{{Short description|Cellular stress response}} The '''integrated stress response''' is a cellular stress response conserved in eukaryotic cells that downregulates protein synthesis and upregulates specific genes in response to internal or environmental stresses.<ref name=":0">{{cite journal |last1=Pakos-Zebrucka |first1=Karolina |last2=Koryga |first2=Izabela |last3=Mnich |first3=Katarzyna |last4=Ljujic |first4=Mila |last5=Samali |first5=Afshin |last6=Gorman |first6=Adrienne M |title=The integrated stress response |journal=EMBO Reports |date=October 2016 |volume=17 |issue=10 |pages=1374–1395 |doi=10.15252/embr.201642195 |pmid=27629041 |pmc=5048378 }}</ref>
== Background == thumb|520x520px The integrated stress response can be triggered within a cell due to either extrinsic or intrinsic conditions. Extrinsic factors include hypoxia, amino acid deprivation, glucose deprivation, viral infection and presence of oxidants. The main intrinsic factor is endoplasmic reticulum stress due to the accumulation of unfolded proteins. It has also been observed that the integrated stress response may trigger due to oncogene activation. The integrated stress response will either cause the expression of genes that fix the damage in the cell due to the stressful conditions, or it will cause a cascade of events leading to apoptosis, which occurs when the cell cannot be brought back into homeostasis.<ref name=":0" />
== eIF2 protein complex ==
Stress signals can cause protein kinases, known as EIF-2 kinases, to phosphorylate the α subunit of a protein complex called translation initiation factor 2 (eIF2), resulting in the gene ATF4 being turned on, which will further affect gene expression.<ref name=":0" /> eIF2 consists of three subunits: eIF2α, eIF2β and eIF2γ. eIF2α contains two binding sites, one for phosphorylation and one for RNA binding.<ref name=":0" /> The kinases work to phosphorylate serine 51 on the α subunit, which is a reversible action.<ref name=":1">{{cite journal |last1=Harding |first1=Heather P. |last2=Zhang |first2=Yuhong |last3=Zeng |first3=Huiquing |last4=Novoa |first4=Isabel |last5=Lu |first5=Phoebe D. |last6=Calfon |first6=Marcella |last7=Sadri |first7=Navid |last8=Yun |first8=Chi |last9=Popko |first9=Brian |last10=Paules |first10=Richard |last11=Stojdl |first11=David F. |last12=Bell |first12=John C. |last13=Hettmann |first13=Thore |last14=Leiden |first14=Jeffrey M. |last15=Ron |first15=David |title=An Integrated Stress Response Regulates Amino Acid Metabolism and Resistance to Oxidative Stress |journal=Molecular Cell |date=March 2003 |volume=11 |issue=3 |pages=619–33 |pmid=12667446 |doi=10.1016/S1097-2765(03)00105-9 |doi-access=free }}</ref> In a cell experiencing normal conditions, eIF2 aids in the initiation of mRNA translation and recognizing the AUG start codon.<ref name=":0" /> However, once eIF2α is phosphorylated, the complex’s activity reduces, causing reduction in translation initiation and protein synthesis, while promoting expression of the ATF4 gene.<ref name=":1" />
== Protein kinases == There are four known mammalian protein kinases that phosphorylate eIF2α, including PKR-like ER kinase (PERK, EIF2AK3), heme-regulated eIF2α kinase (HRI, EIF2AK1), general control non-depressible 2 (GCN2, EIF2AK4) and double stranded RNA dependent protein kinase (PKR, EIF2AK2).<ref name=":0" /><ref name=":2">{{cite journal |last1=Wang |first1=Cheng |last2=Tan |first2=Zhijia |last3=Niu |first3=Ben |last4=Tsang |first4=Kwok Yeung |last5=Tai |first5=Andrew |last6=Chan |first6=Wilson C W |last7=Lo |first7=Rebecca L K |last8=Leung |first8=Keith K H |last9=Dung |first9=Nelson W F |last10=Itoh |first10=Nobuyuki |last11=Zhang |first11=Michael Q |last12=Chan |first12=Danny |last13=Cheah |first13=Kathryn Song Eng |title=Inhibiting the integrated stress response pathway prevents aberrant chondrocyte differentiation thereby alleviating chondrodysplasia |journal=eLife |date=19 July 2018 |volume=7 |article-number=e37673 |doi=10.7554/eLife.37673 |pmid=30024379 |pmc=6053305 |doi-access=free }}</ref>
===PERK=== thumb|201x201px PERK (encoded in humans by the gene ''EIF2AK3'') responds mainly to endoplasmic reticulum stress and has two modes of activation.<ref name=":0" /><ref name=":1" /> This kinase has a unique luminal domain that plays a role in activation. The classical model of activation states that the luminal domain is normally bound to 78-kDa glucose-regulated protein (GRP78). Once there is a buildup of unfolded proteins, GRP78 dissociates from the luminal domain. This causes PERK to dimerize, leading to autophosphorylation and activation. The activated PERK kinase will then phosphorylate eIF2α, causing a cascade of events. Thus, the activation of this kinase is dependent on the aggregation of unfolded proteins in the endoplasmic reticulum. PERK has also been observed to activate in response to activity of the proto-oncogene MYC. This activation causes ATF4 expression, resulting in tumorigenesis and cellular transformation.<ref name=":0" /> thumb|193x193px
=== HRI === HRI (encoded in humans by the gene ''EIF2AK1'') also dimerizes in order to autophosphorylate and activate. This activation is dependent on the presence of heme. HRI has two domains that heme may bind to, including one on the N-terminus and one on the kinase insertion domain. The presence of heme causes a disulfide bond to form between the monomers of HRI, resulting in the structure of an inactive dimer. However, when heme is absent, HRI monomers form an active dimer through non-covalent interactions. Therefore, the activation of this kinase is dependent on heme deficiency. HRI activation can also occur due to other stressors such as heat shock, osmotic stress and proteasome inhibition. Activation of HRI in response to these stressors does not depend on heme, but rather relies on the help of two heat shock proteins (HSP90 and HSP70). HRI is mainly found in the precursors of red blood cells, and has been observed to increase during erythropoiesis.<ref name=":0" /> thumb|198x198px
===GCN2 === thumb|203x203px GCN2 (encoded in humans by the gene ''EIF2AK4'') is activated as a result of amino acid deprivation. The mechanisms regarding this activation are still being researched; however, one mechanism has been studied in yeast.<ref name=":0" /> It was observed that GCN2 binds to uncharged/deacylated tRNA which causes a conformational change, resulting in dimerization.<ref name=":1" /> Dimerization then causes autophosphorylation and activation.<ref name=":1" /> Other stressors have also been reported to activate GCN2. GCN2 activation was observed in glucose deprived tumor cells, although it was suggested that it was an indirect effect due to cells using amino acids as an alternate energy source.<ref name=":0" /> In mouse embryonic fibroblast cells and human keratinocytes, GCN2 was activated due to UV light exposure.<ref>{{cite journal |last1=Ovchinnikova |first1=GA |last2=Pigina |first2=TV |title=[The use of sigetin in the therapy of fetal growth retardation in the rabbit caused by uterine ischemia]. |journal=Akusherstvo I Ginekologiia |date=February 1975 |issue=2 |pages=58–60 |pmid=1217635 }}</ref><ref>{{cite journal |last1=Lu |first1=Wei |last2=László |first2=Csaba F. |last3=Miao |first3=Zhixin |last4=Chen |first4=Hao |last5=Wu |first5=Shiyong |title=The Role of Nitric-oxide Synthase in the Regulation of UVB Light-induced Phosphorylation of the α Subunit of Eukaryotic Initiation Factor 2 |journal=Journal of Biological Chemistry |date=4 September 2009 |volume=284 |issue=36 |pages=24281–24288 |doi=10.1074/jbc.M109.008821 |pmid=19586904 |pmc=2782021 |doi-access=free }}</ref> The pathways for this activation require further research, although multiple models have been proposed, including crosslinking between GCN2 and tRNA.<ref name=":0" />
=== PKR === PKR (encoded in humans by the gene ''EIF2AK2'') activation is mainly dependent on the presence of double-stranded RNA during a viral infection. dsRNA causes PKR to form dimers, resulting in autophosphorylation and activation.<ref name=":0" /> Once activated, PKR will phosphorylate eIF2α which causes a cascade of events that result in viral and host protein synthesis being inhibited. Other stressors that cause the activation of PKR include oxidative stress, endoplasmic reticulum stress, growth factor deprivation and bacterial infection. Caspase activity early on in apoptosis has also been observed to trigger activation of PKR. However, these stressors differ in that they activate PKR without using dsRNA.<ref name=":0" />
== ATF4 == When a cell is subjected to stressful conditions, the ATF4 gene is expressed.<ref name=":0" /> The ATF4 transcription factor has the ability to form dimers with many different proteins that influence gene expression and cell fate. ATF4 binds to C/EBP‐ATF response element (CARE) sequences which work together to increase the transcription of stress-responsive genes. However, when undergoing amino acid starvation, the sequences will act as amino acid response elements instead.<ref name=":0" />
ATF4 will work together with other transcription factors, such as CHOP and ATF3, by forming homodimers or heterodimers, resulting in numerous observed effects.<ref name=":2" /> The proteins that ATF4 interacts with determines the outcome of the cell during the integrated stress response.<ref name=":0" /> For example, ATF4 and ATF3 work to establish homeostasis inside of the cell following stressful conditions.<ref name=":2" /> On the other hand, ATF4 and CHOP work together to induce cell death, as well as regulating amino acid biosynthesis, transport and metabolic processes. The presence of a leucine zipper domain (bZIP) allows ATF4 to work together with many other proteins, thus creating specific responses to different types of stressors. When a cell is undergoing the stress of hypoxia, ATF4 will interact with PHD1 and PHD3 to decrease its transcriptional activity. In addition, when a cell is undergoing amino acid starvation or endoplasmic reticulum stress, TRIP3 also interacts with ATF4 to decrease activity.<ref name=":0" />
One result of ATF4 and stress-response proteins expression is the induction of autophagy.<ref name=":3">{{cite journal |last1=Kroemer |first1=Guido |last2=Mariño |first2=Guillermo |last3=Levine |first3=Beth |title=Autophagy and the Integrated Stress Response |journal=Molecular Cell |date=October 2010 |volume=40 |issue=2 |pages=280–293 |doi=10.1016/j.molcel.2010.09.023 |pmid=20965422 |pmc=3127250 }}</ref> During this process, the cell forms autophagosomes, or double membraned vesicles, that allow for transportation of material throughout the cell.<ref name=":3" /> These autophagosomes can carry unneeded organelles and proteins, as well as damaged or harmful components in an attempt by the cell to maintain homeostasis.<ref name=":3" />
== Termination of integrated stress response == In order to terminate the integrated stress response, dephosphorylation of eIF2α is required. The protein phosphatase 1 complex (PP1) aids in the dephosphorylation of eIF2α. This complex contains a PP1 catalytic subunit as well as two regulatory subunits. This complex is negatively regulated by two proteins: growth arrest and DNA damage‐inducible protein (GADD34), also known as PPP1R15A, or constitutive repressor of eIF2α phosphorylation (CReP), also known as PPP1R15B. CReP acts to keep levels of eIF2α phosphorylation low in cells under normal conditions. GADD34 is produced in response to ATF4 and works to increase dephosphorylation of eIF2α. The dephosphorylation of eIF2α results in the return of normal protein synthesis and cellular function. However, dephosphorylation of eIF2α can also facilitate the production of death-inducing proteins in cases where the cell is so severely damaged that normal functioning cannot be restored.<ref name=":0" />
== Mutations affecting integrated stress response == Mutations that affect the functioning of the integrated stress response may have debilitating effects on cells. For example, cells lacking the ATF4 gene are unable to elicit proper gene expression in response to stressors. This results in cells exhibiting issues with amino acid transport, glutathione biosynthesis and oxidative stress resistance. When a mutation inhibits the functioning of PERK, endogenous peroxides accumulate when the cell experiences endoplasmic reticulum stress.<ref name=":0" /> In mice and humans lacking PERK, there have been observed destruction of secretory cells undergoing high endoplasmic reticulum stress.<ref name=":1" />
== See also == {{Portal|Medicine}} *ISRIB, integrated stress response inhibitor
== References == {{Reflist}}
Category:Cellular processes Category:Eukaryote biology Category:Gene expression Category:Proteins