Figures
- FIGURE 1
The integrated stress response (ISR) is triggered by stress-sensing kinases that phosphorylate eukaryotic initiation factor (eIF)2α, a component of the eIF2 translation initiation complex. Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) responds to endoplasmic reticulum (ER) stress, and so the ISR overlaps with the unfolded protein response (UPR). PKR detects viral double-stranded (ds)RNA. General control non-depressible (GCN)2 is activated by amino acid deficiency. Heme-regulated inhibitor (HRI) responds to iron depletion. Phosphorylated eIF2α binds avidly to eIF2β to inhibit most translation, but some mRNAs including those encoding the transcription factors ATF4 and CHOP are translated more efficiently. The resulting gene expression restores homeostasis by enhancing oxidative protein folding in the ER; promoting amino-acyl transfer (t)RNA synthesis; and inducing antioxidant genes. PPP1R15A (also known as GADD34) is eventually induced and in complex with PP1 and G-actin dephosphorylates eIF2α to terminate the ISR.
- FIGURE 2
The integrated stress response and inflammation. Pathogen-associated molecular patterns (PAMPs) recognised by Toll-like receptors (TLRs) can trigger the innate immune response. Phosphorylation of IκB by IKK promotes its destruction and releases nuclear factor (NF)-κB to transactive pro-inflammatory genes including interleukin (IL)-6, IL-8 and interferons (IFNs). Induction of indoleamine 2,3-dioxygenase (IDO) depletes cells of tryptophan (at least activated macrophages and dendritic cells) to activate general control nondepressible (GCN)2. Activation of GCN2 or protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) can trigger NFκB signalling by reducing the synthesis of IκB, while CHOP increases IL-6 expression directly. Some evidence suggests PERK-mediated p38/ERK signalling might also contribute to inflammatory gene expression. PKR is induced by IFN and further activated if viral double-stranded (ds)RNA is present in the cytoplasm. Inflammatory mediators including C5a fragment can activate PKR, e.g. via C5a receptor (C5aR) signalling. PKR contributes to inflammation by stimulating the NLRP3 inflammasome to generate pro-inflammatory mediators including IL-1β and high mobility group box (HMGB)1.
- FIGURE 3
Variants of EIF2AK4 associated with pulmonary vascular disease. Schematic representation of the general control nondepressible (GCN)2 protein and its domains; boxes correspond to the 39 exons in EIF2AK4. Domains are highlighted: RWD (RING-finger proteins, WD repeat-containing proteins, yeast DEAD-like helicase), pseudokinase, eukaryotic initiation factor (eIF)2α kinase, histidyl-tRNA synthetase-like and carboxy-terminal domain (CTD). Predicted pathogenic variants are shown as lollipops: above the protein are likely pathogenic variants associated with pulmonary arterial hypertension (PAH), below are likely pathogenic variants associated with pulmonary veno-occlusive disease (PVOD) and/or pulmonary capillary haemangiomatosis (PCH). The lollipop length indicates the approximate number of such alleles reported in the literate allowing for incomplete reporting. Note, c.3344C>T (p.P1115L) in exon 23 in at the histidyl-tRNA synthetase-like domain has been reported in five families affected by PAH or PVOD (#). Potentially, 48 alleles have been described, but this may be confounded by overlaps between published reports.
Tables
- TABLE 1
Viral-mediated inhibition of protein kinase R (PKR) activity
Virus(es) Viral protein Mode of PKR inhibition References ssRNA Orthomyxoviridae Influenza A NS1 Direct interaction/dsRNA-mediated interaction [26, 45] Influenza B Coronaviridae Infectious bronchitis virus NSp2 Inhibition of phosphorylation [21] hCoV-229a NSp15 dsRNA sequestration [22] MERS-CoV NS4a dsRNA sequestration [23, 27] Bunyaviridae Rift Valley fever virus NSs Proteasome-mediated degradation [28] Filoviridae Ebolavirus VP35 Unknown/possible dsRNA sequestration [29] Retroviridae HIV Tat Pseudosubstrate/direct interaction [9] TAR RNA Decoy dsRNA [10] Flaviviridae Hepatitis C virus NS5a Blocking of dimerisation/direct interaction [32] E2 Pseudosubstrate/direct interaction [33] Reoviridae Reoviruses σ3/σ4 dsRNA sequestration [34] dsRNA Herpesviridae Human cytomegalovirus pTRS1/pIRS1 Interaction and relocalisation/dsRNA sequestration [35] dsDNA Herpesviridae Herpes simplex virus γ134.5 Dephosphorylation of eIF2α substrate [24, 36, 37] US11 dsRNA sequestration/direct interaction US3/UL13 Inhibition of activation Kaposi's sarcoma herpes virus vIRF2 Direct interaction and inhibition of phosphorylation [38] vIRF3 (LANA2) Inhibition of PKR-induced apoptosis [39] Epstein–Barr virus EBER RNAs Decoy dsRNA [40] SM dsRNA sequestration/direct interaction [41] Poxiviridae Vaccinia virus E3L (p25/p20) dsRNA sequestration/direct interaction [42, 43] K3L Pseudosubstrate/direct interaction [44] Adenoviridae Adenovirus VAI RNA Decoy dsRNA/pseudoactivator [40] Summary of viral-encoded proteins that mediate evasion of PKR-mediated innate immune response. Viruses are classified by family and type of genome. MERS-CoV: Middle East respiratory syndrome-coronavirus; ss: single-stranded; ds: double-stranded; NSp: nonstructural protein; VP: viral protein; TAR: transactivation responsive; vIRF3: viral IRF3-like protein; EBER: Epstein–Barr virus encoded RNAs; VAI: adenovirus-associated RNA-I; eIF: eukaryotic initiation factor.
- TABLE 2
Integrated stress response (ISR)-modifying drugs
Drug Putative mode of action Cautions References ISR inhibitors PERK GSK2656157 Targets ATP binding site of PERK Inhibits RIPK1 [68, 69] GSK2606414 Targets ATP binding site of PERK Inhibits RIPK1 and PKR
Weakly activates GCN2[69, 70] 4-PBA Reduces ER stress by unclear mechanism Affects all arms of the UPR [71] TUDCA Reduces ER stress by unclear mechanism Affects all arms of the UPR [71] HRI Aminopyrazolindane Not commercially available [72] PKR C16 Targets ATP binding site of PKR [73] C22 Targets ATP binding site of PKR [73] 2-Aminopurine Targets ATP binding site of PKR [74] GCN2 6D Targets ATP binding site of GCN2 Not commercially available [75] 6E (aka GCN2iA) Targets ATP binding site of GCN2 Not commercially available [75, 76] eIF2β ISRIB Stablises eIF2β dimers Cell lines can acquire ISRIB resistance mutations [77–79] Dibenzoylmethane Cells insensitive to p-eIF2α Mechanism of action unclear [80] Trazodone Cells insensitive to p-eIF2α Mechanism of action unclear [80] ISR activators PERK CCT020312 Enhances PERK activation Mechanism of action unclear [81] Tunicamycin Induces ER stress: inhibits N-glycosylation Activates all arms of the UPR [82] Bortezomib Induces ER stress: inhibits the proteasome Pleotropic effects of proteasome inhibition [83] Montelukast Enhances PERK signalling
Mechanism unclearLeukotriene receptor antagonist [84] HRI BTdCPU [85] cHAUs [86] PKR Interferon Increases expression of PKR Pleotropic effects of interferon signalling [87] poly I:C RNA mimetic Requires transfection to enter cell [88] BEPP Mechanism of action unclear [89] GCN2 Histidinol Inhibits histidinyl-tRNA synthetase [90] Tryptophanol Inhibits tryptophan-tRNA synthetase [91] Halofuginone Inhibits prolyl-tRNA synthetase [92] L-asparaginase Depletes extracellular asparagine [93] PPP1R15A Salubrinal Putative PPP1R15 inhibitor Concerns that effects may be PPP1R15 independent [67, 94] Guanabenz Putative PPP1R15 inhibitor Concerns that effects may be PPP1R15 independent [67, 95, 96] Sephrin1 Putative PPP1R15 inhibitor Concerns that effects may be PPP1R15 independent [67, 97, 98] PPP1R15A and B Jasplakinolide Depletes G-actin required for PPP1R15 function Pleotropic effects of actin stabilisation [99] PERK: protein kinase R (PKR)-like endoplasmic reticulum kinase; HRI: heme-regulated inhibitor; GCN: general control nondepressible; eIF: eukaryotic initiation factor; 4-PBA: 4-phenylbutyric acid; ER: endoplasmic reticulum; UPR: unfolded protein response; TUDCA: tauroursodeoxycholic acid; C16: CAS 608512-97-6 [6,8-dihydro-8-(1H-imidazol-5-ylmethylene)-7H-pyrrolo[2,3-g]benzothiazol-7-one]; C22: CAS 852547-30-9 (5-chloro-3-[(3,5-dichloro-4-hydroxyphenyl)methylidene]-2,3-dihydro-1H-indol-2-one); ISRIB: integrated stress response inhibitor [trans-2-(4-chlorophenoxy)-N-(4-(2-(4-chlorophenoxy)acetylamino)cyclohexyl)acetamide]; CCT020312: [6-bromo-3-[5-(4-bromo-phenyl)-1-(3-diethylamino-propionyl)-4,5-dihydro-1H-pyrazol-3-yl]-4-phenyl-1H-quinolin-2-one]; cHAUs: [1-((1,4-trans)-4-arylox-ycyclohexyl)-3-arylureas]; polyI:C: polyinosinic-polycytidylic acid; BEPP: [1H-benzimidazole-1-ethanol, 2,3-dihydro-2-imino-α-(phenoxymethyl)-3-(phenylmethyl)-monohydrochloride].
