bims-unfpre Biomed News
on Unfolded protein response
Issue of 2020‒08‒16
eight papers selected by
Susan Logue
University of Manitoba

  1. J Cell Sci. 2020 Aug 11. pii: jcs218107. [Epub ahead of print]133(15):
    Urra H, Pihán P, Hetz C.
      Different perturbations alter the function of the endoplasmic reticulum (ER), resulting in the accumulation of misfolded proteins in its lumen, a condition termed ER stress. To restore ER proteostasis, a highly conserved pathway is engaged, known as the unfolded protein response (UPR), triggering adaptive programs or apoptosis of terminally damaged cells. IRE1α (also known as ERN1), the most conserved UPR sensor, mediates the activation of responses to determine cell fate under ER stress. The complexity of IRE1α regulation and its signaling outputs is mediated in part by the assembly of a dynamic multi-protein complex, named the UPRosome, that regulates IRE1α activity and the crosstalk with other pathways. We discuss several studies identifying components of the UPRosome that have illuminated novel functions in cell death, autophagy, DNA damage, energy metabolism and cytoskeleton dynamics. Here, we provide a theoretical analysis to assess the biological significance of the UPRosome and present the results of a systematic bioinformatics analysis of the available IRE1α interactome data sets followed by functional enrichment clustering. This in silico approach decoded that IRE1α also interacts with proteins involved in the cell cycle, transport, differentiation, response to viral infection and immune response. Thus, defining the spectrum of IRE1α-binding partners will reveal novel signaling outputs and the relevance of the pathway to human diseases.
    Keywords:  Cell fate; ER stress; IRE1α; UPRosome
  2. J Biol Chem. 2020 Aug 11. pii: jbc.RA120.014173. [Epub ahead of print]
    Sharma RB, Darko C, Alonso LC.
      Success or failure of pancreatic beta cell adaptation to ER stress is a determinant of diabetes susceptibility. The ATF6 and IRE1/XBP1 pathways are separate ER stress response effectors important to beta cell health and function. ATF6 and XBP1 direct overlapping transcriptional responses in some cell types. However, the signaling dynamics and interdependence of ATF6α and XBP1 in pancreatic beta cells have not been explored. To assess pathway-specific signal onset, we performed timed exposures of primary mouse islet cells to ER stressors and measured the early transcriptional response. Comparing the time course of induction of ATF6 and XBP1 targets suggested the two pathways have similar response dynamics. The role of ATF6α in target induction was assessed by acute knockdown using islet cells from Atf6αflox/flox mice transduced with adenovirus expressing Cre recombinase. Surprisingly given the mild impact of chronic deletion in mice, acute ATF6α knockdown markedly reduced ATF6-pathway target gene expression under both basal and stressed conditions. Intriguingly, while ATF6α knockdown did not alter Xbp1 splicing dynamics or intensity, it did reduce induction of XBP1 targets. Inhibition of Xbp1 splicing did not decrease induction of ATF6α targets. Taken together, these data suggest that the XBP1 and ATF6 pathways are simultaneously activated in islet cells in response to acute stress, and that ATF6α is required for full activation of XBP1 targets, but XBP1 is not required for activation of ATF6α targets. These observations improve understanding of the ER stress transcriptional response in pancreatic islets.
    Keywords:  diabetes; endoplasmic reticulum stress (ER stress); insulin synthesis; pancreatic beta cell; pancreatic islet; unfolded protein response (UPR)
  3. J Cell Physiol. 2020 Aug 10.
    Tao Y, Qiu T, Yao X, Jiang L, Wang N, Jiang J, Jia X, Wei S, Zhang J, Zhu Y, Tian W, Yang G, Liu X, Liu S, Ding Y, Sun X.
      Liver fibrosis is a severe health problem worldwide, and it is characterized by the activation of hepatic stellate cells (HSCs) and excessive deposition of collagen. Prolonged arsenic exposure can induce HSCs activation and liver fibrosis. In the present study, the results showed that chronic NaAsO2 ingestion could result in liver fibrosis and oxidative stress in Sprague-Dawley rats, along with representative collagen deposition and HSCs activation. In addition, the inositol-requiring enzyme 1α (IRE1α)-endoplasmic reticulum (ER)-stress pathway was activated, and the activity of nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) was upregulated in rat livers. Simultaneously, the excessive production of reactive oxygen species (ROS) could induce HSCs activation, and NOX4 played an important role in generating ROS in vitro. Moreover, ER stress occurred with HSCs activation at the same time under NaAsO2 exposure, and during ER stress, the IRE1α pathway was responsible for NOX4 activation. Therefore, inhibition of IRE1α activation could attenuate the HSCs activation induced by NaAsO2 . In conclusion, the present study manifested that inorganic arsenic exposure could activate HSCs through IRE1α/NOX4-mediated ROS generation.
    Keywords:  NOX4; ROS; arsenic; hepatic stellate cells; liver fibrosis
  4. FASEB J. 2020 Aug 11.
    Yang M, Zhang D, Zhao Z, Sit J, Saint-Sume M, Shabandri O, Zhang K, Yin L, Tong X.
      Prolonged ER stress has been known to be one of the major drivers of impaired lipid homeostasis during the pathogenesis of non-alcoholic liver disease (NAFLD). However, the downstream mediators of ER stress pathway in promoting lipid accumulation remain poorly understood. Here, we present data showing the b-ZIP transcription factor E4BP4 in both the hepatocytes and the mouse liver is potently induced by the chemical ER stress inducer tunicamycin or by high-fat, low-methionine, and choline-deficient (HFLMCD) diet. We showed that such an induction is partially dependent on CHOP, a known mediator of ER stress and requires the E-box element of the E4bp4 promoter. Tunicamycin promotes the lipid droplet formation and alters lipid metabolic gene expression in primary mouse hepatocytes from E4bp4flox/flox but not E4bp4 liver-specific KO (E4bp4-LKO) mice. Compared with E4bp4flox/flox mice, E4bp4-LKO female mice exhibit reduced liver lipid accumulation and partially improved liver function after 10-week HFLMCD diet feeding. Mechanistically, we observed elevated AMPK activity and the AMPKβ1 abundance in the liver of E4bp4-LKO mice. We have evidence supporting that E4BP4 may suppress the AMPK activity via promoting the AMPKβ1 ubiquitination and degradation. Furthermore, acute depletion of the Ampkβ1 subunit restores lipid droplet formation in E4bp4-LKO primary mouse hepatocytes. Our study highlighted hepatic E4BP4 as a key factor linking ER stress and lipid accumulation in the liver. Targeting E4BP4 in the liver may be a novel therapeutic avenue for treating NAFLD.
    Keywords:  ER stress; de novo lipogenesis; high-fat, low-methionine, and choline-deficient diet; lipid accumulation; lipid droplet
  5. Sci Signal. 2020 Aug 11. pii: eabb4749. [Epub ahead of print]13(644):
    Hughes DT, Halliday M, Smith HL, Verity NC, Molloy C, Radford H, Butcher AJ, Mallucci GR.
      Chronic activation of the unfolded protein response (UPR), notably the branch comprising the kinase PERK and the translation initiation factor eIF2α, is a pathological feature of many neurodegenerative diseases caused by protein misfolding. Partial reduction of UPR signaling at the level of phosphorylated eIF2α is neuroprotective and avoids the pancreatic toxicity caused by full inhibition of PERK kinase activity. However, other stress pathways besides the UPR converge on phosphorylated eIF2α in the integrated stress response (ISR), which is critical to normal cellular function. We explored whether partial inhibition of PERK signaling may be a better therapeutic option. PERK-mediated phosphorylation of eIF2α requires its binding to the insert loop within PERK's kinase domain, which is, itself, phosphorylated at multiple sites. We found that, as expected, Akt mediates the phosphorylation of Thr799 in PERK. This phosphorylation event reduced eIF2α binding to PERK and selectively attenuated downstream signaling independently of PERK activity and the broader ISR. Induction of Thr799 phosphorylation with a small-molecule activator of Akt similarly reduced PERK signaling and increased both neuronal and animal survival without measurable pancreatic toxicity in a mouse model of prion disease. Thus, promoting PERK phosphorylation at Thr799 to partially down-regulate PERK-eIF2α signaling while avoiding widespread ISR inhibition may be a safe therapeutic approach in neurodegenerative disease.
  6. FEBS Open Bio. 2020 Aug 08.
    Yu N, Yang L, Ling L, Liu Y, Yu Y, Wu Q, Gu Y, Niu J.
      Podocytes are an important component of the glomerular filtration barrier in the kidneys. The dysfunction and apoptosis of podocytes are important factors that can lead to the progression of chronic kidney disease (CKD). In CKD, angiotensin II is continuously elevated in circulation and is considered to have key roles in inducing podocyte injury and apoptosis. Curcumin is a hydrophobic polyphenolic compound extracted from turmeric. Increasing evidence demonstrates that curcumin has a protective effect on the kidneys in CKD. However, the mechanisms mediating this protective effect remain unclear. The aim of this study was to explore whether curcumin could protect against angiotensin II-induced injury and apoptosis of podocytes. We performed western blotting, immunofluorescence, phalloidin staining, and terminal deoxynulceotidyl transferase nick-end-labeling (TUNEL) staining to observe the expression level of podocyte-specific proteins, apoptosis-related proteins, and the arrangement of F-actin. We found that curcumin could reverse angiotensin II-induced podocyte injury and apoptosis in a dose-dependent manner. In addition, curcumin dose-dependently attenuated a pro-apoptotic pathway, activated by angiotensin II-induced endoplasmic reticulum stress. Conversely, the protective effects of curcumin were impaired upon addition of tunicamycin, an activator of endoplasmic reticulum stress. Thus, we speculate that curcumin protects against angiotensin II-induced podocyte injury and apoptosis, at least partly by inhibiting endoplasmic reticulum stress.
    Keywords:  Angiotensin II; CKD; Curcumin; ER stress; Podocyte
  7. Cancer Metab. 2020 ;8 7
    Panarsky R, Crooks DR, Lane AN, Yang Y, Cassel TA, Fan TW, Linehan WM, Moscow JA.
      Background: The loss-of-function mutation of fumarate hydratase (FH) is a driver of hereditary leiomyomatosis and renal cell carcinoma (HLRCC). Fumarate accumulation results in activation of stress-related mechanisms leading to upregulation of cell survival-related genes. To better understand how cells compensate for the loss of FH in HLRCC, we determined the amino acid nutrient requirements of the FH-deficient UOK262 cell line (UOK262) and its FH-repleted control (UOK262WT).Methods: We determined growth rates and survival of cell lines in response to amino acid depletion and supplementation. RNAseq was used to determine the transcription changes contingent on Asn and Gln supplementation, which was further followed with stable isotope resolved metabolomics (SIRM) using both [U- 13C,15N] Gln and Asn.
    Results: We found that Asn increased the growth rate of both cell lines in vitro. Gln, but not Asn, increased oxygen consumption rates and glycolytic reserve of both cell lines. Although Asn was taken up by the cells, there was little evidence of Asn-derived label in cellular metabolites, indicating that Asn was not catabolized. However, Asn strongly stimulated Gln labeling of uracil and precursors, uridine phosphates and hexosamine metabolites in the UOK262 cells and to a much lesser extent in the UOK262WT cells, indicating an activation of the hexosamine biosynthetic pathway (HBP) by Asn. Asn in combination with Gln, but not Asn or Gln alone, stimulated expression of genes associated with the endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) in UOK262 to a greater extent than in FH-restored cells. The changes in expression of these genes were confirmed by RT-PCR, and the stimulation of the UPR was confirmed orthogonally by demonstration of an increase in spliced XBP1 (sXBP1) in UOK262 cells under these conditions. Asn exposure also increased both the RNA and protein expression of the HBP regulator GFPT2, which is a transcriptional target of sXBP1.
    Conclusions: Asn in the presence of Gln induces an ER stress response in FH-deficient UOK262 cells and stimulates increased synthesis of UDP-acetyl glycans indicative of HBP activity. These data demonstrate a novel effect of asparagine on cellular metabolism in FH-deficient cells that could be exploited therapeutically.
    Keywords:  Asparagine metabolism; Fumarate hydratase; Renal cell carcinoma; Unfolded protein response, SIRM
  8. FEBS Open Bio. 2020 Aug 13.
    Chen P, Chen C, Hu M, Cui R, Liu F, Yu H, Ren Y.
      Drug-induced liver injury (DILI) can lead to acute liver failure, a lethal condition which may require liver transplantation. Hepatotoxicity associated with nonsteroidal anti-inflammatory drugs (NSAIDs) accounts for approximately 10% of all DILI. In the current study, we determined if indomethacin, one of the most commonly used NSAIDS, induced apoptosis in hepatocytes, and investigated the underlying mechanism. Meanwhile, we investigated the protective effect of S-allyl-L-cysteine (SAC), an active garlic derivative, on indomethacin-induced hepatocyte apoptosis, and its implication on endoplasmic reticulum (ER) stress. We found that indomethacin triggered ER stress, as indicated by the elevated expression of phosphorylated eIF2α, CHOP and spliced XBP1 in a rat liver BRL-3A cell line. Following indomethacin treatment, caspase 3 activation and hepatocyte apoptosis were also observed. Inhibition of ER stress by chemical chaperon 4-phenyl butyric acid (4-PBA) alleviated cell apoptosis caused by indomethacin, indicating that ER stress is involved in indomethacin-induced hepatocyte apoptosis. Moreover, SAC abated indomethacin-induced eIF2α phosphorylation, inhibited CHOP upregulation and its nuclear translocation, abrogated the activation of caspase 3 and finally, protected hepatocytes from apoptosis. In conclusion, SAC protects indomethacin-induced hepatocyte apoptosis through mitigating ER stress, and maybe suitable for development into a potential new therapeutic agent for the treatment of DILI.
    Keywords:  S-allyl-L-cysteine; apoptosis; endoplasmic reticulum stress; hepatocyte; indomethacin