Current Molecular Medicine - Volume 6, Issue 1, 2006

The viability of a cell strictly depends on the functional and structural integration between different subcellular compartments. At each organelle, different molecular sentinels permanently sense stressful cellular conditions and initiate a complex molecular response. This response aims either to adapt to the new conditions or to activate specific cell death signaling pathways if a critical threshold of damage has been reached. The endoplasmic reticulum (ER) is the subcellular compartment where membrane-spanning and secreted proteins are synthesized. This organelle is responsible for regulating and executing many post-translational modifications, ensuring proper protein folding and facilitating formation of protein complexes. The ER is also the place where the biosynthesis of steroids, cholesterol, and other lipids occurs, playing a crucial role in organelle biogenesis and signaling through the generation of lipid second messengers. The ER is well-known as a major calcium store in the cells and thus constitutes a signaling organelle that modulates many cellular processes including proliferation, cell death and differentiation via calcium release. A number of stress conditions, such as perturbed calcium homeostasis or redox status, elevated rate of secretory protein synthesis, altered glycosylation and cholesterol overloading, can interfere with ER functioning. These alterations lead to the accumulation of unfolded or misfolded proteins in the ER lumen, which has been referred as a cellular condition denominated "ER stress". ER stress triggers a complex adaptive reaction known as the unfolded protein response (UPR), which aims the restoration of the homeostasis of this organelle. Activation of the UPR affects the expression of proteins involved in nearly every aspect of the secretory pathway, including protein entry into the ER, folding, glycosylation, ER-associated degradation (ERAD), ER biogenesis, lipid metabolism and vesicular trafficking. The UPR restores the folding capacity to decrease unfolded protein load. The protective response of the UPR acts transiently to maintain homeostasis within the ER, but sustained ER stress ultimately leads to apoptosis by the activation of specific cell death programs. Increasing evidence indicates that the UPR is crucial for maintaining tissue homeostasis. Different physiological conditions can induce the UPR by increasing the demand of protein synthesis/secretion or by the generation of excessive misfolded proteins as described for B lymphocytes and pancreatic β cells. Also, abnormal metabolic conditions, such as glucose deprivation can trigger the UPR. Components of the ER stress pathway have been shown to be an important factor for tumor survival and growth due to an adaptation to hypoxia conditions. In addition, in different neurodegenerative conditions associated with protein misfolding (including Huntington's disease, Alzheimer's, Prion-related disorders, Amyotrophic Lateral Sclerosis and others), irreversible alteration of ER homeostasis has been proposed to be a critical mediator of neuronal dysfunction. In higher eukaryotes, ER stress stimulates three distinct signaling pathways mediated by the sensors IRE1α (inositol-requiring transmembrane kinase/endonuclease), PERK (double-stranded RNA activated protein kinase-like ER kinase), and ATF6 (activating transcription factor 6) (Fig. 1A). IRE1α is a Ser/Thr protein kinase/endoribonuclease that upon activation initiates the unconventional splicing of the mRNA encoding the transcriptional factor X-Box-binding protein 1 (XBP-1). This leads to the expression of a more stable and potent transcriptional activator, XBP-1s, a basic leucine zipper (bZIP) transcription factor that controls the upregulation of a subset of UPR-related genes. XBP-1 expression also controls organelle biogenesis. In addition, activated IRE1α can bind to the adaptor protein TRAF2 (TNF-associated factor 2), triggering the activation of the c-Jun N-terminal kinase (JNK) pathway (Fig. 1B). PERK directly phosphorylates and inhibits the translation initiation factor eIF2α decreasing the overload of misfolded proteins in this organelle (Fig. 1B). Conversely, eIF2α phosphorylation activates translation of ATF4 (activating transcription factor 4), a transcription factor that induces expression of genes that function in amino acid metabolism, the antioxidant response and apoptosis. A third UPR pathway is initiated by ATF6 (Fig. 1B), a type II ER transmembrane protein encoding a bZIP transcriptional factor on its cytosolic domain and localized in the ER in unstressed cells. Upon ER stress induction, ATF6 is exported to the Golgi, where it is processed. Cleaved ATF6 then translocates to the nucleus where it increases expression of some ER chaperones and XBP-1 transcription. Current models for the.........

The endoplasmic reticulum (ER) provides unique machinery for the folding and posttranslational modification of many secretory and transmembrane proteins in eukaryotic cells. The unfolded protein response (UPR) is a signal transduction network from the ER to the nucleus activated when the folding demand imposed by nascent, unfolded polypeptide chains exceeds the capacity of the ER protein folding machinery. In all eukaryotes the UPR maintains the physiological balance between folding demand and capacity of the ER by regulating adaptive responses to this stress situation. These include an increase in the folding capacity of the ER through induction of ER resident molecular chaperones and protein foldases, and a decrease in the folding demand on the ER by upregulation of ER associated degradation (ERAD), attenuation of general translation in metazoans, and stimulation of ER synthesis to dilute the unfolded protein load. In higher eukaryotes the UPR gained control over inflammatory and immune responses by controlling the activity of the transcription factor NF-kB to combat viral infections associated with an increased synthesis of viral glycoproteins. Similarly, in multicellular organisms apoptotic programs are controlled by the UPR to eliminate cells whose folding problems in the ER cannot be resolved by coordinated regulation of adaptive, inflammatory, and immune responses. In this review we will summarize our current understanding of signal transduction mechanisms involved in the mammalian UPR, and discuss examples to highlight the regulation of adaptive, inflammatory, immune, and apoptotic responses by the UPR.

Transmissible Spongiform Encephalopathies are fatal and infectious neurodegenerative diseases characterized by extensive neuronal apoptosis and the accumulation of an abnormally folded form of the cellular prion protein (PrP), denoted PrPSC. Compelling evidence suggests the involvement of several signaling pathways in prion pathogenesis, including proteasome dysfunction, alterations in the protein maturation pathways and the unfolded protein response. Recent reports indicate that endoplasmic reticulum stress due to the PrP misfolding may be a critical factor mediating neuronal dysfunction in prion diseases. These findings have applications for developing novel strategies for treatment and early diagnosis of transmissible spongiform encephalopathies and other neurodegenerative diseases.

GRP78, also referred to as BiP, is a central regulator of endoplasmic reticulum (ER) function due to its roles in protein folding and assembly, targeting misfolded protein for degradation, ER Ca2+- binding and controlling the activation of trans-membrane ER stress sensors. Further, due to its antiapoptotic property, stress induction of GRP78 represents an important pro-survival component of the unfolded protein response. GRP78 is induced in a wide variety of cancer cells and cancer biopsy tissues. Recent progress, utilizing overexpression and siRNA approaches, establishes that GRP78 contributes to tumor growth and confers drug resistance to cancer cells. The discovery of GRP78 expression on the cell surface of cancer cells further leads to the development of new therapeutic approaches targeted against cancer, in particular, hypoxic tumors where GRP78 is highly induced. Progress has also been made in understanding how Grp78 is induced by ER stress. The identification of the transcription factors interacting with the ER stress response element leads to the discovery of multiple pathways whereby mammalian cells can sense ER stress and trigger the transcription of Grp78. In addition, advances have been made in understanding how Grp78 expression is regulated in the context of chromatin modification. This review summarizes the transcriptional regulation of Grp78, the molecular basis for the cytoprotective function of GRP78 and its role in cancer progression, drug resistance and potential future cancer therapy.

The development of chronic and fluctuating hypoxic regions in tumors has profound consequences for malignant progression, response to therapy and overall patient survival. Understanding the events involved in hypoxia tolerance will offer new opportunities for antitumor modalities. A universal response of tumor cells to hypoxia is a rapid and substantial decrease in the rates of macromolecular synthesis. Hypoxia induces phosphorylation of the translation initiation factor eIF2α on Ser51 via activation of the endoplasmic reticulum (ER) resident kinase PERK and that this modification is required for the rapid downregulation of global protein synthesis by this hypoxic stress. PERK-dependent phosphorylation of eIF2α is one component of the Unfolded Protein Response (UPR), a coordinated program that promotes cell survival under conditions of ER stress. Inactivation of PERK or eIF2α phosphorylation impairs cell survival under hypoxia, and transformed cells with inactivating PERK or eIF2α mutations form tumors in nude mice that are slower growing, and have higher levels of apoptosis in hypoxic areas compared to tumors with an intact UPR. Expression of the transcription factor ATF4, a downstream effector of eIF2α phosphorylation, is also upregulated by hypoxia in vitro and in human tumors and increases hypoxia tolerance. A second UPR pathway mediated by activation of IRE1 and its downstream target XBP1 is also required for hypoxia tolerance in vitro and for tumor growth. These results reveal a critical role for UPR activation for tumor cell resistance to hypoxia and tumor growth promotion and suggest that the UPR may be an attractive target for anti-tumor modalities.

Increasing evidence suggests that stress signaling pathways emanating from the endoplasmic reticulum (ER) are important to the pathogenesis of both type 1 and type 2 diabetes. Recent observations indicate that ER stress signaling participates in maintaining the ER homeostasis of pancreatic β-cells. Either a high level of ER stress or defective ER stress signaling in β-cells may cause an imbalance in ER homeostasis and lead to β-cell apoptosis and autoimmune response. In addition, it has been suggested that ER stress attributes to insulin resistance in patients with type 2 diabetes. It is necessary to study the relationship between ER stress and diabetes in order to develop new therapeutic approaches to diabetes based on drugs that block the ER stress-mediated cell-death pathway and insulin resistance.

The primary mechanism by which mutations in Cu, Zn-superoxide dismutase (SOD1) contribute to progressive motor neuron loss in familial amyotrophic lateral sclerosis (FALS) remains unknown. Misfolded protein aggregates, ubiquitin-proteasome system impairment and neuronal apoptosis mediated by death receptor or mitochondrial-dependent pathways are implicated in mutant SOD1-induced toxicity. Recent evidence from cellular and transgenic rodent models of FALS proposes activation of a third apoptotic pathway linked to sustained endoplasmic reticulum (ER) stress. Here, we review the emerging role of ER stress and the unfolded protein response (UPR) in the pathogenesis of mutant SOD1-linked FALS. The UPR observed in FALS rodents is described which encompasses induction of key ER-resident chaperones during presymptomatic disease, leading to activation of stress transducers and pro-apoptotic molecules by late stage disease. Importantly, mutant SOD1 coaggregates with UPR components and recruits to the ER, suggesting a direct adverse effect on ER function. By contrast, the opposing neuroprotective effects of wild-type SOD1 overexpression on UPR signalling are also highlighted. In addition, the potential impact of neuronal Golgi apparatus (GA) fragmentation and subsequent disturbances in intracellular protein trafficking on motor neuron survival in FALS is also discussed. We propose that ER stress and UPR may be coupled to GA dysfunction in mutant SOD1-mediated toxicity, promoting ER-initiated cell death signalling in FALS.

Accumulation of unfolded and/or misfolded proteins in the endoplasmic reticulum (ER) lumen induces ER stress. ER stress triggers the unfolded protein response (UPR), which includes the attenuation of general protein synthesis and the transcriptional activation of the genes encoding ERresident chaperones and molecules involved in the ER-associated degradation (ERAD). The UPR coordinately reduces ER stress by restoration of the protein-folding capacity of the ER. However, severe and/or prolonged ER stress eventually leads cells to apoptosis. Several lines of evidence suggest that ER stress-induced apoptosis plays critical roles in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, polyglutamine (polyQ) diseases and amyotrophic lateral sclerosis (ALS). Apoptosis signal-regulating kinase 1 (ASK1), a member of the MAPKKK family that constitutes the JNK and p38 MAP kinase (MAPK) cascades, is activated by physiological and cytotoxic stresses and induces various stress responses including apoptosis. Recent studies have shown that the ASK1-MAPK cascades are involved in ER stressinduced apoptosis and in the neuronal cell death in some model systems of neurodegenerative diseases. This review highlights the current understanding of regulatory mechanisms of ASK1 with a special focus on the ER stress-dependent and -independent neuronal cell death in the context of neurodegenerative diseases.

Irreversible perturbations in the homeostasis of the endoplasmic reticulum (ER) are thought to lead to apoptosis and cell loss in a number of important human diseases, including Alzheimer disease, Parkinson disease, and type 2 diabetes. However, the exact mechanisms that lead from ER stress to cell death remain incompletely understood. Recent work has shown that the BCL-2 family of proteins plays a central role in regulating this form of cell death, both locally at the ER and from a distance at the mitochondrial membrane.

The expanded polyglutamine (polyQ) tracts observed in autosomal dominant neurodegenerative disorders have the tendency to form intracellular aggregates, thus enhancing apoptotic cell death and the formation of autophagic vesicles. PolyQ accumulation inhibits the ERassociated degradation system (ERAD) resulting in reduced retrotranslocation from the ER and increased accumulation of misfolded proteins in the lumen of ER. Autophagy is an early cellular defense mechanism associated with ER stress, but prolonged ER stress may induce autophagic cell death, with destruction of cellular components and apoptotic cell death. Endoplasmic reticulum (ER) stress may be the key signal for both of these events.

Increased cerebral levels of Aβ42 peptide, either as soluble or aggregated forms, are suggested to play a key role in the pathogenesis of Alzheimer's disease (AD). The identification of genetic defects in presenilins and β-amyloid precursor protein (β-APP) has led to the development of cellular and animal models that have helped in understanding aspects of the pathophysiology of the inherited early onset forms of AD. However, the majority of AD cases are sporadic with no clear or defined genetic basis. While genetic mutations are responsible for the accumulation of Aβ in early onset AD, the causative factors for accumulation of Aβ in the late onset AD forms are not known. This raises the possibility that Aβ accumulation in the absence of genetic mutations might result from abnormalities that indirectly affect Aβ production or its clearance. Currently, there is no consensus as to what are the mechanisms by which Aβ accumulates or as to which mechanisms underlie Aβ-induced neuronal death in AD. In this review, I will first describe the physiological role of endoplasmic reticulum in the cell and review some of the data supporting dysfunction of the endoplasmic reticulum as an early event leading to Aβ accumulation in familial AD. I will also discuss the possible role of oxidative stress and other factors as contributors in Aβ accumulation by reducing the clearance of Aβ from the endoplasmic reticulum. Finally, I will summarize data that show the endoplasmic reticulum stress as a mechanism underlying exogenous Aβ neurotoxicity.