Samali - ER stress and UnfoldedProteinResponse PDF

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Summary

ER stress and UPR The ER is an elaborate cellular organelle which is essential for cell function and survival. Newly synthesised proteins undergo PTM, folding and oligomerization in the ER lumen. Conditions that interfere with ER function lead to the accumulation of unfolded proteins which are detec...

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ER stress and UPR The ER is an elaborate cellular organelle which is essential for cell function and survival. Newly synthesised proteins undergo PTM, folding and oligomerization in the ER lumen. Conditions that interfere with ER function lead to the accumulation of unfolded proteins which are detected by the ER transmembrane receptors that initiate the unfolded protein response (UPR) to restore normal ER function. However if this stress is prolonged and adaptive responses fail, this leads to apoptosis. The ER is the primary Ca2+ storage organelle and ca2+ is required for optimum protein folding in the lumen, along with ATP and an oxidising environment. Conditions that lead to ER stress (impair folding, processing and transporting of proteins) include interferences with cellular energy levels, the redox state of ca2+ concentration. The UPR is a complex and highly conserved stress response and is mediated through 3 transmembrane receptors or “stress sensors”: PKR-like ER kinase (PERK), activating transcription factor-6 (ATF-6), and inositol-requiring enzyme-1 (IRE1). Normally these stress sensors are kept inactive by binding to the ER chaperone 78kDa glucose regulated protein (GRP78) or Bip. However, under conditions of ER stress, accumulating unfolded proteins leads to GRP78 dissociation and activation of the receptors triggering the UPR. the UPR is a prosurvival response aimed at tackling the backlog of unfolded proteins and restoring ER function. If this stress is unresolved apoptosis occurs. PERK is a type 1 transmembrane protein with an ER luminal sensor and a cytoplasmic domain and has ser/thr activity. The dissociation of GRP78 from PERK results in its dimerization, autophosphorylation and activation. Active PERK phosphorylates elf2α (eukaryotic initiation factor-2), inhibiting general protein translation. Inhibition of protein translation aids cell survival by decreasing the load of nascent proteins in the ER. this inhibition of protein translation does not always occur; mRNAs carrying certain regulatory sequences in their 5’ untranslated regions can bypass the phospo-elf2a-mediated translational block and can sometimes be translated at higher rates e.g. ATF4. The most studied transcript encodes ATF4, a member of the CCAAT/enhancer binding protein (C/EBP) family of transcription factors. ATF4 translation is upregulated upon elf2a phosphorylation and promotes survival by inducing genes involved in amino acid metabolism, redox reactions, stress response and protein secretion. These target genes work together to resolve ER stress and restore cellular homeostasis. However not all gene induced are prosurvival; the transcription factor CHOP whose induction depends on ATF4 has been linked to apoptosis. NRF2 is also a target of PERK and its phosphorylation frees its from its inhibitor KEAP1, allowing for induction of target genes mainly involved in oxidative stress signalling. ATF6 is a type II transmembrane protein that encodes a basic leucine zipper TF domain in its c-terminus. It differs from IRE1 and PERK as it does not dimerize or does not have a kinase domain. Dissociation of GRP78 from ATF6 allows its translocation to the golgi where is it cleaved to its active form. Active ATF6 then translocates to the nucleus to induce expression of genes with an ER stress response element in their promoter. The targets of ATF6 include ER chaperone proteins such as GRP78, GRP94, protein disulphide isomerase and the TF’s CHOP and X box binding protein-1 (XBP1). IRE1a is a type I transmembrane protein with an N-terminal transmembrane domain, and a c-terminal with a ser/thr kinase domain and an endoribonuclease domain. It is the most conserved ER stress sensor. GRP78 dissociation activates it, and the endonuclease activity

induces rapid turnover of mRNAs encoding membrane and secreted proteins through a pathway referred to as regulated IRE1-dependent decay (RIDD). Another function of the endoribonuclease is the removal of a 26-nucleotide intron from the XBP-1 transcript, which is induced by ATF6. XBP-1 is a TF that is usually unstable and has a short half life. The frameshift mutation splice variant generated, XBP1s, codes for a more stable, active TF. XBP1s is linked to prosurvival responses and targets genes involved in protein folding and maturation and ER associated degradation (ERAD). Recent studies demonstrated that artificial maintenance of IRE1 activity enhanced cell survival under ER stress. IRE1 is the most highly regulated of the 3 ER stress sensors, and this may play a key role in controlling the switch between adaptive responses and the initiation of apoptosis.

During ER stress all 3 branches of the UPR are rapidly activated, and they work together to reduce levels of unfolded proteins. When ER stress persists, the IRE1 pathway is switched off, while PERK and ATF6 are maintained. This results in signalling changing from prosurvival to pro-death. The ER stress-induced death proceeds via the intrinsic death pathway, which is controlled by mitochondrial localised Bcl-2 proteins. Bcl-2 family proteins also reside and are active at the ER membrane, and it has been shown that overexpression of Bcl-2/BclXl also protect against ER stress induced death. Moreover BH3-only proteins (Bax and Bak) tip the balance in favour of apoptosis. Bim, Puma and Noxa have also been reported to be upregulated during ER stress. IRE1 mainly produces pro-survival signals. However it may also contribute to death signals via its ser/thr kinase domain in its c-terminal. IRE1 kinase function can induce pro death signalling through binding TRAF2 which leads to JNK activation. IRE1 also found to associate with ASK1 which stimulates IRE1 dimerisation and JNK activation. JNK phosphorylates Bcl-2 and Bcl-Xl reducing their activity and phosphorylates Bim and Bid which enhances their pro-apoptotic function. Signalling through stress sensors can trigger pro-apoptotic signals during prolonged ER

stress. They do so indirectly through the activation of downstream molecules such as CHOP or JNK which regulate the expression and activity of various pro and anti-apoptotic proteins eg Bcl-2. CHOP is also known as GADD153. The PERK-elf2a-ATF4 arm of the UPR is required to induce CHOP protein expression. CHOP mediates cell death through 2 mechanisms (1) relieving the inhibition of protein translation imposed by PERK via induction of GADD34 expression. GADD34 binds to PP1-a, and facilitates PPI-mediated dephosphorylation of elf2a, thus creating a feedback loop that relieves the transcriptional repression imposed by PERK-dependent phosphorylation of elf2a. This is a key event in ER stress induced apoptosis. (2)altering the transcription of genes involved in apoptosis (Bcl-2). CHOP has been demonstrated to upregulate BH3-only proteins, bim and puma. Also downregulates anti-apoptotic members such as Bcl-2.

Caspases in ER stress induced apoptosis?? - maybe learn incase All upstream signals in apoptosis pathways, such as the activation of transcription factors, kinase pathways and the regulation of BCL-2 family members, ultimately lead to caspase activation, resulting in the ordered and sequential dismantling of the cell. Caspase activation

is a key feature of ER stress-induced apoptosis. It is currently unknown which caspases are directly involved with ER stress-induced apoptosis. Processing of caspases-12, -2, -3, -4, -6, -7, -8 and -9 has been observed in different models of ER stress-induced apoptosis. Although caspase activation is required for the apoptotic process, the identity of the apical caspase is of most interest, yet remains subject to debate. Caspase-12 was proposed as a key mediator of ER stress-induced apoptosis. Caspase-12 is expressed in most mammals; but its human homologue has been rendered inactive by several mutations during evolution in most humans. Caspase-4 has been proposed to fulfill the function of caspase-12 in humans, but this is presently under debate.

ALSO UPR and Cancer ? Tumour microenvironment characterised by hypoxia, glucose deprivation, ischemia

Cancer cells need strategies to survive these harsh environments until sufficient angiogenesis and vascularisation can occur

Increased expression of UPR markers has been reported leading to suggestion tumour cells hijack the UPR as a means by which to ensure their survival...