The endoplasmic reticulum (ER) responds to the accumulation of unfolded proteins in its lumen (ER stress) by activating intracellular signal transduction pathways - cumulatively called the unfolded protein response (UPR). Together, at least three mechanistically distinct arms of the UPR regulate the expression of numerous genes that function within the secretory pathway but also affect broad aspects of cell fate and the metabolism of proteins, amino acids and lipids. The arms of the UPR are integrated to provide a response that remodels the secretory apparatus and aligns cellular physiology to the demands imposed by ER stress.

Signal integration in the endoplasmic reticulum unfolded protein response

Apoptosis, or cellular suicide, is important for normal development and tissue homeostasis, but too much or too little apoptosis can also cause disease. The family of cysteine proteases, the so- called caspases, are critical mediators of programmed cell death, and thus far 14 family members have been identified. Some of these, such as caspase-8, mediate signal transduction downstream of death receptors located on the plasma membrane. Others, such as caspase-9, mediate apoptotic signals after mitochondrial damage. Stress in the endoplasmic reticulum (ER) can also result in apoptosis. Here we show that caspase-12 is localized to the ER and activated by ER stress, including disruption of ER calcium homeostasis and accumulation of excess proteins in ER, but not by membrane- or mitochondrial-targeted apoptotic signals. Mice that are deficient in caspase-12 are resistant to ER stress-induced apoptosis, but their cells undergo apoptosis in response to other death stimuli. Furthermore, we show that caspase-12-deficient cortical neurons are defective in apoptosis induced by amyloid-beta protein but not by staurosporine or trophic factor deprivation. Thus, caspase-12 mediates an ER-specific apoptosis pathway and may contribute to amyloid-beta neurotoxicity.

Caspase-12 mediates endoplasmic-reticulum-specific apoptosis and cytotoxicity by amyloid-beta.

Regulated Translation Initiation Controls Stress-Induced Gene Expression in Mammalian Cells

Endoplasmic reticulum (ER) is the site of synthesis and folding of secretory proteins. Perturbations of ER homeostasis affect protein folding and cause ER stress. ER can sense the stress and respond to it through translational attenuation, upregulation of the genes for ER chaperones and related proteins, and degradation of unfolded proteins by a quality-control system. However, when the ER function is severely impaired, the organelle elicits apoptotic signals. ER stress has been implicated in a variety of common diseases such as diabetes, ischemia and neurodegenerative disorders. One of the components of the ER stress-mediated apoptosis pathway is C/EBP homologous protein (CHOP), also known as growth arrest- and DNA damage-inducible gene 153 (GADD153). Here, we summarize the current understanding of the roles of CHOP/GADD153 in ER stress-mediated apoptosis and in diseases including diabetes, brain ischemia and neurodegenerative disease.

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Roles of CHOP/GADD153 in endoplasmic reticulum stress.

All eukaryotic cells have an extensive membranous labyrinth network of branching tubules and flattened sacs called the endoplasmic reticulum (ER). Approximately one-third of all cellular proteins are translocated into the lumen of the ER where post-translational modification, folding, and oligomerization occurs. The ER provides a unique oxidizing compartment for the folding of membrane and secretory proteins that are destined to the cell surface, as well as for proteins destined to other intracellular organelles, such as lysosomes and the Golgi compartment. Numerous cellular proteins reside within the ER through a mechanism that requires their continuous vesicle-mediated retrieval from post-ER compartments within the early secretory pathway. These ERresident proteins are chaperones and catalysts of protein folding that form a matrix on which newly synthesized proteins attain their final conformation. The ER is also the site of synthesis of cellular lipids and sterols. In addition, the ER is the major signal-transducing organelle within the cell that continuously responds to environmental cues to release calcium. The ER is exquisitely sensitive to alterations in homeostasis, where, upon a variety of different stimuli, signals are transduced from the ER to the cytoplasm and the nucleus to eventually result in adaptation for survival or induction of apoptosis. The immediate response occurs at the translational apparatus, whereas changes in gene expression promote long-term adaptation or apoptotic cell death. Recent evidence supports findings that these signaling pathways influence pathogenesis associated with viral infection and genetic disease. The purpose of this review is to summarize what is presently known about the diversity of molecular signaling mechanisms that coordinate the complex ER stress response at the translational and transcriptional level in yeast and in higher eukaryotic cells.

/pdf/stress-signaling-from-the-lumen-of-the-endoplasmic-reticulum-50luw2lbp2.pdf

Stress signaling from the lumen of the endoplasmic reticulum: coordination of gene transcriptional and translational controls

Cellular stress, particularly in response to toxic and metabolic insults that perturb function of the endoplasmic reticulum (ER stress), is a powerful inducer of the transcription factor CHOP. The role of CHOP in the response of cells to injury associated with ER stress was examined in a murine deficiency model obtained by homologous recombination at the chop gene. Compared with the wild type, mouse embryonic fibroblasts (MEFs) derived from chop -/- animals exhibited significantly less programmed cell death when challenged with agents that perturb ER function. A similar deficit in programmed cells death in response to ER stress was also observed in MEFs that lack CHOP's major dimerization partner, C/EBPbeta, implicating the CHOP-C/EBP pathway in programmed cell death. An animal model for studying the effects of chop on the response to ER stress was developed. It entailed exposing mice with defined chop genotypes to a single sublethal intraperitoneal injection of tunicamycin and resulted in a severe illness characterized by transient renal insufficiency. In chop +/+ and chop +/- mice this was associated with the early expression of CHOP in the proximal tubules followed by the development of a histological picture similar to the human condition known as acute tubular necrosis, a process that resolved by cellular regeneration. In the chop -/- animals, in spite of the severe impairment in renal function, evidence of cellular death in the kidney was reduced compared with the wild type. The proximal tubule epithelium of chop -/- animals exhibited fourfold lower levels of TUNEL-positive cells (a marker for programmed cell death), and significantly less evidence for subsequent regeneration. CHOP therefore has a role in the induction of cell death under conditions associated with malfunction of the ER and may also have a role in cellular regeneration under such circumstances.

/pdf/chop-is-implicated-in-programmed-cell-death-in-response-to-rkxn9wc5w0.pdf

CHOP is implicated in programmed cell death in response to impaired function of the endoplasmic reticulum

Activation of Heat Shock Factor by Alkylating Agents Is Triggered by Glutathione Depletion and Oxidation of Protein Thiols

Hepatocyte growth factor (HGF)-induced tubulogenesis has been demonstrated with renal epithelial cell lines grown in collagen gels but not with primary cultured renal proximal tubular epithelial cells (RPTEs). We show that HGF selectively induces proliferation and branching morphogenesis of primary cultured rat RPTEs. Additional growth factors including fibroblast growth factor (FGF)-1, epidermal growth factor (EGF), FGF-7, or insulin-like growth factor-1 (IGF-1) did not selectively induce tubulogenesis. However, when administered in combination, these factors initiated branching morphogenesis comparable to HGF alone and greatly augmented HGF-induced proliferation and branching. Microscopic analysis revealed that branching RPTEs were undergoing tubulogenesis and formed a polarized epithelium. TGF-β1 blocked HGF- or growth factor cocktail (GFC; HGF, FGF-1, EGF, IGF-1)-induced proliferation and branching morphogenesis. Adding TGF-β1 after GFC-induced tubulogenesis had occurred caused a progressive regression of the tubular structures, a response associated with an increase in apoptosis of the RPTEs. Primary cultured RPTEs are capable of undergoing HGF-induced tubulogenesis. Unlike cell lines, combinations of growth factors differentially augment the response. J. Cell. Physiol. 180:81–90, 1999. © 1999 Wiley-Liss, Inc.

Richard T. Lightfoot

Papers

CHOP is implicated in programmed cell death in response to impaired function of the endoplasmic reticulum

Activation of Heat Shock Factor by Alkylating Agents Is Triggered by Glutathione Depletion and Oxidation of Protein Thiols

Hepatocyte growth factor induces tubulogenesis of primary renal proximal tubular epithelial cells.