Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Eif2ak1

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101653

Synonyms

Historical Background

Heme iron accounts for the majority of the iron in the human body, and hemoglobin contains as much as 70% of the total iron content of a normal adult. Accordingly, iron and heme homeostasis plays very important roles in hemoglobin synthesis and erythropoiesis. Iron deficiency anemia is very common with an incidence of approximately one billion cases worldwide. The hallmark of microcytic hypochromic red blood cells (RBCs) in iron deficiency anemia has led to early studies in the 1950s that demonstrated the stimulation of hemoglobin synthesis by inorganic iron in immature erythroid cells. Iron is inserted into protoporphyrin IX to form heme in the last step of heme biosynthesis. Consequently, iron deficiency also leads to heme deficiency. Further studies in the 1960s showed that heme, not iron per se, was required for protein synthesis in reticulocytes since the iron-chelating agent, desferrioxamine, did not block the stimulatory effect of heme. Heme deficiency causes disaggregation of polysomes in reticulocytes. On addition of hemin, polysomes reform and protein synthesis is restored. Activation of heme-regulated inhibitor (HRI) in heme-deficienct reticulocyte lysates was discovered in 1969. Inhibition of protein synthesis under heme deficiency was associated with a marked decrease in the formation of 40S.eIF2.Met-tRNAiGTP (the 43S pre-initiation complex). Furthemore, addition of purified eIF2 both prevented and reversed the inhibition of protein synthesis in heme deficiency. In 1976 HRI was shown to be the heme-regulated eIF2α kinase that phosphorylates the α-subunit of eIF2 [reviewed in (Chen 2000)].

Depletion of HRI by polyclonal or monoclonal antibodies shows that HRI is principally responsible for the inhibition of protein synthesis during heme deficiency (Chen 2000). This was validated by targeted disruption of the HRI gene in mice (Han et al. 2001). Hri −/− reticulocytes have increased rates of protein synthesis and decreased levels of phosphorylated eIF2α (eIF2αP) in comparison to Hri +/+ reticulocytes. This increase in protein synthesis is accompanied by an increase in larger-sized polysomes in Hri −/− reticulocytes, demonstrating a higher rate of translational initiation. Furthermore, protein synthesis in Hri −/− reticulocytes is no longer dependent on heme. These results establish that HRI is essential for heme-regulated translation in reticulocytes (Han et al. 2001).

The molecular mechanism underlying inhibition of translational initiation by the phosphorylation of eIF2α has been studied extensively [reviewed in (Chen 2007; Sonenberg and Hinnebusch 2009)] and is illustrated in Fig. 1. eIF2 is a heterotrimeric protein which binds GTP, initiating Met-tRNAi and the 40S ribosomal subunit to form the 43S pre-initiation complex. eIF2 exists in two forms, the inactive eIF2-GDP and the active eIF2-GTP. The GTP in the eIF2-GTP complex is hydrolyzed to GDP upon binding of the 60S ribosomal subunit to the 43S pre-initiation complex during translation. The recycling of eIF2 for another round of translational initiation therefore requires the exchange of its bound GDP for GTP. However, eIF2 has a 400-fold greater affinity for GDP than for GTP under physiological conditions. This exchange of tightly bound GDP for GTP requires another initiation factor eIF2B, which is rate limiting with a concentration of 15–25% of that of eIF2. When eIF2α is phosphorylated by HRI at Ser51, the phosphorylated eIF2(αP)-GDP binds much more tightly than eIF2-GDP to eIF2B and inhibits the GDP/GTP exchange activity of eIF2B. Thus, once the amount of phosphorylated eIF2 exceeds that of eIF2B, protein synthesis is shut off. The recovery of protein synthesis following heme repletion requires not only the inactivation of HRI by heme but also the removal of the inhibitory phosphate at Ser51 of eIF2α by type-1 phosphatase (PPase 1) to regenerate the active eIF2-GTP (Fig. 1).
Eif2ak1, Fig. 1

Inhibition of protein synthesis by phosphorylation of eIF2α

Regulation of HRI by Heme

Highly purified HRI is a homodimer and binds hemin (the oxidized form of heme with Fe+3) as illustrated in Fig. 2. ATP binding to HRI is inhibited by prior treatment of HRI with hemin in a concentration-dependent manner. Both autokinase and eIF2α kinase activities are inhibited by submicromolar concentrations of hemin with an apparent Ki (concentration at 50% inhibition) of 0.2 μM (Chen 2000; Rafie-Kolpin et al. 2000). Dissociation of heme from HRI in heme deficiency results in the activation of HRI by multiple autophosphorylation (Fig. 2). Thr485 in the activation loop of mouse HRI is among one of the residues autophosphorylated and is essential for attaining eIF2α kinase activity (Rafie-Kolpin et al. 2003).
Eif2ak1, Fig. 2

Activation of HRI by multiple autophosphorylation and inhibition of translation of globin mRNAs in heme deficiency. Purified heme-reversible HRI is a stable homodimer held together by non-covalent interactions. This HRI is autophosphorylated and has one heme stably bound per subunit to its N-terminus domain and is an active autokinase. In heme abundance, heme binds to the KI domain of HRI and inhibits kinase activity. In heme deficiency, further autophosphorylation of HRI at Thr485 activates its eIF2α kinase activity. Phosphorylation of eIF2α inhibits globin translation by the mechanism illustrated in Fig. 1. During the synthesis of hemoglobin, one molecule of heme is incorporated into each globin chain. HRI, therefore, acts as a heme sensor and a feedback inhibitor of globin synthesis to ensure that no globin is translated in excess of the heme available for the assembly of stable hemoglobin. Heme molecules are represented by red hexagons

Recombinant HRI expressed in Sf9 insect cells purifies as a hemoprotein with the three characteristic absorption peaks in the visible wavelength: the Soret band at 424 nm and the α- and β-bands around 550 nm. Furthermore, there are two distinct types of heme-binding sites per HRI homodimer. One type of binding site is nearly saturated with stably bound endogenous heme co-purified with HRI, while the other binding site is available to bind hemin reversibly (Fig. 2) (Chefalo et al. 1998). This second reversible heme-binding site is responsible for the downregulation of HRI activity by heme. The stoichiometry of two heme molecules per HRI monomer has been confirmed by direct measurement of heme chromophore through alkaline pyridine treatment of homogeneous heme-saturated HRI (Bauer et al. 2001; Chen 2007).

There are three unique regions of HRI in the N-terminus, the kinase insert (KI), and the C-terminus (Fig. 3). The N-terminal 130 amino acids of HRI are necessary for stable high-affinity heme binding to HRI. N-terminally truncated Met2 (Δ103) and Met3 (Δ130) HRI are active eIF2α kinases and autokinases; their specific eIF2α kinase activities are about 50% that of the wild type (Wt) HRI. These results suggest that the N-terminus may be important for achieving a higher specific eIF2α kinase activity although it is not essential for the kinase activity of HRI. Moreover, the N-terminus of HRI is necessary for the high sensitivity to heme regulation since Met2 and Met3 HRI are ten times less sensitive to heme inhibition as compared to Wt HRI. Additionally, both the N-terminus and KI can bind heme, whereas kinase I, kinase II, and C-terminus cannot (Fig. 3). When expressed in the presence of 5 μM hemin, the N-terminus domain, but not the KI, purified as a hemoprotein with a visible pink color (Rafie-Kolpin et al. 2000). Together with the loss of stable heme binding in Met2 and Met3, these results indicate that the KI domain contains the heme-binding site responsible for the reversible heme regulation of HRI (Chefalo et al. 1998; Rafie-Kolpin et al. 2000).
Eif2ak1, Fig. 3

Protein domains and the evolutionary conservation of heme coordination in HRI. HRI protein is divided into five domains as indicated. The amino acid sequence of mouse HRI is used here. Heme molecules are marked in red; S denotes the stable heme-binding site, while R denotes the reversible heme-binding site. * marks the histidine residues that coordinate the heme molecule. ^ marks the positions of CP, putative heme regulated motifs. In the table, single-letter codes of amino acids are used. + denotes absolute conservation of the amino acids, while denotes no conservation

Conserved residues His75 and His120 were identified to be the proximal and distal heme ligand, respectively, in the N-terminus domain of HRI as illustrated in Fig. 3. Mutation of His75 and His120 individually to Ala in full-length HRI resulted in decreased sensitivity to heme inhibition, similar to N-terminally truncated HRI (Yen and Chen unpublished). This finding further underscores the importance of heme coordination in the N-terminus in the downregulation of HRI activity. Thus, HRI is a unique and novel hemoprotein with dual heme-binding domains. Furthermore, there is cooperation between the two heme-binding domains to achieve efficient heme regulation of HRI kinase activities. In this regard, heme has been shown to stabilize the binding of the N-terminus domain with N-terminally truncated HRI as well as to stabilize the binding of the N-terminus domain with the KI domain.

HRI contains two putative heme regulatory motifs (HRMs), Cys409/Pro410 and Cys550/Pro551, in mouse HRI, which are not present in the other three members of eIF2α kinases (Fig. 3). The significance of HRM motifs in heme regulation of HRI is still not clear. These two Cys residues have been mutated individually to Ser with no apparent effect on the heme responsiveness of HRI (Rafie-Kolpin et al. 2000). Additionally, these two Cys residues are not conserved in chicken, Xenopus, or fish HRI, indicating that they may not be important for the heme regulation of HRI. However, Igarashi et al. suggested that His120 and Cys409 are heme ligands in the full-length HRI (Igarashi et al. 2008).

Tissue Expression and Phylogenetic Conservation of HRI

HRI protein and mRNA are highly expressed in erythropoietic tissues of the bone marrow, spleen, and fetal liver. In addition, HRI expression also increases during erythroid differentiation. While there are reports of the expression of HRI in non-erythroid cells, it is important to note that HRI is expressed at two orders of magnitude higher levels in erythroid precursors as compared to macrophages (Liu et al. 2007). There are emerging evidence for the role of HRI in innate immunity and memory acquisition; however, it is beyond the scope of this review.

HRI homologues are present in vertebrates with amino acid sequence identity ranging from 45% in fish to 82% in humans. Importantly, the two heme ligands in the N-terminal heme-binding domain described above, His75 and His120, are conserved in all vertebrate HRI (Fig. 3). Interestingly, Schizosaccharomyces pombe not only has GCN2 but also has two additional eIF2α kinases, which have greater homology to HRI than to other eIF2α kinases. Although these two HRI-related eIF2α kinases can be regulated by heme in vitro, albeit with a higher Ki, these HRI-related kinases do not respond to iron/heme deficiency in vivo (Zhan et al. 2002) as mammalian HRI does (Han et al. 2001). They do, however, respond to oxidative and heat stress (Zhan et al. 2002) like mouse HRI in erythroid precursors (Lu et al. 2001). The pombe-like HRI-related kinases are present in Anopheles gambiae, Aedes aegypti, Tribolium castaneum, Bombyx mori, Branchiostoma floridae, Ciona intestinalis, and Nematostella vectensis. The N-terminus sequences of these HRI-related kinases are much less conserved than those of vertebrate HRI. Furthermore, His75 is no longer conserved, while His120 is replaced with Tyr in these HRI-related kinases (Fig. 3). There is no HRI in Saccharomyces cerevisiae, Caenorhabditis elegans, or Drosophila melanogaster.

The higher degree of conservation of vertebrate HRI from fish to human, the high-level expression of HRI in erythroid cells, and the high sensitivity of HRI to heme regulation support the notion that HRI evolved when diffusion alone could no longer supply the increasing demands for oxygen and when blood circulation was established wherein hemoglobin is carried in red blood cells. Vertebrate HRI retains its ability to respond to oxidative and heat stress similar to the pombe HRI-related kinase.

Role of HRI in Coordinating of Heme and Globin Synthesis and in Iron/Heme Deficiency Anemia

In the formation of stable α2β2 hemoglobins, it is important to keep the concentrations of globin chains and heme balanced. Globin chains misfold and precipitate in the absence of heme. Excess heme causes oxidative stress and is also cytotoxic. As described above, HRI is a hemoprotein with two distinct heme-binding domains, and as such HRI may sense heme availability and serve as a feedback inhibitor of globin synthesis to balance heme and globin synthesis as illustrated in Fig. 1 [reviewed in (Chen 2007)]. This notion was proven by studies using Hri −/− mice to reveal the physiological function of HRI (Han et al. 2001).

The normal adaptive response to iron deficiency in human and mice is the well-characterized microcytic hypochromic anemia with decreased mean corpuscular volume (MCV) and mean cell hemoglobin (MCH) in RBCs (Fig. 4). In Hri −/− mice, this physiological response to iron deficiency is dramatically altered. These mice develop a very unusual pattern of slight hyperchromic macrocytic anemia with an accentuated decrease in RBC counts. Thus, HRI is critical in determining RBC size, cell number, and hemoglobin content per cell. HRI is responsible for the adaptation of microcytic hypochromic anemia in iron deficiency. Moreover, globin inclusions are observed within reticulocytes and to a lesser extent within fully mature RBCs in iron-deficient Hri −/− mice as illustrated in Fig. 4. Together, studies using Hri −/− mice uncover the function of HRI during iron deficiency in coordinating the synthesis of globins in RBC precursors with the concentration of heme in vivo as illustrated in Figs. 1 and 4. HRI normally ensures that no globin chains are translated in excess of what can be assembled into hemoglobin tetramers for the amount of heme available.
Eif2ak1, Fig. 4

Requirement of HRI for the adaptation of microcytic hypochromic anemia in iron deficiency. In iron deficiency, heme concentration declines which leads to HRI activation and inhibition of globin synthesis as illustrated in Fig. 2. This results in decreased hemoglobin and total protein content in Wt RBCs. In the absence of HRI (Hri −/− ), protein synthesis continues in the face of heme deficiency, resulting in excess globins. These heme-free globins are unstable and precipitate as inclusions (highlighted in green) in RBCs and their precursors, causing destruction of these cells. Hri −/− RBCs are macrocytic and hyperchromic. Furthermore, Hri −/− mice exhibit significant decrease in the RBC number with inhibition of erythroid differentiation and splenomegaly in iron deficiency

The importance of HRI in the pathophysiology of heme deficiency disorders is further demonstrated in mice with combined deficiencies of HRI and ferrochelatase (Fech) (Han et al. 2005). Since Fech (the last enzyme of heme biosynthesis) inserts iron into protoporphyrin IX to form heme, Fech-deficient mice are heme deficient. HRI is activated in Fech-deficient reticulocytes, providing the in vivo evidence that HRI is regulated directly by heme and not by iron (Han et al. 2005). Fech-deficient mice develop microcytic hypochromic anemia and erythropoietic protoporphyria. In HRI deficiency, however, these mice display more severe phenotypes and display macrocytic hyperchromic anemia. Furthermore, globin inclusions are observed in reticulocytes from HRI-deficient Fech-deficient animals. The morphologically and biochemically similar red cell abnormalities elicited by the absence of HRI both in iron and heme deficiencies further underscore the importance of HRI in inhibiting protein synthesis to avoid accumulation of excess heme-free globins in heme-deficient states.

Integrated Stress Response of eIF2αP Signaling in Primary Erythroblasts

Phosphorylation of eIF2α by eIF2α kinases elicits an integrated stress response (ISR) under various stress conditions and is conserved from yeast to humans (Ron 2007). In mammalian cells, four eIF2α kinases, HRI, PKR, GCN2, and PERK, are expressed in distinct tissues to combat different stresses. PKR responds to viral infection while GCN2 senses nutrient starvations. PERK is activated by endoplasmic reticulum (ER) stress, and HRI is inhibited by heme. All four eIF2α kinases respond to oxidative and environmental stresses (Chen 2000, 2007).

In addition to inhibiting protein synthesis of highly translated mRNAs, eIF2α phosphorylation also selectively increases translation of certain poorly translated mRNAs for adaptation to stress (Fig. 5). This coordinated translational regulation is coined as ISR (Harding et al. 2003). As illustrated in Fig. 6, translational upregulation by eIF2αP requires upstream open reading frames (uORFs) in the 5′UTR of these unique mRNAs, most notably activating transcription factor 4 (ATF4). Under non-stressed conditions, these uORFs restrict the translation at the downstream-initiating AUG codon encoding ATF4 protein. Upon stress, phosphorylation of eIF2α is the pool of functional eIF2 and slows down the reinitiation at uORFs to permit translation start site at the coding sequence of ATF4 mRNA (Hinnebusch et al. 2016).
Eif2ak1, Fig. 5

Integrated stress response of HRI-eIF2αP-ATF4 signaling in erythroid precursors. HRI is activated in erythroid precursors upon heme deficiency, oxidative and environmental stress, and erythroid diseases of β-thalassemia and erythroid protoporphyria. Phosphorylation of eIF2α inhibits protein synthesis to prevent proteotoxicity of globin inclusions in heme deficiency. In addition, eIF2αP selectively enhances the translation of ATF4 mRNA. ATF4 then initiates an adaptive gene expression to mitigate oxidative stress and to promote erythroid differentiation

Eif2ak1, Fig. 6

Enhanced translation of ATF4 mRNA by eIF2αP upon stress. In the 5′UTR of ATF4 mRNA, there are two uORFs that are preferentially translated under non-stressed conditions and prevent the downstream translational initiation at the coding sequence of ATF4 mRNA. As initiating 40S ribosomal subunits scan from the 5′ cap structure (green dots), translation starts at the uORF1. After termination of translation, the 40S subunit remains associated with mRNA and reinitiates efficiently at uORF2 under non-stressed conditions. Upon stress, elevated eIF2αP impairs the reinitiation of 40S at uORF2 because of limiting functional eIF2. Thus, 40S continues to scan downstream and initiates at the AUG codon of coding sequence of ATF4 mRNA permitting the synthesis of ATF4 protein

A major target gene activated by ATF4 is the transcription factor C/EBP homologous protein-10 (CHOP-10). CHOP is upregulated transcriptionally in a wide variety of cells upon many stresses. Induction of CHOP leads to expression of GADD34 (Fig. 5), which recruits eIF2αP for dephosphorylation by PPase1 (Fig. 1). This action of GADD34 in regenerating active eIF2 is necessary for the recovery of protein synthesis of stress-induced gene expression that occurs late in the stress response (reviewed in (Chen 2014)). Upon ER stress, ISR has been shown to upregulate expression of genes directly involved in redox homeostasis to mitigate oxidative stress (Harding et al. 2003). Increased ROS levels were observed in cells with impaired ISR signaling resulting from mutations in eIF2α phosphorylation (Scheuner et al. 2001) or from deletion of PERK (Harding et al. 2003).

In the erythroid lineage, HRI expression increases during differentiation with higher expression in the hemoglobinized erythroblasts (Fig. 7) (Liu et al. 2008). Starting at the basophilic erythroblast stage, HRI is the predominant eIF2α kinase and is expressed at levels two orders of magnitude higher than the other three eIF2α kinases. Recently, it has been demonstrated that HRI activates the eIF2αP-ATF4 signaling pathway upon oxidative stress in primary erythroblasts (Fig. 5) (Suragani et al. 2012). Hri −/− erythroblasts suffer from increased ROS and apoptosis upon acute oxidative stress induced by exposure to sodium arsenite. During chronic iron deficiency in vivo, HRI is also necessary to reduce oxidative stress. ROS levels in RBCs and erythroid precursors were dramatically elevated during iron deficiency in Hri −/− mice, but not in Hri +/+ mice. Furthermore, the induction of heme oxygenase1 (HO-1) and other antioxidant genes upon acute oxidative stress in erythroblasts is dependent on HRI and ATF4. RBCs from Hri −/− and Atf4 −/− mice were more sensitive to H2O2-induced oxidative stress and exhibited increased ROS levels when compared to RBCs from Hri +/+ and Atf4 +/− mice. These findings indicate that HRI-ATF4 signaling may also be required during erythroid development. Impairment of this pathway may generate RBCs that are more sensitive to oxidative insult (Suragani et al. 2012). Thus, HRI-eIF2αP-ATF4 signaling provides the third signaling axis to combat oxidative stress in addition to the two known pathways mediated by Foxo3 and Nrf2 (Chen 2014).
Eif2ak1, Fig. 7

The HRI-eIF2αP-ATF4 signaling during erythropoiesis. The expression of HRI increases during erythroid differentiation from BFU-E to reticulocytes. HRI is the major eIF2α kinase in the erythroid lineage and is indispensable to coordinate heme and globin synthesis. Additionally, HRI activates ATF4 signaling pathway to mitigate oxidative stress in nucleated erythroblasts. This HRI-ATF4 pathway is activated during erythropoiesis and is necessary to promote erythroid differentiation. At the earlier stages of erythropoiesis before basophilic erythroblasts, other eIF2α kinases and HRI are both required for the regulation of proliferation of erythroid precursors

HRI-eIF2αP-ATF4 Signaling Necessary for Erythroid Differentiation

Beyond regulation of globin translation, HRI is also necessary to reduce ineffective erythropoiesis during iron/heme deficiency and in β-thalassemia (Han et al. 2001, 2005). Recently, it has been shown that the ineffective erythropoiesis occurring in Hri −/− mice during iron deficiency is due primarily to the profound inhibition of erythroid differentiation at the basophilic erythroblast stage (Suragani et al. 2012). This is also observed in several other mouse models of stress erythropoiesis including β-thalassemia and in mice deficient of Rb or Stat5a/5b deficiency [reviewed in (Chen 2014)].

While Hri −/− fetal liver (FL) displayed a mild defect in erythroid differentiation in vivo under normal iron-sufficient conditions (Liu et al. 2008), Hri −/− FL erythroid progenitors showed a significant inhibition of erythroid differentiation at the basophilic erythroblast stage when cultured and differentiated ex vivo (Suragani et al. 2012), recapitulating the inhibition of erythroid differentiation in vivo during iron deficiency (Liu et al. 2008; Suragani et al. 2012). The HRI-eIF2αP-ATF4 pathway is also activated during ex vivo differentiation of erythroid precursors and during erythroid differentiation of mouse erythroleukemic (MEL) cells. Furthermore, knockdown of ATF4 in MEL cells resulted in inhibition of erythroid differentiation (Suragani et al. 2012). Thus, the HRI-ATF4 signaling pathway may be necessary for inducing transcription of genes required for erythropoiesis starting at the basophilic erythroblast stage. As summarized in Fig. 7, HRI not only inhibits globin translation in nucleated erythroblasts but also increases ATF4 translation to mitigate oxidative stress and to promote erythroid differentiation. At the enucleated reticulocyte stage, the role of HRI is to regulate globin translation to prevent excessive globin synthesis, which is cytotoxic and increases oxidative stress. Both of these functions of HRI are necessary for optimal erythroid maturation to prevent anemia.

Summary

HRI is a key regulator of protein synthesis and differentiation in the erythroid lineage under stress conditions of iron deficiency and oxidative stress as well as in disease states of thalassemia and erythropoietic protoporphyria. It senses intracellular concentrations to coordinate globin translation with the heme available for the production of hemoglobin. Additionally, translational upregulation of ATF4 mRNA by HRI-eIF2αP signaling is important not only for mitigating oxidative stress but also for promoting erythroid differentiation. However, ATF4 mRNA may not be the only target of HRI-eIF2αP signaling during erythropoiesis. Ribosome profiling, which assesses ribosome occupancies in translating mRNAs in vivo genome-wide, will be instrumental in uncovering additional translational initiations at uORFs. Defining these translationally regulated and long-sought target uORF containing mRNAs of eIF2αP signaling in Wt and Hri −/− erythroid precursors will reveal novel protein components in regulating erythropoiesis. Further studies on the molecular mechanisms of HRI-eIF2αP-ATF4 signaling in the regulation of erythroid differentiation in iron and heme deficiencies may be illuminated by using mutant mice defective in this signaling pathway such as Atf4 −/− and the erythroid-specific Ala51 substitution of eIF2α in comparison with Hri −/− mice. Finally, HRI-eIF2αP signaling may be exploited for a novel pharmaceutical therapy for hemoglobinopathies.

References

  1. Bauer BN, Rafie-Kolpin M, Lu L, Han A, Chen J-J. Multiple autophosphorylation is essential for the formation of the active and stable homodimer of heme-regulated eIF-2α kinase. Biochemistry. 2001;40:11543–51.PubMedCrossRefGoogle Scholar
  2. Chefalo P, Oh J, Rafie-Kolpin M, Kan B, Chen J-J. Heme-regulated eIF-2α kinase purifies as a hemoprotein. Eur J Biochem. 1998;258:820–30.PubMedCrossRefGoogle Scholar
  3. Chen J-J. Heme-regulated eIF-2α kinase. In: Sonenberg N, JWB H, Mathews MB, editors. Translational control of gene expression. Cold Springs Harbor: Cold Spring Harbor Laboratory Press; 2000. p. 529–46.Google Scholar
  4. Chen JJ. Regulation of protein synthesis by the heme-regulated eIF2alpha kinase: relevance to anemias. Blood. 2007;109:2693–9.PubMedPubMedCentralGoogle Scholar
  5. Chen JJ. Translational control by heme-regulated eIF2alpha kinase during erythropoiesis. Curr Opin Hematol. 2014;21:172–8.PubMedPubMedCentralCrossRefGoogle Scholar
  6. Han AP, Fleming MD, Chen JJ. Heme-regulated eIF2alpha kinase modifies the phenotypic severity of murine models of erythropoietic protoporphyria and beta-thalassemia. J Clin Invest. 2005;115:1562–70.PubMedPubMedCentralCrossRefGoogle Scholar
  7. Han AP, Yu C, Lu L, Fujiwara Y, Browne C, Chin G, Fleming M, Leboulch P, Orkin SH, Chen JJ. Heme-regulated eIF2alpha kinase (HRI) is required for translational regulation and survival of erythroid precursors in iron deficiency. EMBO J. 2001;20:6909–18.PubMedPubMedCentralCrossRefGoogle Scholar
  8. Harding HP, Zhang Y, Zeng H, Novoa I, Lu PD, Calfon M, Sadri N, Yun C, Popko B, Paules R, et al. An integrated stress response regulates amino acid metabolism and resistance to oxidative stress. Mol Cell. 2003;11:619–33.PubMedCrossRefGoogle Scholar
  9. Hinnebusch AG, Ivanov IP, Sonenberg N. Translational control by 5′-untranslated regions of eukaryotic mRNAs. Science. 2016;352:1413–6.PubMedCrossRefGoogle Scholar
  10. Igarashi J, Murase M, Iizuka A, Pichierri F, Martinkova M, Shimizu T. Elucidation of the heme binding site of heme-regulated eukaryotic initiation factor 2alpha kinase and the role of the regulatory motif in heme sensing by spectroscopic and catalytic studies of mutant proteins. J Biol Chem. 2008;283:18782–91.PubMedCrossRefGoogle Scholar
  11. Liu S, Bhattacharya S, Han A, Suragani RN, Zhao W, Fry RC, Chen JJ. Haem-regulated eIF2alpha kinase is necessary for adaptive gene expression in erythroid precursors under the stress of iron deficiency. Br J Haematol. 2008;143:129–37.PubMedPubMedCentralCrossRefGoogle Scholar
  12. Liu S, Suragani RN, Wang F, Han A, Zhao W, Andrews NC, Chen JJ. The function of heme-regulated eIF2alpha kinase in murine iron homeostasis and macrophage maturation. J Clin Invest. 2007;117:3296–305.PubMedPubMedCentralCrossRefGoogle Scholar
  13. Lu L, Han AP, Chen JJ. Translation initiation control by heme-regulated eukaryotic initiation factor 2alpha kinase in erythroid cells under cytoplasmic stresses. Mol Cell Biol. 2001;21:7971–80.PubMedPubMedCentralCrossRefGoogle Scholar
  14. Rafie-Kolpin M, Chefalo PJ, Hussain Z, Hahn J, Uma S, Matts RL, Chen J-J. Two heme-binding domains of heme-regulated eIF-2α kinase: N-terminus and kinase insertion. J Biol Chem. 2000;275:5171–8.PubMedCrossRefGoogle Scholar
  15. Rafie-Kolpin M, Han AP, Chen JJ. Autophosphorylation of threonine 485 in the activation loop is essential for attaining eIF2alpha kinase activity of HRI. Biochemistry. 2003;42:6536–44.PubMedCrossRefGoogle Scholar
  16. Ron D, Harding HP. In: Mathews NSM, Hershey JWB, editors. eIF2α phosphorylation in cellular stress responses and disease. In translational control in biology and medicine. Cols Spring Harbor: Cols Spring Harbor Laboratory Press; 2007.Google Scholar
  17. Scheuner D, Song B, McEwen E, Liu C, Laybutt R, Gillespie P, Saunders T, Bonner-Weir S, Kaufman RJ. Translational control is required for the unfolded protein response and in vivo glucose homeostasis. Mol Cell. 2001;7:1165–76.PubMedCrossRefGoogle Scholar
  18. Sonenberg N, Hinnebusch AG. Regulation of translation initiation in eukaryotes: mechanisms and biological targets. Cell. 2009;136:731–45.PubMedPubMedCentralCrossRefGoogle Scholar
  19. Suragani RN, Zachariah RS, Velazquez JG, Liu S, Sun CW, Townes TM, Chen JJ. Heme-regulated eIF2alpha kinase activated Atf4 signaling pathway in oxidative stress and erythropoiesis. Blood. 2012;119:5276–84.PubMedPubMedCentralCrossRefGoogle Scholar
  20. Zhan K, Vattem KM, Bauer BN, Dever TE, Chen JJ, Wek RC. Phosphorylation of eukaryotic initiation factor 2 by heme-regulated inhibitor kinase-related protein kinases in Schizosaccharomyces pombe is important for fesistance to environmental stresses. Mol Cell Biol. 2002;22:7134–46.PubMedPubMedCentralCrossRefGoogle Scholar

Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute of Medical Engineering and Science, Massachusetts Institute of TechnologyCambridgeUSA