations affecting the ER stress-activated pancreatic ER kinase (PERK) and its downstream effector, the translation initiation complex eukaryotic initiation factor 2 (eIF2), have a profound impact on islet cell development, function, and survival. PERK mutations are associated with the Wolcott-Rallison syndrome of infantile diabetes and mutations that prevent the alpha-subunit of eIF2 from being phosphorylated by PERK, block beta-cell development, and impair gluconeogenesis. We will review this and other rare forms of clinical and experimental diabetes and consider the role of ER stress in the development of more common forms of the disease.

A mitochondrial specific stress response in mammalian cells

Cells respond to a wide variety of stresses through the transcriptional activation of genes that harbour stress elements within their promoters. While many of these elements are shared by genes encoding proteins representative of all subcellular compartments, cells can also respond to stresses that are specific to individual organelles, such as the endoplasmic reticulum un folded protein response. Here we report on the discovery and characterization of a mitochondrial stress response in mammalian cells. We find that the accumulation of unfolded protein within the mitochondrial matrix results in the transcriptional upregulation of nuclear genes encoding mitochondrial stress proteins such as chaperonin 60, chaperonin 10, mtDnaJ and ClpP, but not those encoding stress proteins of the endoplasmic reticulum. Analysis of the chaperonin 60/10 bidirectional promoter identified a CHOP element as the mitochondrial stress response element. Dominant-negative mutant forms of CHOP and overexpression of CHOP revealed that this transcription factor, in association with C/EBPbeta, regulates expression of mitochondrial stress genes in response to the accumulation of unfolded proteins.

Characterization of the human ephrin-A4 promoter

Expression of the ephrin-A4 ligand, a family member of ligands binding the Eph receptor tyrosine kinases, is induced after an antigen-receptor stimulation of lymphocytes. To understand the transcription regulation of the ephrin-A4 gene, its promoter was identified and regulating elements were characterized. The ephrin-A4 promoter contains cis elements directing the cell-specific expression. By deletion studies, three specific regions, which were contributing to the transcription activity in lymphoid cells, were localized. In one of these regions, an inverted CCAAT box was identified and shown to bind the transcription activator nuclear factor-Y (NF-Y). The importance of NF-Y binding for the ephrin-A4 promoter activity is shown by a total abrogation of promoter activity after destruction of its binding site. NF-Y binding and activity are also crucially dependent on the integrity of the surrounding sequence. In addition, electrophoretic mobility-shift assay and serial-mutation analysis of the two remaining regulating regions revealed cis regulatory elements contributing to the transcription activity of the ephrin-A4 promoter.

Distinct roles of activating transcription factor 6 (ATF6) and double-stranded RNA-activated protein kinase-like endoplasmic reticulum kinase (PERK) in transcription during the mammalian unfolded protein response

In response to accumulation of unfolded proteins in the endoplasmic reticulum (ER), a homoeostatic response, termed the unfolded protein response (UPR), is activated in all eukaryotic cells. The UPR involves only transcriptional regulation in yeast, and approx. 6% of all yeast genes, encoding not only proteins to augment the folding capacity in the ER, but also proteins working at various stages of secretion, are induced by ER stress [Travers, Patil, Wodicka, Lockhart, Weissman and Walter (2000) Cell (Cambridge, Mass.) 101, 249-258]. In the present study, we conducted microarray analysis of HeLa cells, although our analysis covered only a small fraction of the human genome. A great majority of human ER stress-inducible genes (approx. 1% of 1800 genes examined) were classified into two groups. One group consisted of genes encoding ER-resident molecular chaperones and folding enzymes, and these genes were directly regulated by the ER-membrane-bound transcription factor activating transcription factor (ATF) 6. The ER-membrane-bound protein kinase double-stranded RNA-activated protein kinase-like ER kinase (PERK)-mediated signalling pathway appeared to be responsible for induction of the remaining genes, which are not involved in secretion, but may be important after cellular recovery from ER stress. In higher eukaryotes, the PERK-mediated translational-attenuation system is known to operate in concert with the transcriptional-induction system. Thus we propose that mammalian cells have evolved a strategy to cope with ER stress different from that of yeast cells.

Genomic sequences necessary for transcriptional activation by amino acid deprivation of mammalian cells

The human genes for C/EBP homology protein (chop) and asparagine synthetase (AS) are model systems to investigate transcription induced by nutrient limitation and endoplasmic reticulum (ER) stress. The genomic cis-elements in the promoters of these two genes that mediate these responses have been identified and partially characterized. Multiple cis-elements are functional in each gene, but differences exist in the molecular mechanisms by which these genes respond to amino acid or glucose deprivation. Whereas chop expression is associated with cell stress and apoptosis, activation of the AS gene by ER stress indicates that asparagine may also be critical for cellular processes other than protein synthesis.

Blimp-1 orchestrates plasma cell differentiation by extinguishing the mature B cell gene expression program

Blimp-1, a transcriptional repressor, drives the terminal differentiation of B cells to plasma cells. Using DNA microarrays, we found that introduction of Blimp-1 into B cells blocked expression of a remarkably large set of genes, while a much smaller number was induced. Blimp-1 initiated this cascade of gene expression changes by directly repressing genes encoding several transcription factors, including Spi-B and Id3, that regulate signaling by the B cell receptor. Blimp-1 also inhibited immunoglobulin class switching by blocking expression of AID, Ku70, Ku86, DNA-PKcs, and STAT6. These findings suggest that Blimp-1 promotes plasmacytic differentiation by extinguishing gene expression important for B cell receptor signaling, germinal center B cell function, and proliferation while allowing expression of important plasma cell genes such as XBP-1.

ER stress regulation of ATF6 localization by dissociation of BiP/GRP78 binding and unmasking of Golgi localization signals

ATF6 is an endoplasmic reticulum (ER) stress-regulated transmembrane transcription factor that activates the transcription of ER molecular chaperones. Upon ER stress, ATF6 translocates from the ER to the Golgi where it is processed to its active form. We have found that the ER chaperone BiP/GRP78 binds ATF6 and dissociates in response to ER stress. Loss of BiP binding correlates with the translocation of ATF6 to the Golgi, which was slowed in cells overexpressing BiP. Two Golgi localization signals (GLSs) were identified in ATF6. Removal of BiP binding sites from ATF6, while retaining a GLS, resulted in its constitutive translocation to the Golgi. These results suggest that BiP retains ATF6 in the ER by inhibiting its GLSs and that dissociation of BiP during ER stress allows ATF6 to be transported to the Golgi.

Coordination of ATF6-mediated transcription and ATF6 degradation by a domain that is shared with the viral transcription factor, VP16

ATF6 is a 670-amino acid endoplasmic reticulum (ER) transmembrane protein that is cleaved in response to ER stress. The resulting N-terminal fragment of approximately 400 amino acids translocates to the nucleus and activates selected ER stress-inducible genes, such as GRP-78 and sarco/endoplasmic reticulum ATPase, which are required for cell survival. In studying the mechanism of ATF6-activated transcription, we found that when HeLa cells were transfected with a plasmid encoding ATF6-(1-373), ER stress-inducible reporter gene activation was high, but ATF6-(1-373) expression was low, unless a proteasome inhibitor was added. In contrast, transfection with a plasmid encoding ATF6-(94-373) resulted in low reporter activation and high expression of ATF6-(94-373), which was independent of the proteasome inhibitor. Thus, the information responsible for transcriptional activation and proteasomal degradation must lie within the N-terminal 93 amino acids of ATF6. This portion of ATF6 was found to be homologous to the herpes simplex viral protein, VP16. One 8-amino acid domain of particular interest in this region of ATF6 is 75% identical to the VN8 region in VP16. VN8 is required for VP16-mediated transcription, as well as rapid degradation of VP16 by proteasomes. Point mutations in the VN8-like region of ATF6 caused a loss of transcription, increased expression levels, and an increase in half-life. Thus, the potent transcriptional activities and rapid degradation of ATF6 and VP16 require the VN8 domains in each protein. Homology searches indicate that ATF6 is the only eukaryotic protein known that possesses an active VN8 domain, raising questions about how this domain evolved and the functional importance underlying its appearance in only these two transcription factors.

The unfolded protein response in nutrient sensing and differentiation

Eukaryotic cells coordinate protein-folding reactions in the endoplasmic reticulum with gene expression in the nucleus and messenger RNA translation in the cytoplasm. As the rate of protein synthesis increases, protein folding can be compromised, so cells have evolved signal-transduction pathways that control transcription and translation -- the 'unfolded protein response'. Recent studies indicate that these pathways also coordinate rates of protein synthesis with nutrient and energy stores, and regulate cell differentiation to survive nutrient-limiting conditions or to produce large amounts of secreted products such as hormones, antibodies or growth factors.

Two distinct stress signaling pathways converge upon the CHOP promoter during the mammalian unfolded protein response

CHOP is a non-ER localized transcription factor that is induced by a variety of adverse physiological conditions including ER stress. Accumulation of unfolded proteins in the ER activates an unfolded protein response pathway that targets both ER resident chaperones (e.g. BiP) and CHOP. Hence, it is unclear if CHOP induction during ER stress occurs through the ER stress response element that is conserved in both CHOP and ER chaperone promoters, or through a separate regulatory pathway conserved among different CHOP inducing cellular stress conditions. We identified a bona fide ER stress element in the hamster CHOP promoter and found that similar transcription complexes containing NF-Y bound to both the CHOP and BiP ER stress response elements. In addition, we demonstrated for the first time the importance of the C/EBP-ATF composite site for CHOP regulation during ER stress. Activation of the ER transmembrane eIF2alpha kinase, PERK, induced ATF4 protein expression, direct binding to the composite site in CHOP promoter, and as a consequence, CHOP protein induction. We propose that this eIF2alpha-kinase/ATF4/C/EBP-ATF composite site pathway is conserved for CHOP regulation during various cellular stress conditions including ER stress. Our data indicate that both the ERSE and the PERK-ATF4 pathways converge on the CHOP promoter during ER stress and provide insights into the similarities and differences between CHOP and ER chaperone expression during normal and stress conditions.

Japanese encephalitis virus infection initiates endoplasmic reticulum stress and an unfolded protein response

The malfunctioning of the endoplasmic reticulum (ER) of cells in hosts ranging from yeast to mammals can trigger an unfolded protein response (UPR). Such malfunctioning can result from a variety of ER stresses, including the inhibition of protein glycosylation and calcium imbalance. To cope with ER stresses, cells may rely on the UPR to send a signal(s) from the ER to the nucleus to stimulate appropriate cellular responses, including induction of chaperone expression. During Japanese encephalitis virus (JEV) infection, the lumen of the ER rapidly accumulates substantial amounts of viral proteins for virus progeny production. In the present study, we demonstrate that as evidenced by certain chaperone inductions, JEV infection triggers the UPR in fibroblast BHK-21 cells and in neuronal N18 and NT-2 cells, in which JEV results in apoptotic cell death. By contrast, no UPR was observed in apoptosis-resistant K562 cells infected by JEV. JEV infection also activates expression of CHOP/GADD153, a distinctive transcription factor often induced by the UPR, and appears to trigger activation of p38 mitogen-activated protein kinase, a posttranslational activator of CHOP. Ectopic enforcement of CHOP expression enhanced JEV-induced apoptosis, whereas treatment with a p38-specific inhibitor, SB203580, partially blocked JEV-induced apoptosis. Interestingly, bcl-2 overexpression and treatment with a pancaspase inhibitor, z-VAD-fmk, inhibited CHOP induction and diminished JEV-induced apoptosis, suggesting that Bcl-2 and caspases could be the upstream regulators of CHOP. Our results thus suggest that virus-induced ER stress may participate, via p38-dependent and CHOP-mediated pathways, in the apoptotic process triggered by JEV infection.

Isolation and characterization of a putative transducer of endoplasmic reticulum stress in Oryza sativa

Following endoplasmic reticulum (ER) stress that prevents correct folding or assembly of ER proteins, at least three responses occur to maintain cell homeostasis: induction of chaperones, attenuation of protein synthesis, and enhancement of lipid synthesis. Transducers that transmit ER stress to the nucleus have already been identified in yeast and mammals. We report here isolation of a cDNA, OsIre1, from rice encoding a putative homolog of Ire1p, a yeast transducer of ER stress. OsIre1 encodes a polypeptide consisting of 893 amino acids, in which two hydrophobic stretches are present in the amino-terminal (N-terminal) and middle regions, possibly serving as a signal peptide and a transmembrane domain, respectively. The carboxyl-terminal (C-terminal) domain was found to possess serine/threonine protein kinase and ribonuclease-like domains showing high similarities with regions in Ire1 homologs from other organisms. A fusion protein of OsIre1 and green fluorescent protein (GFP) expressed in tobacco BY2 cells could be demonstrated to localize to the ER and the N-terminal domain of OsIre1 could substitute for yeast Ire1p in yeast cells. When produced in bacteria as a fusion protein, the C-terminal region of OsIre1 showed autophosphorylation activity. These results thus indicate that OsIre1 encodes a putative plant transducer of ER stress.

The luminal domain of ATF6 senses endoplasmic reticulum (ER) stress and causes translocation of ATF6 from the ER to the Golgi

ATF6 is an endoplasmic reticulum (ER) transmembrane transcription factor that is activated by the ER stress/unfolded protein response by cleavage of its N-terminal half from the membrane. We find that ER stress induces ATF6 to move from the ER to the Golgi, where it is cut in its luminal domain by site 1 protease. ATF6 contains a single transmembrane domain with 272 amino acids oriented in the lumen of the ER. We found that this luminal domain is required for the translocation of ATF6 to the Golgi and its subsequent cleavage, and we have mapped regions required for these properties. These results suggest that the conserved CD1 region is required for translocation, whereas the CD2 region is required for site 1 protease cleavage. We also find that ATF6's luminal domain is sufficient to sense ER stress and cause translocation to the Golgi when fused to LZIP, another ER transmembrane protein. These results show that ATF6 has a mechanism to sense ER stress and respond by translocation to the Golgi.

Nitric oxide-induced apoptosis in RAW 264.7 macrophages is mediated by endoplasmic reticulum stress pathway involving ATF6 and CHOP

Excess nitric oxide (NO) induces apoptosis in some cell types including macrophages; however, the cascade of NO-mediated apoptosis is not fully understood. We investigated the initial steps of NO-mediated apoptosis in mouse macrophage-like RAW 264.7 cells. When cells were treated with bacterial lipopolysaccharide (LPS) plus interferon-gamma (IFN-gamma), NO-mediated apoptosis occurred. Under these conditions, p53 accumulation was not observed, indicating that DNA damage is not the main trigger of NO-mediated apoptosis. On the other hand, mRNA and protein for CHOP, a transcription factor known to be induced by endoplasmic reticulum (ER) stress, were induced. The CHOP induction by LPS/IFN-gamma treatment preceded cytochrome c release from mitochondria. In addition, p90ATF6, an ER membrane-bound transcription factor involved in ER stress response, was cleaved to its active soluble form p50ATF6, which was transported to nucleus and bound to the ER stress response element of the CHOP gene. In the luciferase reporter assay, both the CHOP-binding element of the Rous sarcoma virus long terminal repeat and ER stress response element of the CHOP gene were activated by LPS/IFN-gamma treatment. When RAW 264.7 cells or COS-7 cells were transfected with expression plasmids for CHOP, p90ATF6, or p50ATF6, cell death was observed. In addition, apoptosis induced by p50ATF6 was prevented by a CHOP dominant negative form as well as by an ATF6 dominant negative form, and LPS/IFN-gamma-induced apoptosis was prevented by the CHOP dominant negative form. Peritoneal macrophages from CHOP knockout mice showed resistance to NO-induced apoptosis. These results indicate that the ER stress pathway involving ATF6 and CHOP plays a key role in NO-mediated apoptosis in macrophages.

Transcriptional control of B-cell development

Significant progress has recently been made in our understanding of how transcription factors such as PU.1, Notch1, E2A, EBF, Pax5, Bcl6, Blimp1 and XBP1 control different developmental decisions during the onset and terminal phase of B-lymphopoiesis. One emerging theme is that negative regulatory networks play an important role in suppressing alternative gene programs and their corresponding cell fates throughout B-cell development.

The mammalian endoplasmic reticulum as a sensor for cellular stress

The recent elucidation of the mammalian unfolded protein response pathway has revealed a unique and transcriptionally complex signal transduction pathway that protects cells from a variety of physical and biochemical stresses that can occur during normal development and in disease states. Although the stress conditions are monitored in the endoplasmic reticulum, the beneficial effects of this pathway are extended to other cellular organelles and to the organism itself.

IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response

All eukaryotic cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER) by signaling an adaptive pathway termed the unfolded protein response (UPR). In yeast, a type-I ER transmembrane protein kinase, Ire1p, is the proximal sensor of unfolded proteins in the ER lumen that initiates an unconventional splicing reaction on HAC1 mRNA. Hac1p is a transcription factor required for induction of UPR genes. In higher eukaryotic cells, the UPR also induces site-2 protease (S2P)-mediated cleavage of ER-localized ATF6 to generate an N-terminal fragment that activates transcription of UPR genes. To elucidate the requirements for IRE1alpha and ATF6 for signaling the mammalian UPR, we identified a UPR reporter gene that was defective for induction in IRE1alpha-null mouse embryonic fibroblasts and S2P-deficient Chinese hamster ovary (CHO) cells. We show that the endoribonuclease activity of IRE1alpha is required to splice XBP1 (X-box binding protein) mRNA to generate a new C terminus, thereby converting it into a potent UPR transcriptional activator. IRE1alpha was not required for ATF6 cleavage, nuclear translocation, or transcriptional activation. However, ATF6 cleavage was required for IRE1alpha-dependent induction of UPR transcription. We propose that nuclear-localized IRE1alpha and cytoplasmic-localized ATF6 signaling pathways merge through regulation of XBP1 activity to induce downstream gene expression. Whereas ATF6 increases the amount of XBP1 mRNA, IRE1alpha removes an unconventional 26-nucleotide intron that increases XBP1 transactivation potential. Both processing of ATF6 and IRE1alpha-mediated splicing of XBP1 mRNA are required for full activation of the UPR.

IRE1 couples endoplasmic reticulum load to secretory capacity by processing the XBP-1 mRNA

The unfolded protein response (UPR), caused by stress, matches the folding capacity of endoplasmic reticulum (ER) to the load of client proteins in the organelle. In yeast, processing of HAC1 mRNA by activated Ire1 leads to synthesis of the transcription factor Hac1 and activation of the UPR. The responses to activated IRE1 in metazoans are less well understood. Here we demonstrate that mutations in either ire-1 or the transcription-factor-encoding xbp-1 gene abolished the UPR in Caenorhabditis elegans. Mammalian XBP-1 is essential for immunoglobulin secretion and development of plasma cells, and high levels of XBP-1 messenger RNA are found in specialized secretory cells. Activation of the UPR causes IRE1-dependent splicing of a small intron from the XBP-1 mRNA both in C. elegans and mice. The protein encoded by the processed murine XBP-1 mRNA accumulated during the UPR, whereas the protein encoded by unprocessed mRNA did not. Purified mouse IRE1 accurately cleaved XBP-1 mRNA in vitro, indicating that XBP-1 mRNA is a direct target of IRE1 endonucleolytic activity. Our findings suggest that physiological ER load regulates a developmental decision in higher eukaryotes.

XBP1 mRNA is induced by ATF6 and spliced by IRE1 in response to ER stress to produce a highly active transcription factor

In yeast, the transmembrane protein kinase/endoribonuclease Ire1p activated by endoplasmic reticulum stress cleaves HAC1 mRNA, leading to production of the transcription factor Hac1p that activates the unfolded protein response (UPR). In mammals, no Hac1p counterpart has yet been discovered despite the presence of Ire1p homologs in the endoplasmic reticulum. Instead, the transcription factor ATF6 specific to the mammalian UPR is regulated by intramembrane proteolysis. Here, we identified the transcription factor XBP1, a target of ATF6, as a mammalian substrate of such an unconventional mRNA splicing system and showed that only the spliced form of XBP1 can activate the UPR efficiently. Our results reveal features of the UPR conserved during evolution and clarify the relationship between IRE1- and ATF6-dependent pathways.

Complementary signaling pathways regulate the unfolded protein response and are required for C. elegans development

The unfolded protein response (UPR) is a transcriptional and translational intracellular signaling pathway activated by the accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER). We have used C. elegans as a genetic model system to dissect UPR signaling in a multicellular organism. C. elegans requires ire-1-mediated splicing of xbp-1 mRNA for UPR gene transcription and survival upon ER stress. In addition, ire-1/xbp-1 acts with pek-1, a protein kinase that mediates translation attenuation, in complementary pathways that are essential for worm development and survival. We propose that UPR transcriptional activation by ire-1 as well as translational attenuation by pek-1 maintain ER homeostasis. The results demonstrate that the UPR and ER homeostasis are essential for metazoan development.

CCAAT/enhancer-binding protein-beta is a mediator of the nutrient-sensing response pathway that activates the human asparagine synthetase gene

Transcription from the human asparagine synthetase (AS) gene is increased in response to either amino acid (amino acid response) or glucose (unfolded protein response) deprivation. These two independent pathways converge on the same set of genomic cis-elements within the AS promoter, which are referred to as nutrient-sensing response element (NSRE)-1 and -2, both of which are absolutely necessary for gene activation. The NSRE-1 sequence was used to identify the corresponding transcription factor by yeast one-hybrid screening. Based on those results, electrophoretic mobility shift assays for individual CCAAT/enhancer-binding protein-beta (C/EBP) family members were performed to test for supershifting of complexes by specific antibodies. The results indicated that of all the family members, C/EBPbeta bound to the NSRE-1 sequence to the greatest extent and that the absolute amount of this complex was increased when extracts from amino acid- or glucose-deprived cells were tested. Using electrophoretic mobility shift assays, mutation of the NSRE-1 sequence completely prevented formation of the C/EBPbeta-containing complexes. In contrast, mutation of the NSRE-2 sequence did not block C/EBPbeta binding. Overexpression in HepG2 hepatoma cells of the activating isoform of C/EBPbeta increased AS promoter-driven transcription, whereas the inhibitory dominant-negative isoform of C/EBPbeta blocked enhanced transcription following amino acid or glucose deprivation. Collectively, the results provide both in vitro and in vivo evidence for a role of C/EBPbeta in the transcriptional activation of the AS gene in response to nutrient deprivation.

Sarco/endoplasmic reticulum calcium ATPase-2 expression is regulated by ATF6 during the endoplasmic reticulum stress response: intracellular signaling of calcium stress in a cardiac myocyte model system

The recently described transcription factor, ATF6, mediates the expression of proteins that compensate for potentially stressful changes in the endoplasmic reticulum (ER), such as reduced ER calcium. In cardiac myocytes the maintenance of optimal calcium levels in the sarcoplasmic reticulum (SR), a specialized form of the ER, is required for proper contractility. The present study investigated the hypothesis that ATF6 serves as a regulator of the expression of sarco/endoplasmic reticulum calcium ATPase-2 (SERCA2), a protein that transports calcium into the SR from the cytoplasm. Depletion of SR calcium in cultured cardiac myocytes fostered the translocation of ATF6 from the ER to the nucleus, activated the promoter for rat SERCA2, and led to increased levels of SERCA2 protein. SERCA2 promoter induction by calcium depletion was partially blocked by dominant-negative ATF6, whereas constitutively activated ATF6 led to SERCA2 promoter activation. Mutation analyses identified a promoter-proximal ER stress-response element in the rat SERCA2 gene that was required for maximal induction by ATF6 and calcium depletion. Although this element was shown to be responsible for all of the effects of ATF6 on SERCA2 promoter activation, it was responsible for only a portion of the effects of calcium depletion. Thus, SERCA2 induction in response to calcium depletion appears to be a potentially physiologically important compensatory response to this stress that involves intracellular signaling pathways that are both dependent and independent of ATF6.

Plasma cells: finding new light at the end of B cell development

Plasma cells are cellular factories devoted entirely to the manufacture and export of a single product: soluble immunoglobulin (Ig). As the final mediators of a humoral response, plasma cells play a critical role in adaptive immunity. Although intense effort has been devoted to studying the regulation and requirements for early B cell development, little information has been available on plasma cells. However, more recent work-including studies on genetically altered mice and data from microarray analyses-has begun to identify the regulatory cascades that initiate and maintain the plasma cell phenotype. This review will summarize our current understanding of the molecules that regulate commitment to a plasma cell fate and those that mediate plasma cell function.

Antitumor activity- and gene expression-based profiling of ecteinascidin Et 743 and phthalascidin Pt 650

BACKGROUND

Ecteinascidin 743 (Et 743) is a potent antitumor marine alkaloid currently undergoing phase II clinical trials. The synthetic analog phthalascidin (Pt 650), a designed structural analog of Et 743 displays in vitro potency comparable to Et 743. In this study, we used a panel of 36 human cancer cell lines, flow cytometry and oligonucleotide microarrays to analyze further these two compounds in a parallel fashion with regard to both antitumor activity (phenotype) and gene expression (genotype) bases.

RESULTS

The cancer panel experiment established that activity patterns of Et 743 and Pt 650 were essentially the same with their IC(50) values ranging from pM to low nM. By means of flow cytometric cell cycle analysis using HCT116 cells, they were shown to disrupt S phase progression after a 12-h treatment at 2.0 nM, eventually resulting in the late S and G2/M accumulation at the 24-h time point. Array-based gene expression monitoring also demonstrated that the Et 743 and Pt 650 profiles were highly similar in two distinct cancer cell lines, HCT116 colon and MDA-MB-435 breast. Characteristic changes were observed in subsets of genes involved in DNA damage response, transcription and signal transduction. In HCT116 carrying the wild-type p53 tumor suppressor gene, the up-regulation of several p53-responsive genes was evident. Furthermore, a subset of genes encoding DNA-binding proteins to specific promoter regions (e.g. the CCAAT box) was down-regulated in both cell lines, suggesting one potential mode of action of this series of antitumor agents.

CONCLUSION

A combination of gene expression analysis using oligonucleotide microarrays and flow cytometry confirms an earlier finding that Et 743 and Pt 650 have remarkably similar biological activities.

The membrane proteins, Spt23p and Mga2p, play distinct roles in the activation of Saccharomyces cerevisiae OLE1 gene expression. Fatty acid-mediated regulation of Mga2p activity is independent of its proteolytic processing into a soluble transcription activator

The Saccharomyces OLE1 gene encodes the Delta-9 fatty acid desaturase, an enzyme that converts saturated fatty acyl-CoAs into cis-Delta-9 unsaturated fatty acids. OLE1 gene expression is regulated by unsaturated fatty acids, which repress transcription and destabilize the OLE1 mRNA. Expression of OLE1 is activated by N-terminal proteolytic fragments of two homologous endoplasmic reticulum membrane proteins, Spt23p and Mga2p. Disruption of either gene does not significantly affect cell growth or fatty acid metabolism; cells that contain null alleles of both genes, however, are unsaturated fatty acid auxotrophs. An analysis of spt23Delta and mga2Delta strains shows that Spt23p and Mga2p differentially activate and regulate OLE1 transcription. In glucose-grown cells, both genes activate transcription to similar levels of activity. Expressed alone, Mga2p induces high levels of OLE1 transcription in cells exposed to cobalt or grown in glycerol-containing medium. Spt23p expressed alone activates OLE1 transcription to levels similar to those in wild type cells. OLE1 expression is strongly repressed by unsaturated fatty acids in spt23Delta or mga2Delta cells, under all growth conditions. To test if OLE1 expression is controlled by fatty acids at the level of membrane proteolysis, soluble N-terminal fragments of Spt23p and Mga2p that lack their membrane-spanning regions (Deltatm) were expressed under the control of their native promoters in spt23Delta;mga2Delta cells. Under those conditions, Mga2pDeltatm acts as a powerful transcription activator that is strongly repressed by unsaturated fatty acids. By comparison, the Spt23pDeltatm polypeptide weakly activates transcription and shows little regulation by unsaturated fatty acids. Co-expression of the two soluble fragments results in activation to levels observed with the Mga2pDeltatm protein alone. The fatty acid repression of transcription under those conditions is attenuated by Spt23Deltatm, however, suggesting that the two proteins may interact to modulate OLE1 gene expression.

Disturbed activation of endoplasmic reticulum stress transducers by familial Alzheimer's disease-linked presenilin-1 mutations

Recent studies have shown independently that presenilin-1 (PS1) null mutants and familial Alzheimer's disease (FAD)-linked mutants should both down-regulate signaling of the unfolded protein response (UPR). However, it is difficult to accept that both mutants possess the same effects on the UPR. Furthermore, contrary to these observations, neither loss of PS1 and PS2 function nor expression of FAD-linked PS1 mutants were reported to have a discernable impact on the UPR. Therefore, re-examination and detailed analyses are needed to clarify the relationship between PS1 function and UPR signaling. Here, we report that PS1/PS2 null and dominant negative PS1 mutants, which are mutated at aspartate residue 257 or 385, did not affect signaling of the UPR. In contrast, FAD-linked PS1 mutants were confirmed to disturb UPR signaling by inhibiting activation of both Ire1alpha and ATF6, both of which are endoplasmic reticulum (ER) stress transducers in the UPR. Furthermore, PS1 mutants also disturbed activation of PERK (PKR-like ER kinase), which plays a crucial role in inhibiting translation during ER stress. Taken together, these observations suggested that PS1 mutations could affect signaling pathways controlled by each of the respective ER-stress transducers, possibly through a gain-of-function.

Cooperation and competition between the binding of COUP-TFII and NF-Y on human epsilon- and gamma-globin gene promoters

The nuclear receptor COUP-TFII was recently shown to bind to the promoter of the epsilon- and gamma-globin genes and was identified as the nuclear factor NF-E3. Transgenic experiments and genetic evidence from humans affected with hereditary persistence of fetal hemoglobin suggest that NF-E3 may be a repressor of adult epsilon and gamma expression. We show that, on the epsilon-promoter, recombinant COUP-TFII binds to two sites, the more downstream of which overlaps with an NF-Y binding CCAAT box. Binding occurs efficiently to either the 5' or the 3' COUP-TFII site but not to both sites simultaneously. However, adding recombinant NF-Y induces the formation of a stable COUP-TFII.NF-Y-promoter complex at concentrations of COUP-TFII that would not give significant binding in the absence of NF-Y. Mutations of the promoter indicate that COUP-TFII cooperates with NF-Y when bound to the 5' site, whereas binding at the 3' site is mutually exclusive. Likewise, in the gamma-promoter, COUP-TFII binds to a site overlapping the distal member of a duplicated CCAAT box, competing with NF-Y binding. Transfections in K562 cells show that both the mutation of the 5' COUP-TFII or of the NF-Y site on the epsilon-promoter decrease the activity of a luciferase reporter; the mutation of the 3' COUP-TFII site has little effect. These results, together with transgenic experiments suggesting a repressive activity of COUP-TFII on the epsilon-promoter and the observation that, on the 3' site, COUP-TFII and NF-Y binding is mutually exclusive, suggest that COUP-TFII may exert different effects on epsilon transcription depending on whether it binds to the 5' or to the 3' site. At the 5' site, COUP-TFII might cooperate with NF-Y, forming a stable complex, and stimulate transcription; at the 3' site, COUP-TFII might compete for binding with NF-Y and, directly or indirectly, decrease gene activity.

Differences in HAC1 mRNA processing and translation between yeast and mammalian cells indicate divergence of the eukaryotic ER stress response

Perturbation of normal endoplasmic reticulum (ER) function induces a stress response found throughout eukaryotes, sometimes termed the unfolded protein response (UPR). In yeast, auxotrophic mutants have identified two genes, IRE1 and HAC1, whose products are key components. Normally HAC1 mRNA is not translated owing to a 252-nt "intron." Disruption of ER function activates Ire1p to remove this intron through endogenous endoribonuclease activity. Together with tRNA ligase, cleavage and splicing produces a translatable HAC1 mRNA to give Hac1p, a transcription factor that upregulates the expression of genes responsive to ER stress. No Hac1p homologue has been identified in mammalian cells, but Ire1p homologues exist with endoribonuclease activity required for a fully functional UPR, raising the possibility that the key features of the yeast UPR might be conserved in higher eukaryotic cells. To address this, we expressed yeast HAC1 in HeLa and HEK 293T human cell lines, both on its own and as fusions with yellow fluorescent protein (YFP) to investigate its processing and translation. HAC1 mRNA was not processed, but efficiently translated irrespective of whether the cells were subjected to ER stress. Expression of exogenous HAC1 mRNA constructs in yeast showed UPR-induced splicing required the presence of its 3' UTR. These results suggest that the mammalian ER stress response has diverged from the yeast UPR.

Nitric oxide-induced apoptosis in pancreatic beta cells is mediated by the endoplasmic reticulum stress pathway

Excessive nitric oxide (NO) production in cytokine-activated beta cells has been implicated in beta cell disruption in type 1 diabetes. beta cells are very vulnerable to NO-induced apoptosis. However, the mechanism underlying this phenomenon is unclear. Low concentrations of NO that lead to apoptosis apparently do not cause severe DNA damage in mouse MIN6 beta cells. CHOP, a C/EBP homologous protein that is induced by endoplasmic reticulum (ER) stress and plays a role in growth arrest and cell death, was induced by a NO donor, S-nitroso-N-acetyl-D,L-penicillamine (SNAP). SNAP increased cytosolic Ca(2+), and only agents depleting ER Ca(2+) induced CHOP expression and led to apoptosis, suggesting that NO depletes ER Ca(2+). Overexpression of calreticulin increased the Ca(2+) content of ER and afforded protection to cells against NO-mediated apoptosis. Furthermore, pancreatic islets from CHOP knockout mice showed resistance to NO. We conclude that NO depletes ER Ca(2+), causes ER stress, and leads to apoptosis. Thus, ER Ca(2+) stores are a new target of NO, and the ER stress pathway is a major mechanism of NO-mediated beta cell apoptosis.