bims-unfpre Biomed News
on Unfolded protein response
Issue of 2019–11–24
six papers selected by
Susan Logue, University of Manitoba
  1. Excessive ER-phagy mediated by the autophagy receptor FAM134B results in ER stress, the unfolded protein response, and cell death in HeLa cells.
  2. Proteostasis In The Endoplasmic Reticulum: Road to Cure.
  3. The active nuclear form of SREBP1 amplifies ER stress and autophagy via regulation of PERK.
  4. The ribosomal P-stalk couples amino acid starvation to GCN2 activation in mammalian cells.
  5. Dysregulation of very long chain fatty acid metabolism causes membrane saturation and induction of the unfolded protein response.
  6. Regulation of reticulophagy by the N-degron pathway.
  1. J Biol Chem. 2019 Nov 20. pii: jbc.RA119.008709. [Epub ahead of print]
    Excessive ER-phagy mediated by the autophagy receptor FAM134B results in ER stress, the unfolded protein response, and cell death in HeLa cells.
    Yangjie Liao, Bo Duan, Yufei Zhang, Xinmin Zhang, Bin Xia.
      Autophagy is typically a pro-survival cellular process that promotes the turnover of long-lived proteins and damaged organelles, but it can also induce cell death. We have previously reported that the small molecule Z36 induces autophagy along with autophagic cell death in HeLa cells. In this study, we analyzed differential gene expression in Z36-treated HeLa cells, and found that Z36-induced endoplasmic reticulum (ER)-phagy results in ER stress and the unfolded protein response (UPR). This result is in contrast to the common notion that autophagy is generally activated in response to ER stress and the UPR. We demonstrate that Z36 upregulates the expression levels of FAM134B, LC3, and Atg9, which together mediate excessive ER-phagy, characterized by forming increased numbers of autophagosomes with larger sizes. We noted that the excessive ER-phagy accelerates ER degradation and impairs ER homeostasis, and thereby triggers ER stress and the UPR, as well as ER-phagy-dependent cell death. Interestingly, overexpression of FAM134B alone in HeLa cells is sufficient to impair ER homeostasis and cause ER stress and cell death. These findings suggest a mechanism involving FAM134B activity for ER-phagy to promote cell death.
    Keywords:  Bcl-xL inhibitor; ER-phagy; FAM134B; Z36; autophagy; cell death; endoplasmic reticulum (ER); endoplasmic reticulum stress (ER stress); reticulophagy regulator 1 (RETREG1); unfolded protein response (UPR)
    DOI:  https://doi.org/10.1074/jbc.RA119.008709
  2. Cancers (Basel). 2019 Nov 14. pii: E1793. [Epub ahead of print]11(11):
    Proteostasis In The Endoplasmic Reticulum: Road to Cure.
    Su Min Nam, Young Joo Jeon.
      The endoplasmic reticulum (ER) is an interconnected organelle that is responsible for the biosynthesis, folding, maturation, stabilization, and trafficking of transmembrane and secretory proteins. Therefore, cells evolve protein quality-control equipment of the ER to ensure protein homeostasis, also termed proteostasis. However, disruption in the folding capacity of the ER caused by a large variety of pathophysiological insults leads to the accumulation of unfolded or misfolded proteins in this organelle, known as ER stress. Upon ER stress, unfolded protein response (UPR) of the ER is activated, integrates ER stress signals, and transduces the integrated signals to relive ER stress, thereby leading to the re-establishment of proteostasis. Intriguingly, severe and persistent ER stress and the subsequently sustained unfolded protein response (UPR) are closely associated with tumor development, angiogenesis, aggressiveness, immunosuppression, and therapeutic response of cancer. Additionally, the UPR interconnects various processes in and around the tumor microenvironment. Therefore, it has begun to be delineated that pharmacologically and genetically manipulating strategies directed to target the UPR of the ER might exhibit positive clinical outcome in cancer. In the present review, we summarize recent advances in our understanding of the UPR of the ER and the UPR of the ER-mitochondria interconnection. We also highlight new insights into how the UPR of the ER in response to pathophysiological perturbations is implicated in the pathogenesis of cancer. We provide the concept to target the UPR of the ER, eventually discussing the potential of therapeutic interventions for targeting the UPR of the ER for cancer treatment.
    Keywords:  ER-associated protein degradation (ERAD), protein quality control; cancer; endoplasmic reticulum (ER) stress; proteostasis; therapeutic targets; unfolded protein response (UPR) of the ER
    DOI:  https://doi.org/10.3390/cancers11111793
  3. FEBS J. 2019 Nov 17.
    The active nuclear form of SREBP1 amplifies ER stress and autophagy via regulation of PERK.
    Qin Hu, Yu Mao, Min Liu, Rui Luo, Rong Jiang, Fengjin Guo.
      Endoplasmic reticulum (ER) stress and autophagy dysfunction contribute to the establishment and progression of diverse pathologies. Proteolytic activation of the transcription factor nSREBP1 is induced under ER stress; however, little is known about how SREBP1 and its nuclear active form nSREBP1 influence autophagy and unfolded protein response (UPR) activation in osteosarcoma cells. Our research focused on the effect of SREBP1/nSREBP1 upon apoptosis and autophagy during ER stress and the molecular mechanisms involved. Here, we showed that nSREBP1 binds to the promoter of protein kinase RNA-like endoplasmic reticulum kinase (PERK), then regulates ER stress, cell growth, cell apoptosis and autophagy through the PERK signaling pathway. nSREBP1 increased PERK gene expression and phosphorylation. nSREBP1 was further demonstrated to activate ER stress response through stimulatory effects on PERK signaling. Overexpression of SREBP1 increased its cleavage and release of nSREBP1, therefore, the effect of SREBP1 is achieved through the enhancement of the expression of nSREBP1. Overexpression of SREBP1/nSREBP1 amplifies PERK- associated cell cycle stagnation with G1 phase arresting, S phase reducing and G2-M phase delaying. LV-SREBP1/nSREBP1 can also bolster PERK's ER stress-associated pro-apoptotic effects. LV-SREBP1/nSREBP1 and LV-PERK can activate autophagy in ER stress response, along with the overexpression of SREBP1/nSREBP1 and PERK. This resulted in amplification of PERK-related changes to cell proliferation, ER stress-mediated apoptosis and autophagy, with the biological effect of nSREBP1 relying on PERK, which makes up one of the three branches of the UPR signal pathway. This study reveals important roles for SREBP1/nSREBP1 in PERK signaling under ER stress. Furthermore, nSREBP1, the nuclear active form of SREBP1, is able to robustly augment effects of PERK. Description of the link between PERK and SREBP1/nSREBP1 function offers an improved understanding of the ER stress response and insight into the biological function of SREBP1/nSREBP.
    Keywords:  Autophagy; ER stress; PERK signal pathway; SREBP1; nSREBP1
    DOI:  https://doi.org/10.1111/febs.15144
  4. Elife. 2019 Nov 21. pii: e50149. [Epub ahead of print]8
    The ribosomal P-stalk couples amino acid starvation to GCN2 activation in mammalian cells.
    Heather P Harding, Adriana Ordonez, Felicity Allen, Leopold Parts, Alison J Inglis, Roger L Williams, David Ron.
      The eukaryotic translation initiation factor 2a (eIF2a) kinase GCN2 is activated by amino acid starvation to elicit a rectifying physiological program known as the Integrated Stress Response (ISR). A role for uncharged tRNAs as activating ligands of yeast GCN2 is supported experimentally. However, mouse GCN2 activation has recently been observed in circumstances associated with ribosome stalling with no global increase in uncharged tRNAs. We report on a mammalian CHO cell-based CRISPR-Cas9 mutagenesis screen for genes that contribute to ISR activation by amino acid starvation. Disruption of genes encoding components of the ribosome P-stalk, uL10 and P1, selectively attenuated GCN2-mediated ISR activation by amino acid starvation or interference with tRNA charging without affecting the endoplasmic reticulum unfolded protein stress-induced ISR, mediated by the related eIF2a kinase PERK. Wildtype ribosomes isolated from CHO cells, but not those with P-stalk lesions, stimulated GCN2-dependent eIF2a phosphorylation in vitro. These observations support a model whereby lack of a cognate charged tRNA exposes a latent capacity of the ribosome P-stalk to activate GCN2 in cells and help explain the emerging link between ribosome stalling and ISR activation.
    Keywords:  cell biology; genetics; genomics
    DOI:  https://doi.org/10.7554/eLife.50149
  5. Mol Biol Cell. 2019 Nov 20. mbcE19070392
    Dysregulation of very long chain fatty acid metabolism causes membrane saturation and induction of the unfolded protein response.
    Yagmur Micoogullari, Sankha S Basu, Jessie Ang, Nina Weisshaar, Nicholas D Schmitt, Walid M Abdelmoula, Begona Lopez, Jeffrey N Agar, Nathalie Agar, John Hanna.
      The unfolded protein response (UPR) senses defects in the endoplasmic reticulum and orchestrates a complex program of adaptive cellular remodeling. Increasing evidence suggests an important relationship between lipid homeostasis and the UPR. Defects in the ER membrane induce the UPR, and the UPR in turn controls the expression of some lipid metabolic genes. Among lipid species, the very long chain fatty acids (VLCFAs) are relatively rare and poorly understood. Here we show that loss of the VLCFA-CoA synthetase Fat1, which is essential for VLCFA utilization, results in ER stress with compensatory UPR induction. Comprehensive lipidomic analyses revealed a dramatic increase in membrane saturation in the fat1Δ mutant, likely accounting for UPR induction. In principle, this increased membrane saturation could reflect adaptive membrane remodeling or an adverse effect of VLCFA dysfunction. We provide evidence supporting the latter as the fat1Δ mutant showed defects in the function of Ole1, the sole fatty acyl desaturase in yeast. These results indicate that VLCFAs play essential roles in protein quality control and membrane homeostasis, and suggest an unexpected requirement for VLCFAs in Ole1 function.
    DOI:  https://doi.org/10.1091/mbc.E19-07-0392
  6. Autophagy. 2019 Nov 20.
    Regulation of reticulophagy by the N-degron pathway.
    Chang Hoon Ji, Hee Yeon Kim, Ah Jung Heo, Min Ju Lee, Daniel Youngjae Park, Dong Hyun Kim, Bo Yeon Kim, Yong Tae Kwon.
      Cellular homeostasis requires selective autophagic degradation of damaged or defective organelles, including the endoplasmic reticulum (ER). Previous studies have shown that specific ER transmembrane receptors recruit LC3 on autophagic membranes by using LC3-interacting domains. In this study, we showed that the N-degron pathway mediates ubiquitin (Ub)-dependent reticulophagy. During this 2-step process, the ER transmembrane E3 ligase TRIM13 undergoes auto-ubiquitination via lysine 63 (K63) linkage chains and acts as a ligand for the autophagic receptor SQSTM1/p62 (sequestosome 1). In parallel, ER-residing molecular chaperones, such as HSPA5/GRP78/BiP, are relocated to the cytosol and conjugated with the amino acid L-arginine (Arg) at the N-termini by ATE1 (arginyltransferase 1). The resulting N-terminal Arg (Nt-Arg) binds the ZZ domain of SQSTM1, inducing oligomerization of SQSTM1-TRIM13 complexes and facilitating recruitment of LC3 on phagophores to the sites of reticulophagy. We developed small molecule ligands to the SQSTM1 ZZ domain and demonstrate that these chemical mimics of Nt-Arg facilitate reticulophagy and autophagic protein quality control of misfolded aggregates in the ER.
    Keywords:  ER homeostasis; ER protein quality control; ER stress response; ER-phagy; N-degron pathway; N-terminal arginylation; SQSTM1/p62; TRIM13; alpha1-antitrypsin deficiency; ubiquitination
    DOI:  https://doi.org/10.1080/15548627.2019.1695402
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