Transcription Factor CHOP

Abstract: 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.

Abstract: 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.

Abstract: Unfolded and malfolded client proteins impose a stress on the endoplasmic reticulum (ER), which contributes to cell death in pathophysiological conditions. The transcription factor C/EBP homologous protein (CHOP) is activated by ER stress, and CHOP deletion protects against its lethal consequences. We find that CHOP directly activates GADD34, which promotes ER client protein biosynthesis by dephosphorylating phospho-Ser 51 of the alpha-subunit of translation initiation factor 2 (eIF2alpha) in stressed cells. Thus, impaired GADD34 expression reduces client protein load and ER stress in CHOP(-/-) cells exposed to perturbations that impair ER function. CHOP(-/-) and GADD34 mutant cells accumulate less high molecular weight protein complexes in their stressed ER than wild-type cells. Furthermore, mice lacking GADD34-directed eIF2alpha dephosphorylation, like CHOP(-/-) mice, are resistant to renal toxicity of the ER stress-inducing drug tunicamycin. CHOP also activates ERO1alpha, which encodes an ER oxidase. Consequently, the ER of stressed CHOP(-/-) cells is relatively hypo-oxidizing. Pharmacological and genetic manipulations that promote a hypo-oxidizing ER reduce abnormal high molecular weight protein complexes in the stressed ER and protect from the lethal consequences of ER stress. CHOP deletion thus protects cells from ER stress by decreasing ER client protein load and changing redox conditions within the organelle.

Abstract: Phosphorylation of the α subunit of eukaryotic translation initiation factor 2 (eIF2α) on serine 51 integrates general translation repression with activation of stress-inducible genes such as ATF4, CHOP, and BiP in the unfolded protein response. We sought to identify new genes active in this phospho-eIF2α–dependent signaling pathway by screening a library of recombinant retroviruses for clones that inhibit the expression of a CHOP::GFP reporter. A retrovirus encoding the COOH terminus of growth arrest and DNA damage gene (GADD)34, also known as MYD116 (Fornace, A.J., D.W. Neibert, M.C. Hollander, J.D. Luethy, M. Papathanasiou, J. Fragoli, and N.J. Holbrook. 1989. Mol. Cell. Biol. 9:4196–4203; Lord K.A., B. Hoffman-Lieberman, and D.A. Lieberman. 1990. Nucleic Acid Res. 18:2823), was isolated and found to attenuate CHOP (also known as GADD153) activation by both protein malfolding in the endoplasmic reticulum, and amino acid deprivation. Despite normal activity of the cognate stress-inducible eIF2α kinases PERK (also known as PEK) and GCN2, phospho-eIF2α levels were markedly diminished in GADD34-overexpressing cells. GADD34 formed a complex with the catalytic subunit of protein phosphatase 1 (PP1c) that specifically promoted the dephosphorylation of eIF2α in vitro. Mutations that interfered with the interaction with PP1c prevented the dephosphorylation of eIF2α and blocked attenuation of CHOP by GADD34. Expression of GADD34 is stress dependent, and was absent in PERK−/− and GCN2−/− cells. These findings implicate GADD34-mediated dephosphorylation of eIF2α in a negative feedback loop that inhibits stress-induced gene expression, and that might promote recovery from translational inhibition in the unfolded protein response.

Abstract: We report on the identification of a nuclear protein that serves as a dominant-negative inhibitor of the transcription factors C/EBP and LAP. A 32P-labeled LAP DNA-binding and dimerization domain "zipper probe" was used to isolate a clone that encodes a new C/EBP-homologous protein: CHOP-10. CHOP-10 has strong sequence similarity to C/EBP-like proteins within the bZIP region corresponding to the DNA-binding domain consisting of a leucine zipper and a basic region. Notably, however, CHOP-10 contains 2 prolines substituting for 2 residues in the basic region, critical for binding to DNA. Thus, heterodimers of CHOP-10 and C/EBP-like proteins are unable to bind their cognate DNA enhancer element. CHOP-10 mRNA is expressed in many different rat tissues. Antisera raised against CHOP-10 recognize a nuclear protein with an apparent molecular mass of 29 kD. CHOP-10 is induced upon differentiation of 3T3-L1 fibroblasts to adipocytes, and cytokine-induced dedifferentiation of adipocytes is preceded by the loss of nuclear CHOP-10. Coimmunoprecipitation of CHOP-10 and LAP from transfected COS-1 cells demonstrated a direct interaction between the two proteins, in vivo. Consistent with the structure of its defective basic region, bacterially expressed CHOP-10 inhibits the DNA-binding activity of C/EBP and LAP by forming heterodimers that cannot bind DNA. In transfected HepG2 cells, expression of CHOP-10 attenuates activation of C/EBP- and LAP-driven promoters. We suggest that CHOP-10 is a negative modulator of the activity of C/EBP-like proteins in certain terminally differentiated cells, similar to the regulatory function of Id on the activity of MyoD and MyoD-related proteins important in the development of muscle cells.