Heme-regulated eIF2alpha kinase modifies the phenotypic severity of murine models of erythropoietic protoporphyria and beta-thalassemia

The ATF4-RPS19BP1 axis modulates ribosome biogenesis to promote erythropoiesis

ABSTRACT

Hematopoietic differentiation is controlled by intrinsic regulators and the extrinsic hematopoietic niche. Activating transcription factor 4 (ATF4) plays a crucial role in the function of fetal and adult hematopoietic stem cell maintenance. However, the precise function of ATF4 in the bone marrow (BM) niche and the mechanism by which ATF4 regulates adult hematopoiesis remain largely unknown. Here, we used 4 cell-type-specific mouse Cre lines to achieve conditional knockout of Atf4 in Cdh5+ endothelial cells, Prx1+ BM stromal cells, Osx+ osteoprogenitor cells, and Mx1+ hematopoietic cells and uncovered the role of Atf4 in niche cells and hematopoiesis. Intriguingly, depletion of Atf4 in niche cells did not affect hematopoiesis; however, Atf4-deficient hematopoietic cells exhibited erythroid differentiation defects, leading to hypoplastic anemia. Mechanistically, ATF4 mediated direct regulation of Rps19bp1 transcription, which is, in turn, involved in 40 S ribosomal subunit assembly to coordinate ribosome biogenesis and promote erythropoiesis. Finally, we demonstrate that under conditions of 5-fluorouracil-induced stress, Atf4 depletion impedes the recovery of hematopoietic lineages, which requires efficient ribosome biogenesis. Taken together, our findings highlight the indispensable role of the ATF4-RPS19BP1 axis in the regulation of erythropoiesis.

Surviving and Adapting to Stress: Translational Control and the Integrated Stress Response

Significance: Organisms adapt to changing environments by engaging cellular stress response pathways that serve to restore proteostasis and enhance survival. A primary adaptive mechanism is the integrated stress response (ISR), which features phosphorylation of the α subunit of eukaryotic translation initiation factor 2 (eIF2). Four eIF2α kinases respond to different stresses, enabling cells to rapidly control translation to optimize management of resources and reprogram gene expression for stress adaptation. Phosphorylation of eIF2 blocks its guanine nucleotide exchange factor, eIF2B, thus lowering the levels of eIF2 bound to GTP that is required to deliver initiator transfer RNA (tRNA) to ribosomes. While bulk messenger RNA (mRNA) translation can be sharply lowered by heightened phosphorylation of eIF2α, there are other gene transcripts whose translation is unchanged or preferentially translated. Among the preferentially translated genes is ATF4, which directs transcription of adaptive genes in the ISR. Recent Advances and Critical Issues: This review focuses on how eIF2α kinases function as first responders of stress, the mechanisms by which eIF2α phosphorylation and other stress signals regulate the exchange activity of eIF2B, and the processes by which the ISR triggers differential mRNA translation. To illustrate the synergy between stress pathways, we describe the mechanisms and functional significance of communication between the ISR and another key regulator of translation, mammalian/mechanistic target of rapamycin complex 1 (mTORC1), during acute and chronic amino acid insufficiency. Finally, we discuss the pathological conditions that stem from aberrant regulation of the ISR, as well as therapeutic strategies targeting the ISR to alleviate disease. Future Directions: Important topics for future ISR research are strategies for modulating this stress pathway in disease conditions and drug development, molecular processes for differential translation and the coordinate regulation of GCN2 and other stress pathways during physiological and pathological conditions. Antioxid. Redox Signal. 39, 351-373.

Role of symmetry in 3,3-diphenyl-1,3-dihydroindol-2-one derivatives as inhibitors of translation initiation

The ternary complex (eIF2·GTP·Met-tRNAiMet) and the eIF4F complex assembly are two major regulatory steps in the eukaryotic translation initiation. Inhibition of the ternary complex assembly is therefore a promising target for the development of novel anti-cancer therapeutics. Building on the finding that clotrimazole (CLT), a molecular probe that depletes intracellular Ca2+ stores and subsequently induce eIF2α phosphorylation, inhibit translation initiation, and reduce preferentially the expression of oncoproteins over "housekeeping" ones,1-3 we undertook structure activity relationship (SAR) studies that identified 3,3-diarylindoline-2-one #1181 as an interesting scaffold. Compound #1181 also induce phosphorylation of eIF2α thereby reducing the availability of the ternary complex, which leads to inhibition of translation initiation.4 Our subsequent efforts focused on understanding SAR iterative lead optimization to enhance potency and improve bioavailability. Herein, we report a complementing study focusing on heavily substituted symmetric and asymmetric 3,3-(o,m-disubstituted)diarylindoline-2-ones. These compounds were evaluated by the dual luciferase reporter ternary complex assay that recapitualates phosphorylation of eIF2α in a quantitative manner. We also evaluated all compounds by sulforhodamine B assay, which measures the overall effect of compounds on cell proliferations and/or viability.

The Heme-Regulated Inhibitor Kinase Requires Dimerization for Heme- Sensing Activity

The heme-regulated inhibitor (HRI) is a heme-sensing kinase that regulates mRNA translation in erythroid cells. In heme deficiency, HRI is activated to phosphorylate eukaryotic initiation factor 2α and halt production of globins, thus avoiding accumulation of heme-free globin chains. HRI is inhibited by heme via binding to one or two heme-binding domains within the HRI N-terminal and kinase domains. HRI has recently been found to inhibit fetal hemoglobin (HbF) production in adult erythroid cells. Depletion of HRI increases HbF production, presenting a therapeutically exploitable target for the treatment of patients with sickle cell disease or thalassemia, which benefit from elevated HbF levels. HRI is known to be an oligomeric enzyme that is activated through autophosphorylation, although the exact nature of the HRI oligomer, its relation to autophosphorylation, and its mode of heme regulation remain unclear. Here, we employ biochemical and biophysical studies to demonstrate that HRI forms a dimeric species that is not dependent on autophosphorylation, the C-terminal coiled-coil domain in HRI is essential for dimer formation, and dimer formation facilitates efficient autophosphorylation and activation of HRI. We also employ kinetic studies to demonstrate that the primary avenue by which heme inhibits HRI is through the heme-binding site within the kinase domain, and that this inhibition is relatively independent of binding of ATP and eukaryotic initiation factor 2α substrates. Together, these studies highlight the mode of heme inhibition and the importance of dimerization in human HRI heme-sensing activity.

Translational control by heme-regulated elF2α kinase during erythropoiesis

PURPOSE OF REVIEW

HRI is the heme-regulated elF2α kinase that phosphorylates the α-subunit of elF2. Although the role of HRI in inhibiting globin synthesis in erythroid cells is well established, broader roles of HRI in translation have been uncovered recently. This review is to summarize the new discoveries of HRI in stress erythropoiesis and in fetal γ-globin expression.

RECENT FINDINGS

HRI and activating transcription factor 4 (ATF4) mRNAs are highly expressed in early erythroblasts. Inhibition of protein synthesis by HRI-phosphorylated elF2α (elF2αP) is necessary to maintain protein homeostasis in both the cytoplasm and mitochondria. In addition, HRI-elF2αP specifically enhances translation of ATF4 mRNA leading to the repression of mechanistic target of rapamycin complex 1 (mTORC1) signaling. ATF4-target genes are most highly activated during iron deficiency to maintain mitochondrial function, redox homeostasis, and to enable erythroid differentiation. HRI is therefore a master translation regulator of erythropoiesis sensing intracellular heme concentrations and oxidative stress for effective erythropoiesis. Intriguingly, HRI-elF2αP-ATF4 signaling also inhibits fetal hemoglobin production in human erythroid cells.

SUMMARY

The primary function of HRI is to maintain protein homeostasis accompanied by the induction of ATF4 to mitigate stress. Role of HRI-ATF4 in γ-globin expression raises the potential of HRI as a therapeutic target for hemoglobinopathy.

Nrf2 expands the intracellular pool of the chaperone AHSP in a cellular model of β-thalassemia

In β-thalassemia, free α-globin chains are unstable and tend to aggregate or degrade, releasing toxic heme, porphyrins and iron, which produce reactive oxygen species (ROS). α-Hemoglobin-stabilizing protein (AHSP) is a potential modifier of β-thalassemia due to its ability to escort free α-globin and inhibit the cellular production of ROS. The influence of AHSP on the redox equilibrium raises the question of whether AHSP expression is regulated by components of ROS signaling pathways and/or canonical redox proteins. Here, we report that AHSP expression in K562 cells could be stimulated by NFE2-related factor 2 (Nrf2) and its agonist tert-butylhydroquinone (tBHQ). This tBHQ-induced increase in AHSP expression was also observed in Ter119+ mouse erythroblasts at each individual stage during terminal erythroid differentiation. We further report that the AHSP level was elevated in α-globin-overexpressing K562 cells and staged erythroblasts from β^IVS-2-654 thalassemic mice. tBHQ treatment partially alleviated, whereas Nrf2 or AHSP knockdown exacerbated, α-globin precipitation and ROS production in fetal liver-derived thalassemic erythroid cells. MafG and Nrf2 occupancy at the MARE-1 site downstream of the AHSP transcription start site was detected in K562 cells. Finally, we show that MafG facilitated the activation of the AHSP gene in K562 cells by Nrf2. Our results demonstrate Nrf2-mediated feedback regulation of AHSP in response to excess α-globin, as occurs in β-thalassemia.

CRISPR-Cas9 to induce fetal hemoglobin for the treatment of sickle cell disease

Genome editing is potentially a curative technique available to all individuals with β-hemoglobinopathies, including sickle cell disease (SCD). Fetal hemoglobin (HbF) inhibits sickle hemoglobin (HbS) polymerization, and it is well described that naturally occurring hereditary persistence of HbF (HPFH) alleviates disease symptoms; therefore, reawakening of developmentally silenced HbF in adult red blood cells (RBCs) has long been of interest as a therapeutic strategy. Recent advances in genome editing platforms, particularly with the use of CRISPR-Cas9, have paved the way for efficient HbF induction through the creation of artificial HPFH mutations, editing of transcriptional HbF silencers, and modulating epigenetic intermediates that govern HbF expression. Clinical trials investigating BCL11A enhancer editing in patients with β-hemoglobinopathies have demonstrated promising results, although follow-up is short and the number of patients treated to date is low. While practical, economic, and clinical challenges of genome editing are well recognized by the scientific community, potential solutions to overcome these hurdles are in development. Here, we review the recent progress and obstacles yet to be overcome for the most effective and feasible HbF reactivation practice using CRISPR-Cas9 genome editing as a curative strategy for patients with SCD.

The Hsp70 chaperone system: distinct roles in erythrocyte formation and maintenance

Erythropoiesis is a tightly regulated cell differentiation process in which specialized oxygen- and carbon dioxide-carrying red blood cells are generated in vertebrates. Extensive reorganization and depletion of the erythroblast proteome leading to the deterioration of general cellular protein quality control pathways and rapid hemoglobin biogenesis rates could generate misfolded/aggregated proteins and trigger proteotoxic stresses during erythropoiesis. Such cytotoxic conditions could prevent proper cell differentiation resulting in premature apoptosis of erythroblasts (ineffective erythropoiesis). The heat shock protein 70 (Hsp70) molecular chaperone system supports a plethora of functions that help maintain cellular protein homeostasis (proteostasis) and promote red blood cell differentiation and survival. Recent findings show that abnormalities in the expression, localization and function of the members of this chaperone system are linked to ineffective erythropoiesis in multiple hematological diseases in humans. In this review, we present latest advances in our understanding of the distinct functions of this chaperone system in differentiating erythroblasts and terminally differentiated mature erythrocytes. We present new insights into the protein repair-only function(s) of the Hsp70 system, perhaps to minimize protein degradation in mature erythrocytes to warrant their optimal function and survival in the vasculature under healthy conditions. The work also discusses the modulatory roles of this chaperone system in a wide range of hematological diseases and the therapeutic gain of targeting Hsp70.

HRI depletion cooperates with pharmacologic inducers to elevate fetal hemoglobin and reduce sickle cell formation

Increasing fetal hemoglobin (HbF) provides clinical benefit in patients with sickle cell disease (SCD). We recently identified heme-regulated inhibitor (HRI, EIF2AK1), as a novel HbF regulator. Because HRI is an erythroid-specific protein kinase, it presents a potential target for pharmacologic intervention. We found that maximal HbF induction required >80% to 85% HRI depletion. Because it remains unclear whether this degree of HRI inhibition can be achieved pharmacologically, we explored whether HRI knockdown can be combined with pharmacologic HbF inducers to achieve greater HbF production and minimize potential adverse effects associated with treatments. Strongly cooperative HbF induction was observed when HRI depletion was combined with exposure to pomalidomide or the EHMT1/2 inhibitor UNC0638, but not to hydroxyurea. Mechanistically, reduction in the levels of the HbF repressor BCL11A reflected the cooperativity of HRI loss and pomalidomide treatment, whereas UNC0638 did not modulate BCL11A levels. In conjunction with HRI loss, pomalidomide maintained its HbF-inducing activity at 10-fold lower concentrations, in which condition there were minimal observed detrimental effects on erythroid cell maturation and viability, as well as fewer alterations in the erythroid transcriptome. When tested in cells from patients with SCD, combining HRI depletion with pomalidomide or UNC0638 achieved up to 50% to 60% HbF and 45% to 50% HbF, respectively, as measured by high-performance liquid chromatography, and markedly counteracted cell sickling. In summary, this study provides a foundation for the exploration of combining future small-molecule HRI inhibitors with additional pharmacologic HbF inducers to maximize HbF production and preserve erythroid cell functionality for the treatment of SCD and other hemoglobinopathies.

ATF4 Regulates MYB to Increase γ-Globin in Response to Loss of β-Globin

β-Hemoglobinopathies can trigger rapid production of red blood cells in a process known as stress erythropoiesis. Cellular stress prompts differentiating erythroid precursors to express high levels of fetal γ-globin. However, the mechanisms underlying γ-globin production during cellular stress are still poorly defined. Here, we use CRISPR-Cas genome editing to model the stress caused by reduced levels of adult β-globin. We find that decreased β-globin is sufficient to induce robust re-expression of γ-globin, and RNA sequencing (RNA-seq) of differentiating isogenic erythroid precursors implicates ATF4 as a causal regulator of this response. ATF4 binds within the HBS1L-MYB intergenic enhancer and regulates expression of MYB, a known γ-globin regulator. Overall, the reduction of ATF4 upon β-globin knockout decreases the levels of MYB and BCL11A. Identification of ATF4 as a key regulator of globin compensation adds mechanistic insight to the poorly understood phenomenon of stress-induced globin compensation and could inform strategies to treat hemoglobinopathies.

Beyond pigmentation: signs of liver protection during afamelanotide treatment in Swiss patients with erythropoietic protoporphyria, an observational study

Erythropoietic protoporphyria (EPP) is an ultra-rare inherited disorder with overproduction of protoporphyrin in maturating erythroblasts. This excess protoporphyrin leads to incapacitating phototoxic burns in sunlight exposed skin. Its biliary elimination causes cholestatic liver injury in 20% and terminal liver failure in 4% of EPP patients. Thereby, the risk of liver injury increases with increasing erythrocyte protoporphyrin concentrations. Afamelanotide, an α-melanocyte-stimulating hormone (MSH) analog inducing skin pigmentation, was shown to improve sunlight tolerance in EPP. Beyond this well-known effect on pigmentation, the MSHs have liver-protective effects and improve survival of maturating erythroblasts, effects described in animal or in vitro models to date only. We investigated whether afamelanotide treatment in EPP has effects on erythropoiesis, protoporphyrin concentrations, and liver injury by analyzing retrospectively our long-term safety data.

Methods

From the 47 Swiss EPP-patients treated at our center since 2006, we included those 38 patients in the current analysis who received at least one afamelanotide dose between 2016 and 2018 and underwent regular laboratory testing before and during the treatment. We compared the means of pretreatment measurements with those during the treatment.

Results

Protoporphyrin concentrations dropped from 21.39 ± 11.12 (mean ± SD) before afamelanotide to 16.83 ± 8.24 µmol/L (p < .0001) during treatment. Aspartate aminotransferase decreased from 26.67 ± 13.16 to 22.9 ± 7.76 IU/L (p = .0146). For both entities, patients with higher values showed a more progressive decrease, indicating a risk reduction of EPP-related liver disease. The pre-existing hypochromia and broad mean red-cell distribution width were further augmented under afamelanotide. This was more likely due to an influence of afamelanotide on maturating erythroblasts than due to an exacerbated iron deficiency, as mean zinc-protoporphyrin decreased significantly and ferritin remained unchanged. No serious afamelanotide-related adverse events were observed for a total of 240 treatment years.

Conclusion

Our findings point to a protective effect of afamelanotide on erythroblast maturation and protoporphyrin-induced liver injury.

Plain Language summary

Afamelanotide, a skin tanning hormone, may protect patients with erythropoietic protoporphyria not only from skin burns, but also from liver injury associated with the disease. Patients with erythropoietic protoporphyria (EPP), an inherited metabolic disease, suffer from light-induced skin burns and liver injury elicited by the accumulated light sensitizer protoporphyrin. The excess protoporphyrin is produced in red cell precursors in the bone marrow, and it is eliminated from the body via the liver and bile. A high protoporphyrin excretion burden damages the liver cells, the risk for this increases with higher protoporphyrin concentrations. About 20% of EPP patients show some sign of liver injury and 4% develop life-threatening liver dysfunction.Afamelanotide, closely related to natural α-melanocyte stimulating hormone (MSH), induces skin tanning. This effect protects EPP patients from light-induced skin burns as shown in previous studies. We have treated Swiss EPP patients with afamelanotide since 2006, and we regularly perform safety tests of this treatment.Recent in vitro and animal studies demonstrated α-MSH effects other than skin tanning, including an improved synthesis of red blood cell precursors in the bone-marrow and protection of the liver from experimentally induced damage. Until now, it is unknown whether afamelanotide has similar effects in the human organism.To study this question, we analyzed retrospectively the safety laboratory data of 38 Swiss patients, who received at least one dose of afamelanotide from 2016 to 2019. We found that both, the average protoporphyrin concentrations and aspartate aminotransferase, a test for liver function, improved during afamelanotide treatment as compared to before.We concluded that afamelanotide applied to EPP patients to protect them from light-induced skin burns also may reduce their risk of liver injury.

The Integrated Stress Response and Phosphorylated Eukaryotic Initiation Factor 2α in Neurodegeneration

The proposed molecular mechanisms underlying neurodegenerative pathogenesis are varied, precluding the development of effective therapies for these increasingly prevalent disorders. One of the most consistent observations across neurodegenerative diseases is the phosphorylation of eukaryotic initiation factor 2α (eIF2α). eIF2α is a translation initiation factor, involved in cap-dependent protein translation, which when phosphorylated causes global translation attenuation. eIF2α phosphorylation is mediated by 4 kinases, which, together with their downstream signaling cascades, constitute the integrated stress response (ISR). While the ISR is activated by stresses commonly observed in neurodegeneration, such as oxidative stress, endoplasmic reticulum stress, and inflammation, it is a canonically adaptive signaling cascade. However, chronic activation of the ISR can contribute to neurodegenerative phenotypes such as neuronal death, memory impairments, and protein aggregation via apoptotic induction and other maladaptive outcomes downstream of phospho-eIF2α-mediated translation inhibition, including neuroinflammation and altered amyloidogenic processing, plausibly in a feed-forward manner. This review examines evidence that dysregulated eIF2a phosphorylation acts as a driver of neurodegeneration, including a survey of observations of ISR signaling in human disease, inspection of the overlap between ISR signaling and neurodegenerative phenomenon, and assessment of recent encouraging findings ameliorating neurodegeneration using developing pharmacological agents which target the ISR. In doing so, gaps in the field, including crosstalk of the ISR kinases and consideration of ISR signaling in nonneuronal central nervous system cell types, are highlighted.

Nanosecond pulsed electric fields induce the integrated stress response via reactive oxygen species-mediated heme-regulated inhibitor (HRI) activation

The integrated stress response (ISR) is one of the most important cytoprotective mechanisms and is integrated by phosphorylation of the α subunit of eukaryotic translation initiation factor 2 (eIF2α). Four eIF2α kinases, heme-regulated inhibitor (HRI), double-stranded RNA-dependent protein kinase (PKR), PKR-like endoplasmic reticulum kinase (PERK), and general control nonderepressible 2 (GCN2), are activated in response to several stress conditions. We previously reported that nanosecond pulsed electric fields (nsPEFs) are a potential therapeutic tool for ISR activation. In this study, we examined which eIF2α kinase is activated by nsPEF treatment. To assess the responsible eIF2α kinase, we used previously established eIF2α kinase quadruple knockout (4KO) and single eIF2α kinase-rescued 4KO mouse embryonic fibroblast (MEF) cells. nsPEFs 70 ns in duration with 30 kV/cm electric fields caused eIF2α phosphorylation in wild-type (WT) MEF cells. On the other hand, nsPEF-induced eIF2α phosphorylation was completely abolished in 4KO MEF cells and was recovered by HRI overexpression. CM-H₂DCFDA staining showed that nsPEFs generated reactive oxygen species (ROS), which activated HRI. nsPEF-induced eIF2α phosphorylation was blocked by treatment with the ROS scavenger N-acetyl-L-cysteine (NAC). Our results indicate that the eIF2α kinase HRI is responsible for nsPEF-induced ISR activation and is activated by nsPEF-generated ROS.

Heme-regulated eIF2α kinase in erythropoiesis and hemoglobinopathies

As essential components of hemoglobin, iron and heme play central roles in terminal erythropoiesis. The impairment of this process in iron/heme deficiency results in microcytic hypochromic anemia, the most prevalent anemia globally. Heme-regulated eIF2α kinase, also known as heme-regulated inhibitor (HRI), is a key heme-binding protein that senses intracellular heme concentrations to balance globin protein synthesis with the amount of heme available for hemoglobin production. HRI is activated during heme deficiency to phosphorylate eIF2α (eIF2αP), which simultaneously inhibits the translation of globin messenger RNAs (mRNAs) and selectively enhances the translation of activating transcription factor 4 (ATF4) mRNA to induce stress response genes. This coordinated translational regulation is a universal hallmark across the eIF2α kinase family under various stress conditions and is termed the integrated stress response (ISR). Inhibition of general protein synthesis by HRI-eIF2αP in erythroblasts is necessary to prevent proteotoxicity and maintain protein homeostasis in the cytoplasm and mitochondria. Additionally, the HRI-eIF2αP-ATF4 pathway represses mechanistic target of rapamycin complex 1 (mTORC1) signaling, specifically in the erythroid lineage as a feedback mechanism of erythropoietin-stimulated erythropoiesis during iron/heme deficiency. Furthermore, ATF4 target genes are most highly activated during iron deficiency to maintain mitochondrial function and redox homeostasis, as well as to enable erythroid differentiation. Thus, heme and translation regulate erythropoiesis through 2 key signaling pathways, ISR and mTORC1, which are coordinated by HRI to circumvent ineffective erythropoiesis (IE). HRI-ISR is also activated to reduce the severity of β-thalassemia intermedia in the Hbbth1/th1 murine model. Recently, HRI has been implicated in the regulation of human fetal hemoglobin production. Therefore, HRI-ISR has emerged as a potential therapeutic target for hemoglobinopathies.

Requirement of activating transcription factor 5 for murine fetal liver erythropoiesis

Activating transcription factor 5 (ATF5) is necessary for the development of various tissues, particularly under stress. Dysfunctions of ATF5 have been shown to be involved in many diseases. The exact function of ATF5 is tissue-specific, and its role in erythropoiesis is still unknown. We here employed the loss of function strategy to investigate the role of ATF5 in murine erythropoiesis. We found that knockdown of Atf5 impaired the proliferation of fetal liver erythroid progenitors. Furthermore, erythroid differentiation was inhibited by ATF5 deficiency. Our study suggests that ATF5 may be a potential therapeutic target for treating blood diseases with ineffective erythropoiesis.

Pharmacophore Modeling, Synthesis, Scaffold Hopping and Biological β- Hematin Inhibition Interaction Studies for Anti-malaria Compounds

Backgound:

Exploring potent compounds is critical to generating multi-target drug discovery. Hematin crystallization is an important mechanism of malaria.

Methods:

A series of chloroquine analogues were designed using a repositioning approach to develop new anticancer compounds. Protein-ligand interaction fingerprints and ADMET descriptors were used to assess docking performance in virtual screenings to design chloroquine hybrid β-hematin inhibitors. A PLS algorithm was applied to correlate the molecular descriptors to IC50 values. The modeling presented excellent predictive power with correlation coefficients for calibration and cross-validation of r2 = 0.93 and q2 = 0.72. Using the model, a series of 4-aminoquinlin hybrids were synthesized and evaluated for their biological activity as an external test series. These compounds were evaluated for cytotoxic cell lines and β-hematin inhibition.

Results:

The target compounds exhibited high β-hematin inhibition activity and were 3-9 times more active than the positive control. Furthermore, all the compounds exhibited moderate to high cytotoxic activity. The most potent compound in the dataset was docked with hemoglobin and its pharmacophore features were generated. These features were used as input to the Pharmit server for screening of six databases.

Conclusion:

The compound with the best score from ChEMBL was 2016904, previously reported as a VEGFR-2 inhibitor. The 11 compounds selected presented the best Gold scores with drug-like properties and can be used for drug development.

New activators of eIF2α Kinase Heme-Regulated Inhibitor (HRI) with improved biophysical properties

Heme-regulated inhibitor (HRI), a eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, is critically important for coupling protein synthesis to heme availability in reticulocytes and adaptation to various environmental stressors in all cells. HRI modifies the severity of several hemoglobin misfolding disorders including β-thalassemia. Small molecule activators of HRI are essential for studying normal- and patho-biology of this kinase as well as for the treatment of various human disorders for which activation of HRI or phosphorylation of eIF2α may be beneficial. We previously reported development of 1-((1,4-trans)-4-aryloxycyclohexyl)-3-arylureas (cHAUs) as specific HRI activators and demonstrated their potential as molecular probes for studying HRI biology and as lead compounds for treatment of various human disorders. To develop more druglike cHAUs for in vivo studies and drug development and to expand the chemical space, we undertook bioassay guided structure-activity relationship studies replacing cyclohexyl ring with various 4-6-membered rings and explored further substitutions on the N-phenyl ring. We tested all analogs in the surrogate eIF2α phosphorylation and cell proliferation assays, and a subset of analogs in secondary mechanistic assays that included endogenous eIF2α phosphorylation and expression of C/EBP homologous protein (CHOP), a downstream effector. Finally, we determined specificity of these compounds for HRI by testing their anti-proliferative activity in cells transfected with siRNA targeting HRI or mock. These compounds have significantly improved cLogPs with no loss of potencies, making them excellent candidates for lead optimization for development of investigational new drugs that potently and specifically activate HRI.

Bitopertin, a selective oral GLYT1 inhibitor, improves anemia in a mouse model of β-thalassemia

Anemia of β-thalassemia is caused by ineffective erythropoiesis and reduced red cell survival. Several lines of evidence indicate that iron/heme restriction is a potential therapeutic strategy for the disease. Glycine is a key initial substrate for heme and globin synthesis. We provide evidence that bitopertin, a glycine transport inhibitor administered orally, improves anemia, reduces hemolysis, diminishes ineffective erythropoiesis, and increases red cell survival in a mouse model of β-thalassemia (Hbbth3/+ mice). Bitopertin ameliorates erythroid oxidant damage, as indicated by a reduction in membrane-associated free α-globin chain aggregates, in reactive oxygen species cellular content, in membrane-bound hemichromes, and in heme-regulated inhibitor activation and eIF2α phosphorylation. The improvement of β-thalassemic ineffective erythropoiesis is associated with diminished mTOR activation and Rab5, Lamp1, and p62 accumulation, indicating an improved autophagy. Bitopertin also upregulates liver hepcidin and diminishes liver iron overload. The hematologic improvements achieved by bitopertin are blunted by the concomitant administration of the iron chelator deferiprone, suggesting that an excessive restriction of iron availability might negate the beneficial effects of bitopertin. These data provide important and clinically relevant insights into glycine restriction and reduced heme synthesis strategies for the treatment of β-thalassemia.

Strong correlation of ferrochelatase enzymatic activity with Mitoferrin-1 mRNA in lymphoblasts of patients with protoporphyria

Accumulation of protoporphyrin IX (PPIX) and Zn-PPIX, are the clinical hallmarks of protoporphyria. Phenotypic expression of protoporphyria is due to decreased activity of ferrochelatase (FECH) or to increased activity of aminolevulinic acid synthase (ALAS) in red blood cells. Other genetic defects have been shown to contribute to disease severity including loss of function mutations in the mitochondrial AAA-ATPase, CLPX and mutations in the Iron-responsive element binding protein 2 (IRP2), in mice. It is clear that multiple paths lead to a common phenotype of excess plasma PPIX that causes a phototoxic reaction on sun exposed areas. In this study we examined the association between mitochondrial iron acquisition and utilization with activity of FECH. Our data show that there is a metabolic link between the activity FECH and levels of MFRN1 mRNA. We examined the correlation between FECH activity and MFRN1 mRNA in cell lines established from patients with the classical protoporphyria, porphyria due to defects in ALAS2 mutations. Our data confirm MFRN1 message levels positively correlated with FECH enzymatic activity in all cell types.

Murine models of the human porphyrias: Contributions toward understanding disease pathogenesis and the development of new therapies

Mouse models of the human porphyrias have proven useful for investigations of disease pathogenesis and to facilitate the development of new therapeutic approaches. To date, mouse models have been generated for all major porphyrias, with the exception of X-linked protoporphyria (XLP) and the ultra rare 5-aminolevulinic acid dehydratase deficient porphyria (ADP). Mouse models have been generated for the three autosomal dominant acute hepatic porphyrias, acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), and variegate porphyria (VP). The AIP mice, in particular, provide a useful investigative model as they have been shown to have acute biochemical attacks when induced with the prototypic porphyrinogenic drug, phenobarbital. In addition to providing important insights into the disease pathogenesis of the neurological impairment in AIP, these mice have been valuable for preclinical evaluation of liver-targeted gene therapy and RNAi-mediated approaches. Mice with severe HMBS deficiency, which clinically and biochemically mimic the early-onset homozygous dominant AIP (HD-AIP) patients, have been generated and were used to elucidate the striking phenotypic differences between AIP and HD-AIP. Mice modeling the hepatocutaneous porphyria, porphyria cutanea tarda (PCT), made possible the identification of the iron-dependent inhibitory mechanism of uroporphyrinogen decarboxylase (UROD) that leads to symptomatic PCT. Mouse models for the two autosomal recessive erythropoietic porphyrias, congenital erythropoietic porphyria (CEP) and erythropoeitic protoporphyria (EPP), recapitulate many of the clinical and biochemical features of the severe human diseases and have been particularly useful for evaluation of bone marrow transplantation and hematopoietic stem cell (HSC)-based gene therapy approaches. The EPP mice have also provided valuable insights into the underlying pathogenesis of EPP-induced liver damage and anemia.

Oxidative Stress in β-Thalassemia

Cell oxidative status, which represents the balance between oxidants and antioxidants, is involved in normal functions. Under pathological conditions, there is a shift toward the oxidants, leading to oxidative stress, which is cytotoxic, causing oxidation of cellular components that result in cell death and organ damage. Thalassemia is a hereditary hemolytic anemia caused by mutations in globin genes that cause reduced or complete absence of specific globin chains (commonly, α or β). Although oxidative stress is not the primary etiology of thalassemia, it mediates several of its pathologies. The main causes of oxidative stress in thalassemia are the degradation of the unstable hemoglobin and iron overload-both stimulate the production of excess free radicals. The symptoms aggravated by oxidative stress include increased hemolysis, ineffective erythropoiesis and functional failure of vital organs such as the heart and liver. The oxidative status of each patient is affected by multiple internal and external factors, including genetic makeup, health conditions, nutrition, physical activity, age, and the environment (e.g., air pollution, radiation). In addition, oxidative stress is influenced by the clinical manifestations of the disease (unpaired globin chains, iron overload, anemia, etc.). Application of personalized (theranostics) medicine principles, including diagnostic tests for selecting targeted therapy, is therefore important for optimal treatment of the oxidative stress of these patients. We summarize the role of oxidative stress and the current and potential antioxidative therapeutics in β-thalassemia and describe some methodologies, mostly cellular, that might be helpful for application of a theranostics approach to therapy.

Translational regulation and deregulation in erythropoiesis

Translational regulation plays a critical role in erythropoiesis, as it reflects the translational needs of enucleated mature erythroid cells in the absence of transcription and the large translational demands of balanced globin chain synthesis during erythroid maturation. In addition, red blood cells need to respond quickly to changes in their environment and the demands of the organism. Translational regulation occurs at several levels in erythroid cells, including the differential utilization of upstream open reading frames during differentiation and in response to signaling and the employment of RNA-binding proteins in an erythroid cell-specific fashion. Translation initiation is a critical juncture for translational regulation in response to environmental signals such as heme and iron availability, whereas regulatory mechanisms for ribosome recycling are consistent with recent observations highlighting the importance of maintaining adequate ribosome levels in differentiating erythroid cells. Translational deregulation in erythroid cells leads to disease associated with ineffective erythropoiesis, further highlighting the pivotal role translational regulation in erythropoiesis plays in human physiology and homeostasis. Overall, erythropoiesis has served as a unique model that has provided invaluable insight into translational regulation.

RNA Binding Proteins and Regulation of mRNA Translation in Erythropoiesis

Control of gene expression in erythropoiesis has to respond to signals that may emerge from intracellular processes or environmental factors. Control of mRNA translation allows for relatively rapid modulation of protein synthesis from the existing transcriptome. For instance, the protein synthesis rate needs to be reduced when reactive oxygen species or unfolded proteins accumulate in the cells, but also when iron supply is low or when growth factors are lacking in the environment. In addition, regulation of mRNA translation can be important as an additional layer of control on top of gene transcription, in which RNA binding proteins (RBPs) can modify translation of a set of transcripts to the cell’s actual protein requirement. The 5′ and 3′ untranslated regions of mRNA (5′UTR, 3′UTR) contain binding sites for general and sequence specific translation factors. They also contain secondary structures that may hamper scanning of the 5′UTR by translation complexes or may help to recruit translation factors. In addition, the term 5′UTR is not fully correct because many transcripts contain small open reading frames in their 5′UTR that are translated and contribute to regulation of mRNA translation. It is becoming increasingly clear that the transcriptome only partly predicts the proteome. The aim of this review is (i) to summarize how the availability of general translation initiation factors can selectively regulate transcripts because the 5′UTR contains secondary structures or short translated sequences, (ii) to discuss mechanisms that control the length of the mRNA poly(A) tail in relation to mRNA translation, and (iii) to give examples of sequence specific RBPs and their targets. We focused on transcripts and RBPs required for erythropoiesis. Whereas differentiation of erythroblasts to erythrocytes is orchestrated by erythroid transcription factors, the production of erythrocytes needs to respond to the availability of growth factors and nutrients, particularly the availability of iron.

Polymorphism studies on microRNA targetome of thalassemia

Thalassemia is one of the most prevalent hemoglobin disorders. It is caused by the decreased or absent synthesis of one globin chain that leads to moderate to severe hemolytic anemia in clinical complications. Some genetic factors cause these phenotypic variations by the alteration of gene expression. MicroRNAs (miRNAs) are post-transcriptional regulators in gene expression. Therefore, variations in 3'-untranslated region (3'-UTR) of target genes may affect gene expression. It is of interest to evaluate the impact of noncoding SNPs in thalassemia related genes on miRNA: mRNA interactions in the severity of thalassemia. Polymorphisms that alter miRNA: mRNA interactions were predicted using PolymiRTS and Mirsnpscore tools. Then, the effect of predicted target SNPs on thermodynamic stability, local RNA structure and regulatory elements was investigated using RNAhybrid, RNAsnp and RegulomeDB, respectively. The molecular functions and the Biological process of candidate genes were extracted and interaction network was created. Forty-six SNPs were predicted to affect 188 miRNA interactions. These results suggest that 3'-UTR SNP may affect gene expression and cause phenotypic variation in thalassemia patients.

Experimental Methods for Studying Cellular Heme Signaling

The study of heme is important to our understanding of cellular bioenergetics, especially in cancer cells. The function of heme as a prosthetic group in proteins such as cytochromes is now well-documented. Less is known, however, about its role as a regulator of metabolic and energetic pathways. This is due in part to some inherent difficulties in studying heme. Due to its slightly amphiphilic nature, heme is a "sticky" molecule which can easily bind non-specifically to proteins. In addition, heme tends to dimerize, oxidize, and aggregate in purely aqueous solutions; therefore, there are constraints on buffer composition and concentrations. Despite these difficulties, our knowledge of heme's regulatory role continues to grow. This review sums up the latest methods used to study reversible heme binding. Heme-regulated proteins will also be reviewed, as well as a system for imaging the cellular localization of heme.

What can we learn from ineffective erythropoiesis in thalassemia?

Erythropoiesis is a dynamic process regulated at multiple levels to balance proliferation, differentiation and survival of erythroid progenitors. Ineffective erythropoiesis is a key feature of various diseases, including β-thalassemia. The pathogenic mechanisms leading to ineffective erythropoiesis are complex and still not fully understood. Altered survival and decreased differentiation of erythroid progenitors are both critical processes contributing to reduced production of mature red blood cells. Recent studies have identified novel important players and provided major advances in the development of targeted therapeutic approaches. In this review, β-thalassemia is used as a paradigmatic example to describe our current knowledge on the mechanisms leading to ineffective erythropoiesis and novel treatments that may have the potential to improve the clinical phenotype of associated diseases in the future.

Pathophysiology and treatment of patients with beta-thalassemia - an update

Thalassemia (thal) is an autosomal recessive, hereditary, chronic hemolytic anemia due to a partial or complete deficiency in the synthesis of α-globin chains (α-thal) or β-globin chains (β-thal) that compose the major adult hemoglobin (α ₂β _2). It is caused by one or more mutations in the corresponding genes. The unpaired globin chains are unstable; they precipitate intracellularly, resulting in hemolysis, premature destruction of red blood cell [RBC] precursors in the bone marrow, and a short life-span of mature RBCs in the circulation. The state of anemia is treated by frequent RBC transfusions. This therapy results in the accumulation of iron (iron overload), a condition that is exacerbated by the breakdown products of hemoglobin (heme and iron) and the increased iron uptake for the chronic accelerated, but ineffective, RBC production. Iron catalyzes the generation of reactive oxygen species, which in excess are toxic, causing damage to vital organs such as the heart and liver and the endocrine system. Herein, we review recent findings regarding the pathophysiology underlying the major symptoms of β-thal and potential therapeutic modalities for the amelioration of its complications, as well as new modalities that may provide a cure for the disease.

HRI coordinates translation by eIF2αP and mTORC1 to mitigate ineffective erythropoiesis in mice during iron deficiency

Iron deficiency (ID) anemia is a prevalent disease, yet molecular mechanisms by which iron and heme regulate erythropoiesis are not completely understood. Heme-regulated eIF2α kinase (HRI) is a key hemoprotein in erythroid precursors that sense intracellular heme concentrations to balance globin synthesis with the amount of heme available for hemoglobin production. HRI is activated by heme deficiency and oxidative stress, and it phosphorylates eIF2α (eIF2αP), which inhibits the translation of globin messenger RNAs (mRNAs) and selectively enhances the translation of activating transcription factor 4 (ATF4) mRNA to induce stress response genes. Here, we generated a novel mouse model (eAA) with the erythroid-specific ablation of eIF2αP and demonstrated that eIF2αP is required for induction of ATF4 protein synthesis in vivo in erythroid cells during ID. We show for the first time that both eIF2αP and ATF4 are necessary to promote erythroid differentiation and to reduce oxidative stress in vivo during ID. Furthermore, the HRI-eIF2αP-ATF4 pathway suppresses mTORC1 signaling specifically in the erythroid lineage. Pharmacologic inhibition of mTORC1 significantly increased red blood cell counts and hemoglobin content in the blood, improved erythroid differentiation, and reduced splenomegaly of iron-deficient Hri^-/- and eAA mice. However, globin inclusions and elevated oxidative stress remained, demonstrating the essential nonredundant role of HRI-eIF2αP in these processes. Dietary iron repletion completely reversed ID anemia and ineffective erythropoiesis of Hri^-/- , eAA, and Atf4^-/- mice by inhibiting both HRI and mTORC1 signaling. Thus, HRI coordinates 2 key translation-regulation pathways, eIF2αP and mTORC1, to circumvent ineffective erythropoiesis, highlighting heme and translation in the regulation of erythropoiesis.

The eIF2-alpha kinase HRI: a potential target beyond the red blood cell

INTRODUCTION

The eIF2α kinase heme-regulated inhibitor (HRI) is one of four well-described kinases that phosphorylate eIF2α in response to various cell stressors, resulting in reduced ternary complex formation and attenuation of mRNA translation. Although HRI is well known for its role as a heme sensor in erythroid progenitors, pharmacologic activation of HRI has been demonstrated to have anti-cancer activity across a wide range of tumor sub-types. Here, the potential of HRI activators as novel cancer therapeutics is explored. Areas covered: We provide an introduction to eIF2 signaling pathways in general, and specifically review data on the eIF2α kinase HRI in erythroid and non-erythroid cells. We review aspects of targeting eIF2 signaling in cancer and highlight promising data using HRI activators against tumor cells. Expert opinion: Pharmacologic activation of HRI inhibits tumor growth as a single agent without appreciable toxicity in vivo. The ability of HRI activators to provide direct and sustained eIF2α phosphorylation without inducing oxidative stress or broad eIF2α kinase activation may be especially advantageous for tolerability. Combination therapy with established therapeutics may further augment anti-cancer activity to overcome disease resistance.

Modulation of mitochondrial dysfunction and endoplasmic reticulum stress are key mechanisms for the wide-ranging actions of epoxy fatty acids and soluble epoxide hydrolase inhibitors

The arachidonic acid cascade is arguably the most widely known biologic regulatory pathway. Decades after the seminal discoveries involving its cyclooxygenase and lipoxygenase branches, studies of this cascade remain an active area of research. The third and less widely known branch, the cytochrome P450 pathway leads to highly active oxygenated lipid mediators, epoxy fatty acids (EpFAs) and hydroxyeicosatetraenoic acids (HETEs), which are of similar potency to prostanoids and leukotrienes. Unlike the COX and LOX branches, no pharmaceuticals currently are marketed targeting the P450 branch. However, data support therapeutic benefits from modulating these regulatory lipid mediators. This is being approached by stabilizing or mimicking the EpFAs or even by altering the diet. These approaches lead to predominantly beneficial effects on a wide range of apparently unrelated states resulting in an enigma of how this small group of natural chemical mediators can have such diverse effects. EpFAs are degraded by soluble epoxide hydrolase (sEH) and stabilized by inhibiting this enzyme. In this review, we focus on interconnected aspects of reported mechanisms of action of EpFAs and inhibitors of soluble epoxide hydrolase (sEHI). The sEHI and EpFAs are commonly reported to maintain homeostasis under pathological conditions while remaining neutral under normal physiological conditions. Here we provide a conceptual framework for the unique and broad range of biological activities ascribed to epoxy fatty acids. We argue that their mechanism of action pivots on their ability to prevent mitochondrial dysfunction, to reduce subsequent ROS formation and to block resulting cellular signaling cascades, primarily the endoplasmic reticulum stress. By stabilizing the mitochondrial - ROS - ER stress axis, the range of activity of EpFAs and sEHI display an overlap with the disease conditions including diabetes, fibrosis, chronic pain, cardiovascular and neurodegenerative diseases, for which the above outlined mechanisms play key roles.

Development of 1-((1,4-trans)-4-Aryloxycyclohexyl)-3-arylurea Activators of Heme-Regulated Inhibitor as Selective Activators of the Eukaryotic Initiation Factor 2 Alpha (eIF2α) Phosphorylation Arm of the Integrated Endoplasmic Reticulum Stress Response

Heme-regulated inhibitor (HRI), an eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, plays critical roles in cell proliferation, differentiation, adaptation to stress, and hemoglobin disorders. HRI phosphorylates eIF2α, which couples cellular signals, including endoplasmic reticulum (ER) stress, to translation. We previously identified 1,3-diarylureas and 1-((1,4-trans)-4-aryloxycyclohexyl)-3-arylureas (cHAUs) as specific activators of HRI that trigger the eIF2α phosphorylation arm of ER stress response as molecular probes for studying HRI biology and its potential as a druggable target. To develop drug-like cHAUs needed for in vivo studies, we undertook bioassay-guided structure-activity relationship studies and tested them in the surrogate eIF2α phosphorylation and cell proliferation assays. We further evaluated some of these cHAUs in endogenous eIF2α phosphorylation and in the expression of the transcription factor C/EBP homologous protein (CHOP) and its mRNA, demonstrating significantly improved solubility and/or potencies. These cHAUs are excellent candidates for lead optimization for development of investigational new drugs that potently and specifically activate HRI.

Disturbed iron metabolism in erythropoietic protoporphyria and association of GDF15 and gender with disease severity

Patients with erythropoietic protoporphyria (EPP) have reduced activity of the enzyme ferrochelatase that catalyzes the insertion of iron into protoporphyrin IX (PPIX) to form heme. As the result of ferrochelatase deficiency, PPIX accumulates and causes severe photosensitivity. Among different patients, the concentration of PPIX varies considerably. In addition to photosensitivity, patients frequently exhibit low serum iron and a microcytic hypochromic anemia. The aims of this study were to (1) search for factors related to PPIX concentration in EPP, and (2) characterize anemia in EPP, i.e., whether it is the result of an absolute iron deficiency or the anemia of chronic disease (ACD). Blood samples from 67 EPP patients (51 Italian and 16 Swiss) and 21 healthy volunteers were analyzed. EPP patients had lower ferritin (p = 0.021) and hepcidin (p = 0.031) concentrations and higher zinc-protoporphyrin (p < 0.0001) and soluble-transferrin-receptor (p = 0.0007) concentrations compared with controls. This indicated that anemia in EPP resulted from an absolute iron deficiency. Among EPP patients, PPIX concentrations correlated with both growth differentiation factor (GDF) 15 (p = 0.012) and male gender (p = 0.015). Among a subgroup of patients who were iron replete, hemoglobin levels were normal, which suggested that iron but not ferrochelatase is the limiting factor in heme synthesis of individuals with EPP.

2017 Clinical trials update in new treatments of β-thalassemia

The underlying basis of β-thalassemia pathology is the diminished β-globin synthesis leading to α-globin accumulation and premature apoptotic destruction of erythroblasts, causing oxidative stress-induced ineffective erythropoiesis, bone marrow hyperplasia, splenomegaly, and increased intestinal iron absorption with progressive iron overload. Better understanding of the molecular mechanisms underlying this disease led to the recognition of new targets with potential therapeutic utility. Agents such as JAK2 inhibitors and TGF-β ligand traps that reduce the ineffective erythropoiesis process are already being tested in clinical trials with promising results. Other agents that aim to reduce oxidative stress (activators of Foxo3, HRI-eIF2aP, Prx2, Hsp70, and PK anti-oxidant systems and inhibitors of HO-1) and to decrease iron overload (hepcidin agonists, erythroferrone inhibitors and exogenous transferrin) are also under experimental investigation. Significant progress has also been made in the area of allogeneic hematopoietic stem cell transplantation with several ongoing clinical trials examining new condition regimens as well as different donor selection and stem cell source options. Gene therapy has reached a critical point and phase 1 clinical trials have recently been launched to examine the effectiveness and especially long term safety. Epigenetic manipulation and genomic editing of the γ- or β-globin gene are novel and promising experimental gene therapy approaches for β-thalassemia giving hope for cure for this chronic disease. This review outlines the key points of the molecular mechanisms underlying β-thalassemia in relation to the development of new therapies and an update is given both at the pre-clinical and clinical level. Am. J. Hematol. 91:1135-1145, 2016. © 2016 Wiley Periodicals, Inc.

Biology of Heme in Mammalian Erythroid Cells and Related Disorders

Heme is a prosthetic group comprising ferrous iron (Fe(2+)) and protoporphyrin IX and is an essential cofactor in various biological processes such as oxygen transport (hemoglobin) and storage (myoglobin) and electron transfer (respiratory cytochromes) in addition to its role as a structural component of hemoproteins. Heme biosynthesis is induced during erythroid differentiation and is coordinated with the expression of genes involved in globin formation and iron acquisition/transport. However, erythroid and nonerythroid cells exhibit distinct differences in the heme biosynthetic pathway regulation. Defects of heme biosynthesis in developing erythroblasts can have profound medical implications, as represented by sideroblastic anemia. This review will focus on the biology of heme in mammalian erythroid cells, including the heme biosynthetic pathway as well as the regulatory role of heme and human disorders that arise from defective heme synthesis.

Oxidative stress in β-thalassaemia and sickle cell disease

Sickle cell disease and β-thalassaemia are inherited haemoglobinopathies resulting in structural and quantitative changes in the β-globin chain. These changes lead to instability of the generated haemoglobin or to globin chain imbalance, which in turn impact the oxidative environment both intracellularly and extracellularly. The ensuing oxidative stress and the inability of the body to adequately overcome it are, to a large extent, responsible for the pathophysiology of these diseases. This article provides an overview of the main players and control mechanisms involved in the establishment of oxidative stress in these haemoglobinopathies.

Heme and erythropoieis: more than a structural role

Erythropoiesis is the biological process that consumes the highest amount of body iron for heme synthesis. Heme synthesis in erythroid cells is finely coordinated with that of alpha (α) and beta (β)-globin, resulting in the production of hemoglobin, a tetramer of 2α- and 2β-globin chains, and heme as the prosthetic group. Heme is not only the structural component of hemoglobin, but it plays multiple regulatory roles during the differentiation of erythroid precursors since it controls its own synthesis and regulates the expression of several erythroid-specific genes. Heme is synthesized in developing erythroid progenitors by the stage of proerythroblast, through a series of eight enzymatic reactions divided between mitochondria and cytosol. Defects of heme synthesis in the erythroid lineage result in sideroblastic anemias, characterized by microcytic anemia associated to mitochondrial iron overload, or in erythropoietic porphyrias, characterized by porphyrin deposition in erythroid cells. Here, we focus on the heme biosynthetic pathway and on human erythroid disorders due to defective heme synthesis. The regulatory role of heme during erythroid differentiation is discussed as well as the heme-mediated regulatory mechanisms that allow the orchestration of the adaptive cell response to heme deficiency.

Genetic determinants of β-thalassemia intermedia in Pakistan

This study covers the molecular characterization of clinically diagnosed β-thalassemia intermedia (β-TI) patients in Pakistan. Blood samples of β-TI patients were collected from all four provinces of Pakistan throughout the period of 2011-2013. The study was carried out using allele-specific primers through polymerase chain reaction or sequencing to determine both α- and β-thalassemia (α- and β-thal) mutations, and restriction enzymes for the characterization of β-globin gene arrangements. In a total of 63 patients, the IVS-I-5 (G > C) was the most frequent mutation (33.88%). The codon 30 (G > A) and IVS-II-1 (T > C) mutations were found only in the Punjabi ethnic group, while the codon 30 (G > C) and Hb S (HBB: c.20A > T) mutations were found only in the Pashtoon and Sindhi ethnic groups, respectively. In case of α-globin genotypes, 44 patients were normal (αα/αα), six patients carried the αα/-α(3.7) genotype, 12 patients carried the -α(3.7)/-α(3.7) genotype, while one patient had the αα/ααα(anti 3.7) genotype. We found that haplotype I was the most frequent, mostly associated with the codons 8/9 (+G) mutation, while the Saudi haplotype was found only with Hb S.

Translational control by heme-regulated eIF2α kinase during erythropoiesis

PURPOSE OF REVIEW

This review will provide an overview of the translational regulation of globin mRNAs and integrated stress response (ISR) during erythropoiesis by heme-regulated eIF2α kinase (HRI). HRI is an intracellular heme sensor that coordinates heme and globin synthesis in erythropoiesis by inhibiting protein synthesis of globins and heme biosynthetic enzymes during heme deficiency.

RECENT FINDINGS

It has been demonstrated recently that HRI also activates the eIF2αP-activating transcription factor 4 (ATF4) ISR in primary erythroid precursors to combat oxidative stress. During chronic iron/heme deficiency in vivo, this HRI-eIF2αP-ATF4 signaling is necessary both to reduce oxidative stress and to promote erythroid differentiation. Augmenting eIF2αP signaling by the small molecule salubrinal, which inhibits dephosphorylation of eIF2αP, reduces excess α-globin synthesis and enhances translation of ATF4 mRNA in mouse β-thalassemic erythroid precursors. Intriguingly, salubrinal treatment of differentiating human CD34⁺ cells in culture increases fetal hemoglobin production with a concomitant decrease of adult hemoglobin by a posttranscriptional mechanism.

SUMMARY

HRI-eIF2αP-ATF4 stress signaling is important not only to inhibit excess globin synthesis during erythropoiesis, but is also critical for adaptation to oxidative stress and for enhancing effective erythropoiesis. Modulation of this signaling pathway with small chemicals may provide a novel therapy for hemoglobinopathy.

[Role of HRI in apoptosis resistance]

When exposed to environmental stresses, cells activate defence mechanisms to adapt stress and inhibit apoptotic pathways leading to their survival. Stressed cells also reduce their general metabolism in part by inhibiting mRNA translation, thereby saving energy needed to repair stress-induced damages. Under stress conditions, the inhibition of mRNA translation occurs mainly at its initiation step through the phosphorylation of the translation initiation factor eIF2α. One of the four kinases known to phosphorylate eIF2α is heme-regulated inhibitor (HRI). The activation of HRI occurs under conditions of heme deficiency, oxidative stress and treatment with anti-cancer drugs such as proteasome inhibitors. In this article, we discuss the role of HRI in promoting cell resistance to stress-mediated apoptosis.

Stimulators of translation identified during a small molecule screening campaign

In screening a library of natural and synthetic products for eukaryotic translation modulators, we identified two natural products, isohymenialdisine and hymenialdisine, that exhibit stimulatory effects on translation. The characterization of these compounds led to the insight that mRNA used to program the translation extracts during high-throughput assay setup was leading to phosphorylation of eIF2α, a potent negative regulatory event that is mediated by one of four kinases. We identified double-stranded RNA-dependent protein kinase (PKR) as the eIF2α kinase that was being activated by exogenously added mRNA template. Characterization of the mode of action of isohymenialdisine revealed that it directly acts on PKR by inhibiting autophosphorylation, perturbs the PKR-eIF2α phosphorylation axis, and can be modeled into the PKR ATP binding site. Our results identify a source of "false positives" for high-throughput screen campaigns using translation extracts, raising a cautionary note for this type of screen.

KLF1 mutations are relatively more common in a thalassemia endemic region and ameliorate the severity of β-thalassemia

Mutations in human Krüppel-like factor 1 (KLF1) have recently been reported to be responsible for increased fetal hemoglobin (HbF) and hemoglobin A2 (HbA2). Because increased HbF and HbA2 levels are important features of β-thalassemia, we examined whether there is any relationship between KLF1 mutation and β-thalassemia in China. To do this, we first studied the incidence of KLF1 mutations in 2 Chinese populations: 3839 individuals from a thalassemia endemic region in south China and 1190 individuals from a non-thalassemia endemic region in north China. Interestingly, we found that the prevalence of KLF1 mutations is significantly higher in the thalassemia endemic region than that in non-thalassemia endemic region (1.25% vs 0.08%). Furthermore, we identified 7 functional variants including 4 previously reported (p.Gly176AlafsX179, p.Ala298Pro, p.Thr334Arg, and c.913+1G>A) and 3 novel variants (p.His299Asp, p.Cys341Tyr, and p.Glu5Lys) in southern China. The 2 most common mutations, p.Gly176AlafsX179 and p.His299Asp, accounted for 90.6% of the total. We found that zinc-finger mutations in KLF1 were selectively represented in 12 β-thalassemia intermedia patients and resulted in significantly different transfusion-free survival curves. Our findings suggest that KLF1 mutations occur selectively in the presence of β-thalassemia to increase the production of HbF, which in turn ameliorates the clinical severity of β-thalassemia.

Modified activin receptor IIB ligand trap mitigates ineffective erythropoiesis and disease complications in murine β-thalassemia

In β-thalassemia, unequal production of α- and β-globin chains in erythroid precursors causes apoptosis and inhibition of late-stage erythroid differentiation, leading to anemia, ineffective erythropoiesis (IE), and dysregulated iron homeostasis. Here we used a murine model of β-thalassemia intermedia (Hbb(th1/th1) mice) to investigate effects of a modified activin receptor type IIB (ActRIIB) ligand trap (RAP-536) that inhibits Smad2/3 signaling. In Hbb(th1/th1) mice, treatment with RAP-536 reduced overactivation of Smad2/3 in splenic erythroid precursors. In addition, treatment of Hbb(th1/th1) mice with RAP-536 reduced α-globin aggregates in peripheral red cells, decreased the elevated reactive oxygen species present in erythroid precursors and peripheral red cells, and alleviated anemia by promoting differentiation of late-stage erythroid precursors and reducing hemolysis. Notably, RAP-536 treatment mitigated disease complications of IE, including iron overload, splenomegaly, and bone pathology, while reducing erythropoietin levels, improving erythrocyte morphology, and extending erythrocyte life span. These results implicate signaling by the transforming growth factor-β superfamily in late-stage erythropoiesis and reveal potential of a modified ActRIIB ligand trap as a novel therapeutic agent for thalassemia syndrome and other red cell disorders characterized by IE.

Abnormal erythropoiesis and the pathophysiology of chronic anemia

Erythropoiesis, the bone marrow production of erythrocytes by the proliferation and differentiation of hematopoietic cells, replaces the daily loss of 1% of circulating erythrocytes that are senescent. This daily output increases dramatically with hemolysis or hemorrhage. When erythrocyte production rate of erythrocytes is less than the rate of loss, chronic anemia develops. Normal erythropoiesis and specific abnormalities of erythropoiesis that cause chronic anemia are considered during three periods of differentiation: a) multilineage and pre-erythropoietin-dependent hematopoietic progenitors, b) erythropoietin-dependent progenitor cells, and c) terminally differentiating erythroblasts. These erythropoietic abnormalities are discussed in terms of their pathophysiological effects on the bone marrow cells and the resultant changes that can be detected in the peripheral blood using a clinical laboratory test, the complete blood count.

Modulation of hepcidin as therapy for primary and secondary iron overload disorders: preclinical models and approaches

In this article, the authors discuss new approaches to treating iron overload diseases using hepcidin mimetics or by modulating endogenous hepcidin expression. In particular, the authors discuss lipid nanoparticle encapsulated siRNA and antisense oligonucleotide-mediated inhibition of TMPRSS6, an upstream regulator of hepcidin, and treatment with transferrin or hepcidin mimetics, including the recently described minihepcidins. In each case, in animal models of β-thalassemia, not only do the interventions affect iron absorption but they also act as disease-modifying agents that ameliorate the ineffective erythropoiesis.

Explorations of substituted urea functionality for the discovery of new activators of the heme-regulated inhibitor kinase

Heme-regulated inhibitor kinase (HRI), a eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, plays critical roles in cell proliferation, differentiation, and adaptation to cytoplasmic stress. HRI is also a critical modifier of hemoglobin disorders such as β-thalassemia. We previously identified N,N'-diarylureas as potent activators of HRI suitable for studying the biology of this important kinase. To expand the repertoire of chemotypes that activate HRI, we screened a ∼1900 member N,N'-disubstituted urea library in the surrogate eIF2α phosphorylation assay, identifying N-aryl,N'-cyclohexylphenoxyurea as a promising scaffold. We validated hit compounds as a bona fide HRI activators in secondary assays and explored the contributions of substitutions on the N-aryl and N'-cyclohexylphenoxy groups to their activity by studying focused libraries of complementing analogues. We tested these N-aryl,N'-cyclohexylphenoxyureas in the surrogate eIF2α phosphorylation and cell proliferation assays, demonstrating significantly improved bioactivities and specificities. We consider these compounds to represent lead candidates for the development of potent and specific HRI activators.

A protective role of heme-regulated eIF2α kinase in cadmium-induced toxicity in erythroid cells

Although a number of studies have demonstrated that cadmium (Cd) can incur damage to mature red cells, the potential injuries of Cd to erythroid progenitor cells have not been investigated thus far. Heme-regulated eIF2α kinase (Hri) is essential for translational regulation and survival of erythroid precursors in the setting of iron deficiency. Hri has been demonstrated to activate Atf4 signaling in reducing oxidative stress and in promoting erythroid differentiation during stress erythropoiesis. Here, we demonstrated that Cd significantly provoked cell death and suppressed erythroid differentiation of erythroid progenitor cells. Importantly, our results established a crucial role of Hri in ameliorating Cd-induced impairment to erythropoiesis. Upon Cd treatment, Hri-eIF2αP-Atf4 signaling was activated to protect cells from cell death and differentiation attenuation in Wt fetal liver erythroblasts; in contrast, Hri(-/-) erythroblasts suffered from enhanced oxidative stress, as evidenced by increased levels of reactive oxygen species (ROS) and consequentially elevated apoptosis. As for Cd administration in vivo, impaired erythropoiesis in bone marrow and dramatic extramedullary erythropoiesis in spleen were observed in Hri(-/-) mice. Taken together, our combined data highlighted a crucial role of Hri in protecting survival and differentiation of erythroid progenitor cells upon Cd treatment.

The eIF2α kinases: their structures and functions

Cell signaling in response to an array of diverse stress stimuli converges on the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2). Phosphorylation of eIF2α on serine 51 results in a severe decline in de novo protein synthesis and is an important strategy in the cell's armory against stressful insults including viral infection, the accumulation of misfolded proteins, and starvation. The phosphorylation of eIF2α is carried out by a family of four kinases, PERK (PKR-like ER kinase), PKR (protein kinase double-stranded RNA-dependent), GCN2 (general control non-derepressible-2), and HRI (heme-regulated inhibitor). Each primarily responds to a distinct type of stress or stresses. Thus, while significant sequence similarity exists between the eIF2α kinases in their kinase domains, underlying their common role in phosphorylating eIF2α, additional unique features determine the regulation of these four proteins, that is, what signals activate them. This review will describe the structure of each eIF2α kinase and discuss how this is linked to their activation and function. In parallel to the general translational attenuation elicited by eIF2α kinase activation the translation of stress-induced mRNAs, most notably activating transcription factor 4 (ATF4) is enhanced and these set in motion cascades of gene expression constituting the integrated stress response (ISR), which seek to remediate stress and restore homeostasis. Depending on the cellular context and concurrent signaling pathways active, however, translational attenuation can also facilitate apoptosis. Accordingly, the role of the kinases in determining cell fate will also be discussed.

Eukaryotic initiation factor 2α phosphorylation mediates fetal hemoglobin induction through a post-transcriptional mechanism

Strategies to increase fetal hemoglobin (HbF) levels can ameliorate symptoms and improve the lives of β-hemoglobinopathy patients. Although most studies have focused on induction of γ-globin gene expression as an approach to induce HbF, we hypothesized that post-transcriptional regulation of HbF plays an underappreciated yet important role in controlling HbF levels. In the present study, we investigated whether increasing eukaryotic initiation factor 2α (eIF2α) phosphorylation, a key regulator of protein translation, could enhance HbF post-transcriptionally in human primary erythroid cells. Initial analysis using a known inhibitor of eIF2α dephosphorylation, salubrinal, revealed that elevated eIF2α phosphorylation enhanced HbF production without changing globin gene expression, proliferation, or cell differentiation. These results were further supported by the post-transcriptional induction of HbF by other pharmacologic activators of the eIF2α pathway and by genetic inactivation of the negative regulators, GADD34 and CReP. Additionally, we found that this novel mechanism of increasing HbF could be combined with clinically relevant transcriptional activators of γ-globin gene expression to additively enhance HbF. Taken together, these findings identify eIF2α phosphorylation as a post-transcriptional regulator of HbF induction that may be pharmacologically targeted, either alone or in combination, in β-hemoglobinopathy patients.

Ineffective erythropoiesis in β -thalassemia

In humans, β-thalassemia dyserythropoiesis is characterized by expansion of early erythroid precursors and erythroid progenitors and then ineffective erythropoiesis. This ineffective erythropoiesis is defined as a suboptimal production of mature erythrocytes originating from a proliferating pool of immature erythroblasts. It is characterized by (1) accelerated erythroid differentiation, (2) maturation blockade at the polychromatophilic stage, and (3) death of erythroid precursors. Despite extensive knowledge of molecular defects causing β-thalassemia, less is known about the mechanisms responsible for ineffective erythropoiesis. In this paper, we will focus on the underlying mechanisms leading to premature death of thalassemic erythroid precursors in the bone marrow.

Heme-regulated eIF2α kinase activated Atf4 signaling pathway in oxidative stress and erythropoiesis

Heme-regulated eIF2α kinase (Hri) is necessary for balanced synthesis of heme and globin. In addition, Hri deficiency exacerbates the phenotypic severity of β-thalassemia intermedia in mice. Activation of Hri during heme deficiency and in β-thalassemia increases eIF2α phosphorylation and inhibits globin translation. Under endoplasmic reticulum stress and nutrient starvation, eIF2α phosphorylation also induces the Atf4 signaling pathway to mitigate stress. Although the function of Hri in regulating globin translation is well established, its role in Atf4 signaling in erythroid precursors is not known. Here, we report the role of the Hri-activated Atf4 signaling pathway in reducing oxidative stress and in promoting erythroid differentiation during erythropoiesis. On acute oxidative stress, Hri(-/-) erythroblasts suffered from increased levels of reactive oxygen species (ROS) and apoptosis. During chronic iron deficiency in vivo, Hri is necessary both to reduce oxidative stress and to promote erythroid differentiation. Hri(-/-) mice developed ineffective erythropoiesis during iron deficiency with inhibition of differentiation at the basophilic erythroblast stage. This inhibition is recapitulated during ex vivo differentiation of Hri(-/-) fetal liver erythroid progenitors. Importantly, the Hri-eIF2αP-Atf4 pathway was activated and required for erythroid differentiation. We further demonstrate the potential of modulating Hri-eIF2αP-Atf4 signaling with chemical compounds as pharmaceutical therapies for β-thalassemia.