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J Biol Chem. 2019 May 17;294(20):8197-8217. doi: 10.1074/jbc.RA118.002829. Epub 2019 Mar 29.

The kinase PERK and the transcription factor ATF4 play distinct and essential roles in autophagy resulting from tunicamycin-induced ER stress.

Author information

1
From the Centre for Molecular Medicine Norway (NCMM), Nordic EMBL Partnership for Molecular Medicine, University of Oslo, P.O. Box 1137 Blindern, 0318 Oslo, Norway.
2
the Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Montebello, 0379 Oslo, Norway.
3
CanCell, University of Oslo, 0310 Oslo, Norway.
4
the Department of Biosciences, University of Oslo, 0310 Oslo, Norway, and.
5
the Department of Haematology and.
6
Norwegian Center for Stem Cell Research, Department of Immunology, Oslo University Hospital, 0372 Oslo, Norway.
7
From the Centre for Molecular Medicine Norway (NCMM), Nordic EMBL Partnership for Molecular Medicine, University of Oslo, P.O. Box 1137 Blindern, 0318 Oslo, Norway, nikolai.engedal@ncmm.uio.no.

Abstract

Endoplasmic reticulum (ER) stress is thought to activate autophagy via unfolded protein response (UPR)-mediated transcriptional up-regulation of autophagy machinery components and modulation of microtubule-associated protein 1 light chain 3 (LC3). The upstream UPR constituents pancreatic EIF2-α kinase (PERK) and inositol-requiring enzyme 1 (IRE1) have been reported to mediate these effects, suggesting that UPR may stimulate autophagy via PERK and IRE1. However, how the UPR and its components affect autophagic activity has not been thoroughly examined. By analyzing the flux of LC3 through the autophagic pathway, as well as the sequestration and degradation of autophagic cargo, we here conclusively show that the classical ER stressor tunicamycin (TM) enhances autophagic activity in mammalian cells. PERK and its downstream factor, activating transcription factor 4 (ATF4), were crucial for this induction, but surprisingly, IRE1 constitutively suppressed autophagic activity. TM-induced autophagy required autophagy-related 13 (ATG13), Unc-51-like autophagy-activating kinases 1/2 (ULK1/ULK2), and GABA type A receptor-associated proteins (GABARAPs), but interestingly, LC3 proteins appeared to be redundant. Strikingly, ATF4 was activated independently of PERK in both LNCaP and HeLa cells, and our further examination revealed that ATF4 and PERK regulated autophagy through separate mechanisms. Specifically, whereas ATF4 controlled transcription and was essential for autophagosome formation, PERK acted in a transcription-independent manner and was required at a post-sequestration step in the autophagic pathway. In conclusion, our results indicate that TM-induced UPR activates functional autophagy, and whereas IRE1 is a negative regulator, PERK and ATF4 are required at distinct steps in the autophagic pathway.

KEYWORDS:

GABA type A receptor-associated protein (GABARAP); activating transcription factor 4 (ATF4); autophagic degradation; autophagic sequestration; autophagy; endoplasmic reticulum stress (ER stress); inositol-requiring enzyme 1 (IRE1); microtubule-associated protein 1 light chain 3 (LC3); pancreatic EIF2-α kinase (PERK); protein degradation; signal transduction; tunicamycin (TM); unfolded protein response (UPR)

PMID:
30926605
PMCID:
PMC6527152
[Available on 2020-05-17]
DOI:
10.1074/jbc.RA118.002829

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