scholarly journals Lipid disequilibrium disrupts ER proteostasis by impairing ERAD substrate glycan trimming and dislocation

2017 ◽  
Vol 28 (2) ◽  
pp. 270-284 ◽  
Author(s):  
Milton To ◽  
Clark W. H. Peterson ◽  
Melissa A. Roberts ◽  
Jessica L. Counihan ◽  
Tiffany T. Wu ◽  
...  
Keyword(s):  
Fatty Acid ◽  
Mammalian Cells ◽  
26S Proteasome ◽  
Unfolded Protein ◽  
Acid Analogue ◽  
Triacsin C ◽  
Chain Acyl ◽  
Dependent Cell ◽  
Outstanding Question ◽  
The Unfolded Protein Response

The endoplasmic reticulum (ER) mediates the folding, maturation, and deployment of the secretory proteome. Proteins that fail to achieve their native conformation are retained in the ER and targeted for clearance by ER-associated degradation (ERAD), a sophisticated process that mediates the ubiquitin-dependent delivery of substrates to the 26S proteasome for proteolysis. Recent findings indicate that inhibition of long-chain acyl-CoA synthetases with triacsin C, a fatty acid analogue, impairs lipid droplet (LD) biogenesis and ERAD, suggesting a role for LDs in ERAD. However, whether LDs are involved in the ERAD process remains an outstanding question. Using chemical and genetic approaches to disrupt diacylglycerol acyltransferase (DGAT)–dependent LD biogenesis, we provide evidence that LDs are dispensable for ERAD in mammalian cells. Instead, our results suggest that triacsin C causes global alterations in the cellular lipid landscape that disrupt ER proteostasis by interfering with the glycan trimming and dislocation steps of ERAD. Prolonged triacsin C treatment activates both the IRE1 and PERK branches of the unfolded protein response and ultimately leads to IRE1-dependent cell death. These findings identify an intimate relationship between fatty acid metabolism and ER proteostasis that influences cell viability.

scholarly journals Homocysteine as a Risk Factor for Atherosclerosis: Is Its Conversion toS-Adenosyl-L-Homocysteine the Key to Deregulated Lipid Metabolism?

2011 ◽  
Vol 2011 ◽  
pp. 1-11 ◽  
Author(s):  
Oksana Tehlivets
Keyword(s):  
Risk Factor ◽  
Lipid Metabolism ◽  
Mammalian Cells ◽  
Yeast Cells ◽  
Sterol Synthesis ◽  
Unfolded Protein ◽  
Strong Product ◽  
The Unfolded Protein Response ◽  
Protein Response

Homocysteine (Hcy) has been recognized for the past five decades as a risk factor for atherosclerosis. However, the role of Hcy in the pathological changes associated with atherosclerosis as well as the pathological mechanisms triggered by Hcy accumulation is poorly understood. Due to the reversal of the physiological direction of the reaction catalyzed byS-adenosyl-L-homocysteine hydrolase Hcy accumulation leads to the synthesis ofS-adenosyl-L-homocysteine (AdoHcy). AdoHcy is a strong product inhibitor ofS-adenosyl-L-methionine (AdoMet)-dependent methyltransferases, and to date more than 50 AdoMet-dependent methyltransferases that methylate a broad spectrum of cellular compounds including nucleic acids, proteins and lipids have been identified. Phospholipid methylation is the major consumer of AdoMet, both in mammals and in yeast. AdoHcy accumulation induced either by Hcy supplementation or due toS-adenosyl-L-homocysteine hydrolase deficiency results in inhibition of phospholipid methylation in yeast. Moreover, yeast cells accumulating AdoHcy also massively accumulate triacylglycerols (TAG). Similarly, Hcy supplementation was shown to lead to increased TAG and sterol synthesis as well as to the induction of the unfolded protein response (UPR) in mammalian cells. In this review a model of deregulation of lipid metabolism in response to accumulation of AdoHcy in Hcy-associated pathology is proposed.

scholarly journals HY5, a positive regulator of light signaling, negatively controls the unfolded protein response inArabidopsis

2017 ◽  
Vol 114 (8) ◽  
pp. 2084-2089 ◽  
Author(s):  
Ganesh M. Nawkar ◽  
Chang Ho Kang ◽  
Punyakishore Maibam ◽  
Joung Hun Park ◽  
Young Jun Jung ◽  
...  
Keyword(s):  
Stress Response ◽  
Er Stress ◽  
Unfolded Protein Response ◽  
Molecular Mechanism ◽  
26S Proteasome ◽  
Light Signaling ◽  
Unfolded Protein ◽  
Er Stress Response ◽  
The Unfolded Protein Response ◽  
Protein Response

Light influences essentially all aspects of plant growth and development. Integration of light signaling with different stress response results in improvement of plant survival rates in ever changing environmental conditions. Diverse environmental stresses affect the protein-folding capacity of the endoplasmic reticulum (ER), thus evoking ER stress in plants. Consequently, the unfolded protein response (UPR), in which a set of molecular chaperones is expressed, is initiated in the ER to alleviate this stress. Although its underlying molecular mechanism remains unknown, light is believed to be required for the ER stress response. In this study, we demonstrate that increasing light intensity elevates the ER stress sensitivity of plants. Moreover, mutation of the ELONGATED HYPOCOTYL 5 (HY5), a key component of light signaling, leads to tolerance to ER stress. This enhanced tolerance ofhy5plants can be attributed to higher expression of UPR genes. HY5 negatively regulates the UPR by competing with basic leucine zipper 28 (bZIP28) to bind to the G-box–like element present in the ER stress response element (ERSE). Furthermore, we found that HY5 undergoes 26S proteasome-mediated degradation under ER stress conditions. Conclusively, we propose a molecular mechanism of crosstalk between the UPR and light signaling, mediated by HY5, which positively mediates light signaling, but negatively regulates UPR gene expression.

scholarly journals Ire1-mediated decay in mammalian cells relies on mRNA sequence, structure, and translational status

2015 ◽  
Vol 26 (16) ◽  
pp. 2873-2884 ◽  
Author(s):  
Kristin Moore ◽  
Julie Hollien
Keyword(s):  
Er Stress ◽  
Mammalian Cells ◽  
Mrna Sequence ◽  
Sequence Structure ◽  
Stem Loop ◽  
Unfolded Protein ◽  
Target Mrnas ◽  
Specific Manner ◽  
The Unfolded Protein Response ◽  
Protein Response

Endoplasmic reticulum (ER) stress occurs when misfolded proteins overwhelm the capacity of the ER, resulting in activation of the unfolded protein response (UPR). Ire1, an ER transmembrane nuclease and conserved transducer of the UPR, cleaves the mRNA encoding the transcription factor Xbp1 at a dual stem-loop (SL) structure, leading to Xbp1 splicing and activation. Ire1 also cleaves other mRNAs localized to the ER membrane through regulated Ire1-dependent decay (RIDD). We find that during acute ER stress in mammalian cells, Xbp1-like SLs within the target mRNAs are necessary for RIDD. Furthermore, depletion of Perk, a UPR transducer that attenuates translation during ER stress, inhibits RIDD in a substrate-specific manner. Artificially blocking translation of the SL region of target mRNAs fully restores RIDD in cells depleted of Perk, suggesting that ribosomes disrupt SL formation and/or Ire1 binding. This coordination between Perk and Ire1 may serve to spatially and temporally regulate RIDD.

XBP-1 Levels Predict Sensitivity of Myelomas to Proteasome Inhibitor Bortezomib.

Blood ◽  
2007 ◽  
Vol 110 (11) ◽  
pp. 1473-1473
Author(s):  
Silvia C. Ling ◽  
Edwin Lau ◽  
Lye L. Ho ◽  
Joy Ho ◽  
Douglas E. Joshua ◽  
...  
Keyword(s):  
Unfolded Protein Response ◽  
Cell Lines ◽  
Cancer Cell ◽  
Cancer Cell Lines ◽  
Myeloma Cell ◽  
26S Proteasome ◽  
Mrna Levels ◽  
Unfolded Protein ◽  
The Unfolded Protein Response ◽  
Protein Response

Abstract Background: Proteasome inhibitors (PI) are remarkably effective in relapsed and refractory myeloma but the origin of this peculiar sensitivity remains unclear. Myeloma is dependent on the unfolded protein response (UPR) and its regulator, transcription factor XBP-1. PI perturbs the unfolded protein response (UPR) by inhibition of the 26S proteasome-the main pathway for protein degradation. We hypothesize that the dependence on the UPR and XBP-1 mediates sensitivity to PI and the level of XBP-1 correlates with sensitivity to PI. The aim of this study is to correlate Bortezomib sensitivity with XBP-1 in vitro and in myeloma patients; to check the effect of manipulating XBP-1 on Bortezomib sensitivity and develop Bortezomib-resistant myeloma cell lines to ascertain the effects on XBP-1 and the UPR. Methods and Results: Sensitivity to Bortezomib was measured by growth inhibition assay. XBP-1 mRNA levels and its isoforms were measured by a two-step quantitative QPCR assay, in 6 myeloma cell lines and 17 other cancer cell lines. There is a strong inverse correlation in myeloma cell lines between total or unspliced XBP-1 with Bortezomib sensitivity (r = −0.9) but not in other cancer cell lines. 23 marrow biopsies from 11 Bortezomib-treated myeloma patients were analysed for XBP-1 expression. Myeloma cells (CD38 hi, CD14 lo, kappa or lambda light chain +ve) were purified by flow cytometry. XBP-1 levels in myeloma cell lines were manipulated by shRNA-mediated knockdown and overexpression by retroviral transduction and had little effect on Bortezomib sensitivity. Bortezomib-resistant myeloma lines were developed. The mechanism of resistance was elucidated (XBP-1, ATF6, P-EIF2a, P58 INK and immunogloblin production). Marked downregulation of XBP-1 was demonstrated. Conclusion: XBP-1 is a surrogate marker of Bortezomib sensitivity and its clinical utility is being tested now. Sensitivity to PI is related to the dependence on the UPR, reflected in the level of XBP-1. Bortezomib resistance is mediated by downregulation of the UPR.

scholarly journals Molecular basis of fatty acid selectivity in the zDHHC family of S-acyltransferases revealed by click chemistry

2017 ◽  
Vol 114 (8) ◽  
pp. E1365-E1374 ◽  
Author(s):  
Jennifer Greaves ◽  
Kevin R. Munro ◽  
Stuart C. Davidson ◽  
Matthieu Riviere ◽  
Justyna Wojno ◽  
...  
Keyword(s):  
Fatty Acid ◽  
Click Chemistry ◽  
Mammalian Cells ◽  
Transmembrane Domain ◽  
Acyl Chain ◽  
Fatty Acid Selectivity ◽  
Molecular Determinants ◽  
Chain Acyl ◽  
Domain 3 ◽  
Acylated Proteins

S-acylation is a major posttranslational modification, catalyzed by the zinc finger DHHC domain containing (zDHHC) enzyme family. S-acylated proteins can be modified by different fatty acids; however, very little is known about how zDHHC enzymes contribute to acyl chain heterogeneity. Here, we used fatty acid-azide/alkyne labeling of mammalian cells, showing their transformation into acyl-CoAs and subsequent click chemistry-based detection, to demonstrate that zDHHC enzymes have marked differences in their fatty acid selectivity. This difference in selectivity was apparent even for highly related enzymes, such as zDHHC3 and zDHHC7, which displayed a marked difference in their ability to use C18:0 acyl-CoA as a substrate. Furthermore, we identified isoleucine-182 in transmembrane domain 3 of zDHHC3 as a key determinant in limiting the use of longer chain acyl-CoAs by this enzyme. This study uncovered differences in the fatty acid selectivity profiles of cellular zDHHC enzymes and mapped molecular determinants governing this selectivity.

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