Regulation of erythroid 5-aminolevulinate synthase expression during erythropoiesis

Brain tissue haemoglobin expression in saline-perfused vs non-perfused rodents

Haemoglobin beta (Hbb) and delta-aminolevulinate synthase 2 (Alas2) messenger RNA (mRNA) is mainly found in immature red blood cells, reticulocytes, and not in mature erythrocytes. However, these are also expressed in other tissues such as brain cells, mostly neurons. Therefore, exact quantification of neural tissue homogenates may be confounded by remaining blood in the brain vasculature that may give falsely high values of Hbb/Alas2 expression. To investigate and compare the contribution of local Hbb/Alas2 expression, we investigated mRNA expression locally in the hippocampus and prefrontal cortex, in post-sacrifice saline-perfused and non-perfused mice and rats. Although there was a higher level of Hbb/Alas2 transcripts in the non-perfused animals, there was a significant mRNA expression in perfused brains that could at most partially be explained by remaining blood. Finally, we suggest that saline-perfusion should be recommended for quantification of brain Hbb/Alas2 transcripts in homogenates.

Liver transcriptomic and methylomic analyses identify transcriptional mitogen-activated protein kinase regulation in facultative hibernation of Syrian hamster

Hibernation consists of alternating torpor-arousal phases, during which animals cope with repetitive hypothermia and ischaemia-reperfusion. Due to limited transcriptomic and methylomic information for facultative hibernators, we here conducted RNA and whole-genome bisulfide sequencing in liver of hibernating Syrian hamster (Mesocricetus auratus). Gene ontology analysis was performed on 844 differentially expressed genes and confirmed the shift in metabolic fuel utilization, inhibition of RNA transcription and cell cycle regulation as found in seasonal hibernators. Additionally, we showed a so far unreported suppression of mitogen-activated protein kinase (MAPK) and protein phosphatase 1 pathways during torpor. Notably, hibernating hamsters showed upregulation of MAPK inhibitors (dual-specificity phosphatases and sproutys) and reduced levels of MAPK-induced transcription factors (TFs). Promoter methylation was found to modulate the expression of genes targeted by these TFs. In conclusion, we document gene regulation between hibernation phases, which may aid the identification of pathways and targets to prevent organ damage in transplantation or ischaemia-reperfusion.

Protective effects of electroacupuncture on polycystic ovary syndrome in rats: Down-regulating Alas2 to inhibit apoptosis, oxidative stress, and mitochondrial dysfunction in ovarian granulosa cells

Polycystic ovary syndrome (PCOS) is a heterogeneous endocrine disorder affecting women at reproductive age. The therapeutic effect of electroacupuncture (EA) on PCOS has been revealed, while the anti-PCOS mechanisms of EA have not been fully explored. In this study, PCOS were induced in rats by daily injection with dehydroepiandrosterone (DHEA) for 20 days and EA treatment was performed for 5 weeks. The mRNA expression profiles in ovarian tissues from control, PCOS, and EA-treated rats were examined by high-throughput mRNA sequencing. 5'-aminolevulinate synthase 2 (Alas2), a vital rate-limiting enzyme of the heme synthesis pathway, was selected to be further studied. PCOS led to the upregulation of Alas2 mRNA, whereas EA treatment restored this change. In vitro, primary ovarian granulosa cells (GCs) were challenged with H₂O₂ to mimic the oxidative stress (OS) state in PCOS. H₂O₂ induced apoptosis, OS, mitochondrial dysfunction, as well as Alas2 overexpression in GCs, while lentivirus-mediated Alas2 knockdown evidently restrained the above impairments. In summary, this study highlights the crucial role of Alas2 in cell apoptosis, OS, and mitochondrial dysfunction of PCOS GCs and provides potential therapeutic candidates for further investigation on PCOS treatment.

Mutation analysis of the GSDME gene in a Chinese family with non-syndromic hearing loss

Background

Hearing loss is considered one of the most common sensory nervous system defects, about 60% of which are caused by genetic factors. Mutations in the GSDME gene are responsible for post-lingual, progressive, autosomal dominant hearing loss. This study aimed to characterize the genetic mutations and clinical features of a Chinese GSDME family.

Methods

After clinical evaluations, high-throughput DNA sequencing was conducted using DNA samples from this family. Sanger sequencing was performed to verify the suspected variants. A detailed genotype and phenotype analysis were carried out. Gene set enrichment analysis (GSEA) was performed to identify the signaling pathway associated with GSDME expression.

Results

A known hotspot heterozygous splice-site variation (c.991-15_991_13delTTC) was identified and shown to segregate with the hearing loss phenotype in the family. This pathogenic splice-site variant results in skipping of exon 8. GSEA analysis identified changes in regulation of the cell cycle checkpoint, peroxisome, and amino acid metabolism signaling pathways.

Conclusions

We identified a reported mutation in the GSDME gene. Our findings support the 3 bp deletion (c.991-15_991-13del) was a hotspot variation, and it emerged as an essential contributor to autosomal dominant progressive hearing loss in East Asians. GSDME gene is closely associated with a range of signaling pathways. These characterized findings may provide new evidence for pathogenesis.

Iron Metabolism in the Disorders of Heme Biosynthesis

Given its remarkable property to easily switch between different oxidative states, iron is essential in countless cellular functions which involve redox reactions. At the same time, uncontrolled interactions between iron and its surrounding milieu may be damaging to cells and tissues. Heme—the iron-chelated form of protoporphyrin IX—is a macrocyclic tetrapyrrole and a coordination complex for diatomic gases, accurately engineered by evolution to exploit the catalytic, oxygen-binding, and oxidoreductive properties of iron while minimizing its damaging effects on tissues. The majority of the body production of heme is ultimately incorporated into hemoglobin within mature erythrocytes; thus, regulation of heme biosynthesis by iron is central in erythropoiesis. Additionally, heme is a cofactor in several metabolic pathways, which can be modulated by iron-dependent signals as well. Impairment in some steps of the pathway of heme biosynthesis is the main pathogenetic mechanism of two groups of diseases collectively known as porphyrias and congenital sideroblastic anemias. In porphyrias, according to the specific enzyme involved, heme precursors accumulate up to the enzyme stop in disease-specific patterns and organs. Therefore, different porphyrias manifest themselves under strikingly different clinical pictures. In congenital sideroblastic anemias, instead, an altered utilization of mitochondrial iron by erythroid precursors leads to mitochondrial iron overload and an accumulation of ring sideroblasts in the bone marrow. In line with the complexity of the processes involved, the role of iron in these conditions is then multifarious. This review aims to summarise the most important lines of evidence concerning the interplay between iron and heme metabolism, as well as the clinical and experimental aspects of the role of iron in inherited conditions of altered heme biosynthesis.

Heme-dependent recognition of 5-aminolevulinate synthase by the human mitochondrial molecular chaperone ClpX

The caseinolytic mitochondrial matrix peptidase chaperone subunit (ClpX) plays an important role in the heme-dependent regulation of 5-aminolevulinate synthase (ALAS1), a key enzyme in heme biosynthesis. However, the mechanisms underlying the role of ClpX in this process remain unclear. In this in vitro study, we confirmed the direct binding between ALAS1 and ClpX in a heme-dependent manner. The substitution of C¹⁰⁸ P¹⁰⁹ [CP motif 3 (CP3)] with A¹⁰⁸ A¹⁰⁹ in ALAS1 resulted in a loss of ability to bind ClpX. Computational disorder analyses revealed that CP3 was located in a potential intrinsically disordered protein region (IDPR). Thus, we propose that conditional disorder-to-order transitions in the IDPRs of ALAS1 may represent key mechanisms underlying the heme-dependent recognition of ALAS1 by ClpX.

Iron chelation rescues hemolytic anemia and skin photosensitivity in congenital erythropoietic porphyria

Congenital erythropoietic porphyria (CEP) is an inborn error of heme synthesis resulting from uroporphyrinogen III synthase (UROS) deficiency and the accumulation of nonphysiological porphyrin isomer I metabolites. Clinical features are heterogeneous among patients with CEP but usually combine skin photosensitivity and chronic hemolytic anemia, the severity of which is related to porphyrin overload. Therapeutic options include symptomatic strategies only and are unsatisfactory. One promising approach to treating CEP is to reduce the erythroid production of porphyrins through substrate reduction therapy by inhibiting 5-aminolevulinate synthase 2 (ALAS2), the first and rate-limiting enzyme in the heme biosynthetic pathway. We efficiently reduced porphyrin accumulation after RNA interference-mediated downregulation of ALAS2 in human erythroid cellular models of CEP disease. Taking advantage of the physiological iron-dependent posttranscriptional regulation of ALAS2, we evaluated whether iron chelation with deferiprone could decrease ALAS2 expression and subsequent porphyrin production in vitro and in vivo in a CEP murine model. Treatment with deferiprone of UROS-deficient erythroid cell lines and peripheral blood CD34+-derived erythroid cultures from a patient with CEP inhibited iron-dependent protein ALAS2 and iron-responsive element-binding protein 2 expression and reduced porphyrin production. Furthermore, porphyrin accumulation progressively decreased in red blood cells and urine, and skin photosensitivity in CEP mice treated with deferiprone (1 or 3 mg/mL in drinking water) for 26 weeks was reversed. Hemolysis and iron overload improved upon iron chelation with full correction of anemia in CEP mice treated at the highest dose of deferiprone. Our findings highlight, in both mouse and human models, the therapeutic potential of iron restriction to modulate the phenotype in CEP.

Integrated multiomics analysis of hepatoblastoma unravels its heterogeneity and provides novel druggable targets

Although hepatoblastoma is the most common pediatric liver cancer, its genetic heterogeneity and therapeutic targets are not well elucidated. Therefore, we conducted a multiomics analysis, including mutatome, DNA methylome, and transcriptome analyses, of 59 hepatoblastoma samples. Based on DNA methylation patterns, hepatoblastoma was classified into three clusters exhibiting remarkable correlation with clinical, histological, and genetic features. Cluster F was largely composed of cases with fetal histology and good outcomes, whereas clusters E1 and E2 corresponded primarily to embryonal/combined histology and poor outcomes. E1 and E2, albeit distinguishable by different patient age distributions, were genetically characterized by hypermethylation of the HNF4A/CEBPA-binding regions, fetal liver-like expression patterns, upregulation of the cell cycle pathway, and overexpression of NQO1 and ODC1. Inhibition of NQO1 and ODC1 in hepatoblastoma cells induced chemosensitization and growth suppression, respectively. Our results provide a comprehensive description of the molecular basis of hepatoblastoma and rational therapeutic strategies for high-risk cases.

Identification of Key Genes and Pathways in Myeloma side population cells by Bioinformatics Analysis

Background: Multiple myeloma (MM) is the second most common hematological malignancy, which is still incurable and relapses inevitably, highlighting further understanding of the possible mechanisms. Side population (SP) cells are a group of enriched progenitor cells showing stem-like phenotypes with a distinct low-staining pattern with Hoechst 33342. Compared to main population (MP) cells, the underlying molecular characteristics of SP cells remain largely unclear. This bioinformatics analysis aimed to identify key genes and pathways in myeloma SP cells to provide novel biomarkers, predict MM prognosis and advance potential therapeutic targets. Methods: The gene expression profile GSE109651 was obtained from Gene Expression Omnibus database, and then differentially expressed genes (DEGs) with P-value <0.05 and |log2 fold-change (FC)| > 2 were selected by the comparison of myeloma light-chain (LC) restricted SP (LC/SP) cells and MP CD138⁺ cells. Subsequently, gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analysis, protein-protein interaction (PPI) network analysis were performed to identify the functional enrichment analysis of the DEGs and screen hub genes. Cox proportional hazards regression was used to select the potential prognostic DEGs in training dataset (GSE2658). The prognostic value of the potential prognostic genes was evaluated by Kaplan-Meier curve and validated in another external dataset (MMRF-CoMMpass cohort from TCGA). Results: Altogether, 403 up-regulated and 393 down-regulated DEGs were identified. GO analysis showed that the up-regulated DEGs were significantly enriched in innate immune response, inflammatory response, plasma membrane and integral component of membrane, while the down-regulated DEGs were mainly involved in protoporphyrinogen IX and heme biosynthetic process, hemoglobin complex and erythrocyte differentiation. KEGG pathway analysis suggested that the DEGs were significantly enriched in osteoclast differentiation, porphyrin and chlorophyll metabolism and cytokine-cytokine receptor interaction. The top 10 hub genes, identified by the plug-in cytoHubba of the Cytoscape software using maximal clique centrality (MCC) algorithm, were ITGAM, MMP9, ITGB2, FPR2, C3AR1, CXCL1, CYBB, LILRB2, HP and FCER1G. Modules and corresponding GO enrichment analysis indicated that myeloma LC/SP cells were significantly associated with immune system, immune response and cell cycle. The predictive value of the prognostic model including TFF3, EPDR1, MACROD1, ARHGEF12, AMMECR1, NFATC2, HES6, PLEK2 and SNCA was identified, and validated in another external dataset (MMRF-CoMMpass cohort from TCGA). Conclusions: In conclusion, this study provides reliable molecular biomarkers for screening, prognosis, as well as novel therapeutic targets for myeloma LC/SP cells.

Growth Hormone and Neuronal Hemoglobin in the Brain-Roles in Neuroprotection and Neurodegenerative Diseases

In recent years, evidence for hemoglobin (Hb) synthesis in both animal and human brains has been accumulating. While circulating Hb originating from cerebral hemorrhage or other conditions is toxic, there is also substantial production of neuronal Hb, which is influenced by conditions such as ischemia and regulated by growth hormone (GH), insulin-like growth factor-I (IGF-I), and other growth factors. In this review, we discuss the possible functions of circulating and brain Hb, mainly the neuronal form, with respect to the neuroprotective activities of GH and IGF-I against ischemia and neurodegenerative diseases. The molecular pathways that link Hb to the GH/IGF-I system are also reviewed, although the limited number of reports on this topic suggests a need for further studies. In summary, GH and/or IGF-I appear to be significant determinants of systemic and local brain Hb concentrations through mediating responses to oxygen and metabolic demand, as part of the neuroprotective effects exerted by GH and IGF-I. The nature and quantity of the latter deserve further exploration in specific experiments.

Delta-aminolevulinic acid synthase 2 expression in combination with iron as modifiers of disease severity in erythropoietic protoporphyria

Deficiency in ferrochelatase (FECH), the last enzyme in the heme biosynthetic pathway, leads to an accumulation of protoporphyrin IX (PPIX) that causes a severely painful phototoxic reaction of the skin in patients with erythropoietic protoporphyria (EPP). Besides phototoxicity of the skin, EPP patients often present with symptoms of iron deficiency in form of a microcytic and hypochromic anemia with low serum iron and ferritin. In addition, elevated aminolevulinic acid synthase 2 (ALAS2) both at the mRNA and protein levels have been observed among EPP patients. ALAS is the first enzyme in the pathway and exists in two isoforms, whereby the isoform 2 (ALAS2) is expressed exclusively in erythropoiesis. The mRNA of ALAS2 contains an iron response element (IRE) at its 5'UTR. When iron is limited, iron response element binding protein 2 (IRP2) binds to the IRE of ALAS2 mRNA and suppresses its translation. In this study, we demonstrated that iron deprivation increased the amount of ALAS2 mRNA as well as the ratio of ALAS2 to FECH mRNAs in cultured erythroleukemic K562 cells. At the protein level, however, iron deprivation in the cell line caused reductions in both enzymes as shown by the Western blot analysis. A comparable increase in the ratio of ALAS2 to FECH mRNAs was also found in EPP patients indicating an imbalance in heme biosynthesis. As iron cannot be completely missing from an organism, we assume that in EPP patients, a certain amount of ALAS2 mRNA is translated despite a partial deficiency of FECH. The increase in ALAS2 enzyme contributes to the accumulation in PPIX in the patients. Targeted inhibition of ALAS2 could therefore be a treatment option for EPP.

Zebrafish as a model system to delineate the role of heme and iron metabolism during erythropoiesis

Coordination of iron acquisition and heme synthesis is required for effective erythropoiesis. The small teleost zebrafish (Danio rerio) is an ideal vertebrate animal model to replicate various aspects of human physiology and provides an efficient and cost-effective way to model human pathophysiology. Importantly, zebrafish erythropoiesis largely resembles mammalian erythropoiesis. Gene discovery by large-scale forward mutagenesis screening has identified key components in heme and iron metabolism. Reverse genetic screens, using morpholino-knockdown and CRISPR/Cas9, coupled with the genetic tractability of the developing embryo have further accelerated functional studies. Ultimately, the ex utero development of zebrafish embryos combined with their transparency and developmental plasticity could provide a deeper understanding of the role of iron and heme metabolism during early vertebrate embryonic development.

Regulation and tissue-specific expression of δ-aminolevulinic acid synthases in non-syndromic sideroblastic anemias and porphyrias

Recently, new genes and molecular mechanisms have been identified in patients with porphyrias and sideroblastic anemias (SA). They all modulate either directly or indirectly the δ-aminolevulinic acid synthase (ALAS) activity. ALAS, is encoded by two genes: the erythroid-specific (ALAS2), and the ubiquitously expressed (ALAS1). In the liver, ALAS1 controls the rate-limiting step in the production of heme and hemoproteins that are rapidly turned over in response to metabolic needs. Several heme regulatory targets have been identified as regulators of ALAS1 activity: 1) transcriptional repression via a heme-responsive element, 2) post-transcriptional destabilization of ALAS1 mRNA, 3) post-translational inhibition via a heme regulatory motif, 4) direct inhibition of the activity of the enzyme and 5) breakdown of ALAS1 protein via heme-mediated induction of the protease Lon peptidase 1. In erythroid cells, ALAS2 is a gatekeeper of production of very large amounts of heme necessary for hemoglobin synthesis. The rate of ALAS2 synthesis is transiently increased during the period of active heme synthesis. Its gene expression is determined by trans-activation of nuclear factor GATA1, CACC box and NF-E2-binding sites in the promoter areas. ALAS2 mRNA translation is also regulated by the iron-responsive element (IRE)/iron regulatory proteins (IRP) binding system. In patients, ALAS enzyme activity is affected in most of the mutations causing non-syndromic SA and in several porphyrias. Decreased ALAS2 activity results either directly from loss-of-function ALAS2 mutations as seen in X-linked sideroblastic anemia (XLSA) or from defect in the availability of one of its two mitochondrial substrates: glycine in SLC25A38 mutations and succinyl CoA in GLRX5 mutations. Moreover, ALAS2 gain of function mutations is responsible for X-linked protoporphyria and increased ALAS1 activity lead to acute attacks of hepatic porphyrias. A missense dominant mutation in the Walker A motif of the ATPase binding site in the gene coding for the mitochondrial protein unfoldase CLPX also contributes to increasing ALAS and subsequently protoporphyrinemia. Altogether, these recent data on human ALAS have informed our understanding of porphyrias and sideroblastic anemias pathogeneses and may contribute to new therapeutic strategies.

Role of Heme Oxygenase as a Modulator of Heme-Mediated Pathways

The heme oxygenase (HO) system is essential for heme and iron homeostasis and necessary for adaptation to cell stress. HO degrades heme to biliverdin (BV), carbon monoxide (CO) and ferrous iron. Although mostly beneficial, the HO reaction can also produce deleterious effects, predominantly attributed to excessive product formation. Underrated so far is, however, that HO may exert effects additionally via modulation of the cellular heme levels. Heme, besides being an often-quoted generator of oxidative stress, plays also an important role as a signaling molecule. Heme controls the anti-oxidative defense, circadian rhythms, activity of ion channels, glucose utilization, erythropoiesis, and macrophage function. This broad spectrum of effects depends on its interaction with proteins ranging from transcription factors to enzymes. In degrading heme, HO has the potential to exert effects also via modulation of heme-mediated pathways. In this review, we will discuss the multitude of pathways regulated by heme to enlarge the view on HO and its role in cell physiology. We will further highlight the contribution of HO to pathophysiology, which results from a dysregulated balance between heme and the degradation products formed by HO.

Single-cell analysis of progenitor cell dynamics and lineage specification in the human fetal kidney

The mammalian kidney develops through reciprocal interactions between the ureteric bud and the metanephric mesenchyme to give rise to the entire collecting system and the nephrons. Most of our knowledge of the developmental regulators driving this process arises from the study of gene expression and functional genetics in mice and other animal models. In order to shed light on human kidney development, we have used single-cell transcriptomics to characterize gene expression in different cell populations, and to study individual cell dynamics and lineage trajectories during development. Single-cell transcriptome analyses of 6414 cells from five individual specimens identified 11 initial clusters of specific renal cell types as defined by their gene expression profile. Further subclustering identifies progenitors, and mature and intermediate stages of differentiation for several renal lineages. Other lineages identified include mesangium, stroma, endothelial and immune cells. Novel markers for these cell types were revealed in the analysis, as were components of key signaling pathways driving renal development in animal models. Altogether, we provide a comprehensive and dynamic gene expression profile of the developing human kidney at the single-cell level.

Summary: New markers for specific cell types in the developing human kidney are identified and computational approaches infer developmental trajectories and interrogate the complex network of signaling pathways and cellular transitions.

Alas1 is essential for neutrophil maturation in zebrafish

Neutrophils play essential roles in innate immunity and are the first responders to kill foreign micro-organisms, a function that partially depends on their granule content. The complicated regulatory network of neutrophil development and maturation remains largely unknown. Here we utilized neutrophil-deficient zebrafish to identify a novel role of Alas1, a heme biosynthesis pathway enzyme, in neutrophil development. We showed that Alas1-deficient zebrafish exhibited proper neutrophil initiation, but further neutrophil maturation was blocked due to heme deficiency, with lipid storage and granule formation deficiencies, and loss of heme-dependent granule protein activities. Consequently, Alas1-deficient zebrafish showed impaired bactericidal ability and augmented inflammatory responses when challenged with Escherichia coli. These findings demonstrate the important role of Alas1 in regulating neutrophil maturation and physiological function through the heme. Our study provides an in vivo model of Alas1 deficiency and may be useful to evaluate the progression of heme-related disorders in order to facilitate the development of drugs and treatment strategies for these diseases.

Molecular investigation of genetic assimilation during the rapid adaptive radiations of East African cichlid fishes

Adaptive radiations are characterized by adaptive diversification intertwined with rapid speciation within a lineage resulting in many ecologically specialized, phenotypically diverse species. It has been proposed that adaptive radiations can originate from ancestral lineages with pronounced phenotypic plasticity in adaptive traits, facilitating ecologically driven phenotypic diversification that is ultimately fixed through genetic assimilation of gene regulatory regions. This study aimed to investigate how phenotypic plasticity is reflected in gene expression patterns in the trophic apparatus of several lineages of East African cichlid fishes, and whether the observed patterns support genetic assimilation. This investigation used a split brood experimental design to compare adaptive plasticity in species from within and outside of adaptive radiations. The plastic response was induced in the crushing pharyngeal jaws through feeding individuals either a hard or soft diet. We find that nonradiating, basal lineages show higher levels of adaptive morphological plasticity than the derived, radiated lineages, suggesting that these differences have become partially genetically fixed during the formation of the adaptive radiations. Two candidate genes that may have undergone genetic assimilation, gif and alas1, were identified, in addition to alterations in the wiring of LPJ patterning networks. Taken together, our results suggest that genetic assimilation may have dampened the inducibility of plasticity related genes during the adaptive radiations of East African cichlids, flattening the reaction norms and canalizing their feeding phenotypes, driving adaptation to progressively more narrow ecological niches.

Mode of GH administration and gene expression in the female rat brain

The endogenous secretion of growth hormone (GH) is sexually dimorphic in rats with females having a more even and males a more pulsatile secretion and low trough levels. The mode of GH administration, mimicking the sexually dimorphic secretion, has different systemic effects. In the brains of male rats, we have previously found that the mode of GH administration differently affects neuron-haemoglobin beta (Hbb) expression whereas effects on other transcripts were moderate. The different modes of GH administration could have different effects on brain transcripts in female rats. Hypophysectomised female rats were given GH either as injections twice daily or as continuous infusion and GH-responsive transcripts were assessed by quantitative reverse transcription polymerase chain reaction in the hippocampus and parietal cortex (cortex). The different modes of GH-administration markedly increased Hbb and 5'-aminolevulinate synthase 2 (Alas2) in both brain regions. As other effects were relatively moderate, a mixed model analysis (MMA) was used to investigate general effects of the treatments. In the hippocampus, MMA showed that GH-infusion suppressed glia- and neuron-related transcript expression levels, whereas GH-injections increased expression levels. In the cortex, GH-infusion instead increased neuron-related transcripts, whereas GH-injections had no significant effect. Interestingly, this contrasts to previous results obtained from male rat cortex where GH-infusion generally decreased expression levels. In conclusion, the results indicate that there is a small but significant difference in response to mode of GH administration in the hippocampus as compared to the cortex. For both modes of GH administration, there was a robust effect on Hbb and Alas2.

Transcriptome Profiling Reveals Disruption of Innate Immunity in Chronic Heavy Ethanol Consuming Female Rhesus Macaques

It is well established that heavy ethanol consumption interferes with the immune system and inflammatory processes, resulting in increased risk for infectious and chronic diseases. However, these processes have yet to be systematically studied in a dose and sex-dependent manner. In this study, we investigated the impact of chronic heavy ethanol consumption on gene expression using RNA-seq in peripheral blood mononuclear cells isolated from female rhesus macaques with daily consumption of 4% ethanol available 22hr/day for 12 months resulting in average ethanol consumption of 4.3 g/kg/day (considered heavy drinking). Differential gene expression analysis was performed using edgeR and gene enrichment analysis using MetaCore™. We identified 1106 differentially expressed genes, meeting the criterion of ≥ two-fold change and p-value ≤ 0.05 in expression (445 up- and 661 down-regulated). Pathway analysis of the 879 genes with characterized identifiers showed that the most enriched gene ontology processes were "response to wounding", "blood coagulation", "immune system process", and "regulation of signaling". Changes in gene expression were seen despite the lack of differences in the frequency of any major immune cell subtype between ethanol and controls, suggesting that heavy ethanol consumption modulates gene expression at the cellular level rather than altering the distribution of peripheral blood mononuclear cells. Collectively, these observations provide mechanisms to explain the higher incidence of infection, delay in wound healing, and increase in cardiovascular disease seen in subjects with Alcohol use disorder.

Blood transcriptional signature of recombinant human erythropoietin administration and implications for antidoping strategies

Recombinant human erythropoietin (rHuEPO) is frequently abused by athletes as a performance-enhancing drug, despite being prohibited by the World Anti-Doping Agency. Although the methods to detect blood doping, including rHuEPO injections, have improved in recent years, they remain imperfect. In a proof-of-principle study, we identified, replicated, and validated the whole blood transcriptional signature of rHuEPO in endurance-trained Caucasian males at sea level ( n = 18) and Kenyan endurance runners at moderate altitude ( n = 20), all of whom received rHuEPO injections for 4 wk. Transcriptional profiling shows that hundreds of transcripts were altered by rHuEPO in both cohorts. The main regulated expression pattern, observed in all participants, was characterized by a “rebound” effect with a profound upregulation during rHuEPO and a subsequent downregulation up to 4 wk postadministration. The functions of the identified genes were mainly related to the functional and structural properties of the red blood cell. Of the genes identified to be differentially expressed during and post-rHuEPO, we further confirmed a whole blood 34-transcript signature that can distinguish between samples collected pre-, during, and post-rHuEPO administration. By providing biomarkers that can reveal rHuEPO use, our findings represent an advance in the development of new methods for the detection of blood doping.

Protoporphyrin IX: the Good, the Bad, and the Ugly

Protoporphyrin IX (PPIX) is ubiquitously present in all living cells in small amounts as a precursor of heme. PPIX has some biologic functions of its own, and PPIX-based strategies have been used for cancer diagnosis and treatment (the good). PPIX serves as the substrate for ferrochelatase, the final enzyme in heme biosynthesis, and its homeostasis is tightly regulated during heme synthesis. Accumulation of PPIX in human porphyrias can cause skin photosensitivity, biliary stones, hepatobiliary damage, and even liver failure (the bad and the ugly). In this work, we review the mechanisms that are associated with the broad aspects of PPIX. Because PPIX is a hydrophobic molecule, its disposition is by hepatic rather than renal excretion. Large amounts of PPIX are toxic to the liver and can cause cholestatic liver injury. Application of PPIX in cancer diagnosis and treatment is based on its photodynamic effects.

Increased Heme Levels in the Heart Lead to Exacerbated Ischemic Injury

Background

Heme is an essential iron-containing molecule for cardiovascular physiology, but in excess it may increase oxidative stress. Failing human hearts have increased heme levels, with upregulation of the rate-limiting enzyme in heme synthesis, δ-aminolevulinic acid synthase 2 (ALAS2), which is normally not expressed in cardiomyocytes. We hypothesized that increased heme accumulation (through cardiac overexpression of ALAS2) leads to increased oxidative stress and cell death in the heart.

Methods and Results

We first showed that ALAS2 and heme levels are increased in the hearts of mice subjected to coronary ligation. To determine the causative role of increased heme in the development of heart failure, we generated transgenic mice with cardiac-specific overexpression of ALAS2. While ALAS2 transgenic mice have normal cardiac function at baseline, their hearts display increased heme content, higher oxidative stress, exacerbated cell death, and worsened cardiac function after coronary ligation compared to nontransgenic littermates. We confirmed in cultured cardiomyoblasts that the increased oxidative stress and cell death observed with ALAS2 overexpression is mediated by increased heme accumulation. Furthermore, knockdown of ALAS2 in cultured cardiomyoblasts exposed to hypoxia reversed the increases in heme content and cell death. Administration of the mitochondrial antioxidant MitoTempo to ALAS2-overexpressing cardiomyoblasts normalized the elevated oxidative stress and cell death levels to baseline, indicating that the effects of increased ALAS2 and heme are through elevated mitochondrial oxidative stress. The clinical relevance of these findings was supported by the finding of increased ALAS2 induction and heme accumulation in failing human hearts from patients with ischemic cardiomyopathy compared to nonischemic cardiomyopathy.

Conclusions

Heme accumulation is detrimental to cardiac function under ischemic conditions, and reducing heme in the heart may be a novel approach for protection against the development of heart failure.

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.

Regulatory gene networks that shape the development of adaptive phenotypic plasticity in a cichlid fish

Phenotypic plasticity is the ability of organisms with a given genotype to develop different phenotypes according to environmental stimuli, resulting in individuals that are better adapted to local conditions. In spite of their ecological importance, the developmental regulatory networks underlying plastic phenotypes often remain uncharacterized. We examined the regulatory basis of diet-induced plasticity in the lower pharyngeal jaw (LPJ) of the cichlid fish Astatoreochromis alluaudi, a model species in the study of adaptive plasticity. Through raising juvenile A. alluaudi on either a hard or soft diet (hard-shelled or pulverized snails) for between 1 and 8 months, we gained insight into the temporal regulation of 19 previously identified candidate genes during the early stages of plasticity development. Plasticity in LPJ morphology was first detected between 3 and 5 months of diet treatment. The candidate genes, belonging to various functional categories, displayed dynamic expression patterns that consistently preceded the onset of morphological divergence and putatively contribute to the initiation of the plastic phenotypes. Within functional categories, we observed striking co-expression, and transcription factor binding site analysis was used to examine the prospective basis of their coregulation. We propose a regulatory network of LPJ plasticity in cichlids, presenting evidence for regulatory crosstalk between bone and muscle tissues, which putatively facilitates the development of this highly integrated trait. Through incorporating a developmental time-course into a phenotypic plasticity study, we have identified an interconnected, environmentally responsive regulatory network that shapes the development of plasticity in a key innovation of East African cichlids.

Expression of murine 5-aminolevulinate synthase variants causes protoporphyrin IX accumulation and light-induced mammalian cell death

5-Aminolevulinate synthase (ALAS; EC 2.3.1.37) catalyzes the first committed step of heme biosynthesis in animals. The erythroid-specific ALAS isozyme (ALAS2) is negatively regulated by heme at the level of mitochondrial import and, in its mature form, certain mutations of the murine ALAS2 active site loop result in increased production of protoporphyrin IX (PPIX), the precursor for heme. Importantly, generation of PPIX is a crucial component in the widely used photodynamic therapies (PDT) of cancer and other dysplasias. ALAS2 variants that cause high levels of PPIX accumulation provide a new means of targeted, and potentially enhanced, photosensitization. In order to assess the prospective utility of ALAS2 variants in PPIX production for PDT, K562 human erythroleukemia cells and HeLa human cervical carcinoma cells were transfected with expression plasmids for ALAS2 variants with greater enzymatic activity than the wild-type enzyme. The levels of accumulated PPIX in ALAS2-expressing cells were analyzed using flow cytometry with fluorescence detection. Further, cells expressing ALAS2 variants were subjected to white light treatments (21–22 kLux) for 10 minutes after which cell viability was determined. Transfection of HeLa cells with expression plasmids for murine ALAS2 variants, specifically for those with mutated mitochondrial presequences and a mutation in the active site loop, caused significant cellular accumulation of PPIX, particularly in the membrane. Light treatments revealed that ALAS2 expression results in an increase in cell death in comparison to aminolevulinic acid (ALA) treatment producing a similar amount of PPIX. The delivery of stable and highly active ALAS2 variants has the potential to expand and improve upon current PDT regimes.

Molecular evolution of multiple-level control of heme biosynthesis pathway in animal kingdom

Adaptation of enzymes in a metabolic pathway can occur not only through changes in amino acid sequences but also through variations in transcriptional activation, mRNA splicing and mRNA translation. The heme biosynthesis pathway, a linear pathway comprised of eight consecutive enzymes in animals, provides researchers with ample information for multiple types of evolutionary analyses performed with respect to the position of each enzyme in the pathway. Through bioinformatics analysis, we found that the protein-coding sequences of all enzymes in this pathway are under strong purifying selection, from cnidarians to mammals. However, loose evolutionary constraints are observed for enzymes in which self-catalysis occurs. Through comparative genomics, we found that in animals, the first intron of the enzyme-encoding genes has been co-opted for transcriptional activation of the genes in this pathway. Organisms sense the cellular content of iron, and through iron-responsive elements in the 5′ untranslated regions of mRNAs and the intron-exon boundary regions of pathway genes, translational inhibition and exon choice in enzymes may be enabled, respectively. Pathway product (heme)-mediated negative feedback control can affect the transport of pathway enzymes into the mitochondria as well as the ubiquitin-mediated stability of enzymes. Remarkably, the positions of these controls on pathway activity are not ubiquitous but are biased towards the enzymes in the upstream portion of the pathway. We revealed that multiple-level controls on the activity of the heme biosynthesis pathway depend on the linear depth of the enzymes in the pathway, indicating a new strategy for discovering the molecular constraints that shape the evolution of a metabolic pathway.

Heme levels are increased in human failing hearts

OBJECTIVES

The goal of this study was to characterize the regulation of heme and non-heme iron in human failing hearts.

BACKGROUND

Iron is an essential molecule for cellular physiology, but in excess it facilitates oxidative stress. Mitochondria are the key regulators of iron homeostasis through heme and iron-sulfur cluster synthesis. Because mitochondrial function is depressed in failing hearts and iron accumulation can lead to oxidative stress, we hypothesized that iron regulation may also be impaired in heart failure (HF).

METHODS

We measured mitochondrial and cytosolic heme and non-heme iron levels in failing human hearts retrieved during cardiac transplantation surgery. In addition, we examined the expression of genes regulating cellular iron homeostasis, the heme biosynthetic pathway, and micro-RNAs that may potentially target iron regulatory networks.

RESULTS

Although cytosolic non-heme iron levels were reduced in HF, mitochondrial iron content was maintained. Moreover, we observed a significant increase in heme levels in failing hearts, with corresponding feedback inhibition of the heme synthetic enzymes and no change in heme degradation. The rate-limiting enzyme in heme synthesis, delta-aminolevulinic acid synthase 2 (ALAS2), was significantly upregulated in HF. Overexpression of ALAS2 in H9c2 cardiac myoblasts resulted in increased heme levels, and hypoxia and erythropoietin treatment increased heme production through upregulation of ALAS2. Finally, increased heme levels in cardiac myoblasts were associated with excess production of reactive oxygen species and cell death, suggesting a maladaptive role for increased heme in HF.

CONCLUSIONS

Despite global mitochondrial dysfunction, heme levels are maintained above baseline in human failing hearts.

The long history of iron in the Universe and in health and disease

BACKGROUND

Not long after the Big Bang, iron began to play a central role in the Universe and soon became mired in the tangle of biochemistry that is the prima essentia of life. Since life's addiction to iron transcends the oxygenation of the Earth's atmosphere, living things must be protected from the potentially dangerous mix of iron and oxygen. The human being possesses grams of this potentially toxic transition metal, which is shuttling through his oxygen-rich humor. Since long before the birth of modern medicine, the blood-vibrant red from a massive abundance of hemoglobin iron-has been a focus for health experts.

SCOPE OF REVIEW

We describe the current understanding of iron metabolism, highlight the many important discoveries that accreted this knowledge, and describe the perils of dysfunctional iron handling.

GENERAL SIGNIFICANCE

Isaac Newton famously penned, "If I have seen further than others, it is by standing upon the shoulders of giants". We hope that this review will inspire future scientists to develop intellectual pursuits by understanding the research and ideas from many remarkable thinkers of the past.

MAJOR CONCLUSIONS

The history of iron research is a long, rich story with early beginnings, and is far from being finished. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Lineage regulators direct BMP and Wnt pathways to cell-specific programs during differentiation and regeneration

BMP and Wnt signaling pathways control essential cellular responses through activation of the transcription factors SMAD (BMP) and TCF (Wnt). Here, we show that regeneration of hematopoietic lineages following acute injury depends on the activation of each of these signaling pathways to induce expression of key blood genes. Both SMAD1 and TCF7L2 co-occupy sites with master regulators adjacent to hematopoietic genes. In addition, both SMAD1 and TCF7L2 follow the binding of the predominant lineage regulator during differentiation from multipotent hematopoietic progenitor cells to erythroid cells. Furthermore, induction of the myeloid lineage regulator C/EBPα in erythroid cells shifts binding of SMAD1 to sites newly occupied by C/EBPα, whereas expression of the erythroid regulator GATA1 directs SMAD1 loss on nonerythroid targets. We conclude that the regenerative response mediated by BMP and Wnt signaling pathways is coupled with the lineage master regulators to control the gene programs defining cellular identity.

Heme-oxygenases during erythropoiesis in K562 and human bone marrow cells

In mammalian cells, heme can be degraded by heme-oxygenases (HO). Heme-oxygenase 1 (HO-1) is known to be the heme inducible isoform, whereas heme-oxygenase 2 (HO-2) is the constitutive enzyme. Here we investigated the presence of HO during erythroid differentiation in human bone marrow erythroid precursors and K562 cells. HO-1 mRNA and protein expression levels were below limits of detection in K562 cells. Moreover, heme was unable to induce HO-1, at the protein and mRNA profiles. Surprisingly, HO-2 expression was inhibited upon incubation with heme. To evaluate the physiological relevance of these findings, we analyzed HO expression during normal erythropoiesis in human bone marrow. Erythroid precursors were characterized by lack of significant expression of HO-1 and by progressive reduction of HO-2 during differentiation. FLVCR expression, a recently described heme exporter found in erythroid precursors, was also analyzed. Interestingly, the disruption in the HO detoxification system was accompanied by a transient induction of FLVCR. It will be interesting to verify if the inhibition of HO expression, that we found, is preventing a futile cycle of concomitant heme synthesis and catabolism. We believe that a significant feature of erythropoiesis could be the replacement of heme breakdown by heme exportation, as a mechanism to prevent heme toxicity.

Oxidative stress modulates heme synthesis and induces peroxiredoxin-2 as a novel cytoprotective response in β-thalassemic erythropoiesis

BACKGROUND

β-thalassemic syndromes are inherited red cell disorders characterized by severe ineffective erythropoiesis and increased levels of reactive oxygen species whose contribution to β-thalassemic anemia is only partially understood.

DESIGN AND METHODS

We studied erythroid precursors from normal and β-thalassemic peripheral CD34(+) cells in two-phase liquid culture by proteomic, reverse transcriptase polymerase chain reaction and immunoblot analyses. We measured intracellular reactive oxygen species, heme levels and the activity of δ-aminolevulinate-synthase-2. We exposed normal cells and K562 cells with silenced peroxiredoxin-2 to H(2)O(2) and generated a recombinant peroxiredoxin-2 for kinetic measurements in the presence of H(2)O(2) or hemin.

RESULTS

In β-thalassemia the increased production of reactive oxygen species was associated with down-regulation of heme oxygenase-1 and biliverdin reductase and up-regulation of peroxiredoxin-2. In agreement with these observations in β-thalassemic cells we found decreased heme levels related to significantly reduced activity of the first enzyme of the heme pathway, δ-aminolevulinate synthase-2 without differences in its expression. We demonstrated that the activity of recombinant δ-aminolevulinate synthase-2 is inhibited by both reactive oxygen species and hemin as a protective mechanism in β-thalassemic cells. We then addressed the question of the protective role of peroxiredoxin-2 in erythropoiesis by exposing normal cells to oxidative stress and silencing peroxiredoxin-2 in human erythroleukemia K562 cells. We found that peroxiredoxin-2 expression is up-regulated in response to oxidative stress and required for K562 cells to survive oxidative stress. We then showed that peroxiredoxin-2 binds heme in erythroid precursors with high affinity, suggesting a possible multifunctional cytoprotective role of peroxiredoxin-2 in β-thalassemia.

CONCLUSIONS

In β-thalassemic erythroid cells the reduction of δ-aminolevulinate synthase-2 activity and the increased expression of peroxiredoxin-2 might represent two novel stress-response protective systems.

Regulation of iron pathways in response to hypoxia

Constituting an integral part of a heme's porphyrin ring, iron is essential for supplying cells and tissues with oxygen. Given tight links between oxygen delivery and iron availability, it is not surprising that iron deprivation and oxygen deprivation (hypoxia) have very similar consequences at the molecular level. Under hypoxia, the expression of major iron homeostasis genes including transferrin, transferrin receptor, ceruloplasmin, and heme oxygenase-1 is activated by hypoxia-inducible factors to provide increased iron availability for erythropoiesis in an attempt to enhance oxygen uptake and delivery to hypoxic cells. Iron-response proteins (IRP1 and IRP2) and "cap-n-collar" bZIP transcriptional factors (NE-F2 p45; Nrf1, 2, and 3; Bach1 and 2) also control gene and protein expression of the key iron homeostasis proteins. In this article, we give an overview of the mechanisms by which iron pathways are regulated by hypoxia at multiple levels. In addition, potential clinical benefits of manipulating iron pathways in the hypoxia-related conditions anemia and ischemia are discussed.

Hypoxic induction of human erythroid-specific δ-aminolevulinate synthase mediated by hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF1) is a heterodimeric basic helix-loop-helix transcription factor that regulates many key genes. δ-Aminolevulinate synthase (ALAS) catalyzes the first and rate-limiting reaction in the heme biosynthetic pathway. In this study, we show that hypoxia-induced expression of erythroid-specific ALAS2 is mediated by HIF1 in erythroid cells. Under hypoxic conditions, significantly increased ALAS2 mRNA and protein levels were detected in K562 cells and erythroid induction cultures of CD34+ hematopoietic stem/progenitor cells. Enforced HIF1α expression increased the level of ALAS2 expression, while HIF1α knockdown by RNA interference decreased the level of ALAS2 expression. In silico analysis revealed three potential hypoxia-response elements (HREs) that are located 611, 621, and 741 bp downstream of the ALAS2 gene. The results from reporter gene and mutation analysis suggested that these elements are necessary for a maximal hypoxic response. Chromatin immunoprecipitation and polymerase chain reaction showed that the HREs could be recognized and bound by HIF1α in vivo. These results demonstrate that the upregulation of ALAS2 during hypoxia is directly mediated by HIF1. We hypothesize that HIF1-mediated ALAS2 upregulation promotes erythropoiesis to satisfy the needs of an organism under hypoxic conditions. This may be accomplished via increased heme levels and an interaction between ALAS2 and erythropoietin.

Hemoglobin expression in rat experimental granulation tissue

The general opinion that hemoglobin is only a carrier protein for oxygen and carbon dioxide has been challenged by several recent studies showing hemoglobin expression in other cells than those of the erythroid series, for example, in macrophages. We discovered β-globin expression in rat experimental granulation tissue induced by subcutaneously implanted cellulose sponges. Closer investigation revealed also α-globin expression. The first peak of the biphasic globin expression noticed during granulation tissue formation correlated with the invasion of monocytes/macrophages, whereas the second one seemed to be connected to the appearance of hematopoietic progenitors. Data presented in this study indicate globin expression both in macrophages and in immature erythroid cells as validated by erythroid-specific markers.

hem6: an ENU-induced recessive hypochromic microcytic anemia mutation in the mouse

Mouse models have proven invaluable for understanding erythropoiesis. Here, we describe an autosomal recessive, inherited anemia in the mouse mutant hem6. Hematologic and transplantation analyses reveal a mild, congenital, hypochromic, microcytic anemia intrinsic to the hematopoietic system that is associated with a decreased red blood cell zinc protoporphyrin to heme ratio, indicative of porphyrin insufficiency. Intercross matings show that hem6 can suppress the porphyric phenotype of mice with erythropoietic protoporphyria (EPP). Furthermore, iron uptake studies in hem6 reticulocytes demonstrate defective incorporation of iron into heme that can be partially corrected by the addition of porphyrin precursors. Gene expression and enzymatic assays indicate that erythroid 5-aminolevulinic acid synthase (Alas2) is decreased in hem6 animals, suggesting a mechanism that could account for the anemia. Overall, these data lead to the hypothesis that hem6 encodes a protein that directly or indirectly regulates the expression of Alas2.

Pleiotropic effects and compensation mechanisms determine tissue specificity in mitochondrial myopathy and sideroblastic anemia (MLASA)

The tissue specificity of mitochondrial diseases is poorly understood. Recently, tissue-specific quantitative differences of the components of the mitochondrial translation system have been found to correlate with disease presentation in fatal hepatopathy caused by mutations in mitochondrial translation factor EFG1. MLASA is an autosomal recessive inherited progressive oxidative phosphorylation disorder that affects muscle and erythroid cells. The disease is caused by the homozygous point mutation C656T (R116W) in the catalytic domain of the pseudouridylate synthase 1 (PUS1) gene, which leads to a complete lack of pseudouridylation at the expected sites in mitochondrial and cytoplasmic tRNAs. Despite the presence of these altered tRNAs, most tissues are unaffected, and even in muscle and erythroid cells the disease phenotype only slowly emerges over the course of years. In order to elucidate intracellular pathways through which the homozygous mutation leads to tissue-restricted phenotype, we performed microarray expression analysis of EBV-transformed lymphoblasts from MLASA patients, heterozygous parents, and controls using human Beadchip microarray with 47,296 transcripts. Genes coding for proteins involved in DNA transcription and its regulation, and metal binding proteins, demonstrated major differences in expression between patients and all other individuals with normal phenotype. Genes coding for ribosomal proteins differed significantly between individual with at least one copy of the mutated PUS1 gene and controls. These findings indicate that the lack of tRNA pseudouridylation can be overcome by compensatory changes in levels of ribosomal proteins, and that the disease phenotype in affected tissues is likely due to pleiotropic effects of PUS1p on non-tRNA molecules involved in DNA transcription and iron metabolism. Similar combinations of mechanisms may play a role in the tissue specificity of other mitochondrial disorders.

Histone acetyltransferase p300 regulates the transcription of human erythroid-specific 5-aminolevulinate synthase gene

Erythroid-specific 5-aminolevulinate synthase (ALAS2) catalyzes the rate-limiting step in heme biosynthesis of erythroid cells. Here, we show that treatment of erythroid K562 cells with HDAC inhibitors sodium butyrate or Trichostatin A gave rise to a significant increase in ALAS2 gene transcripts, with a concurrent increase in acetylation level of histone H4 at the ALAS2 gene promoter. Histone acetyltransferase p300 bound withALAS2 promoter and overexpression of p300 increased both the promoter reporter expression and endogenous mRNA level of ALAS2. Additionally, two functional Sp1 sites located in ALAS2 promoter were identified. Both of the GATA-1 sites and all the Sp1 sites at the ALAS2 promoter contributed to the transcription synergistic action with p300. These data implicated a close relationship between the acetylation modification of histone at the ALAS2 promoter and the regulation of this gene. Meanwhile, this work identified that ALAS2 is a novel target gene for p300/CBP action as histone acetyltransferases.

Erythroid-specific 5-aminolevulinate synthase protein is stabilized by low oxygen and proteasomal inhibition

5-Aminolevulinate synthase (ALAS; E.C. 2.3.1.37) catalyzes the first and rate-limiting step of heme synthesis within the mitochondria. Two isozymes of ALAS, encoded by separate genes, exist. ALAS1 is ubiquitously expressed and provides heme for cytochromes and other hemoproteins. ALAS2 is expressed exclusively in erythroid cells and synthesizes heme specifically for haemoglobin. A database search for proteins potentially regulated by oxygen tension revealed that ALAS2 contained a sequence of amino acids (LXXLAP where L is leucine, X is any amino acid, A is alanine, and P is proline) not occurring in ALAS1, which may be hydroxylated under normoxic conditions (21% O2) and target the enzyme for ubiquitination and degradation by the proteasome. We examined protein turnover of ALAS2 in the presence of cycloheximide in K562 cells. Normoxic ALAS2 had a turnover time of approximately 36 h. Hypoxia (1% O2) and inhibition of the proteasome increased both the stability and the specific activity of ALAS2 (greater than 2- and 7-fold, respectively, over 72 h of treatment). Mutation of a key proline within the LXXLAP sequence of ALAS2 also stabilized the protein beyond 36 h under normoxic conditions. The von Hippel-Lindau (vHL) protein was immunoprecipitated with FLAG epitope-tagged ALAS2 produced in normoxic cells but not in hypoxic cells, suggesting that the ALAS2 is hydroxylated under normoxic conditions and targeted for ubiquitination by the E3 ubiquitin ligase system. ALAS2 could also be ubiquitinated under normoxia using an in vitro ubiquitination assay. The present study provides evidence that ALAS2 is broken down under normoxic conditions by the proteasome and that the prolyl-4-hydroxylase/vHL E3 ubiquitin ligase pathway may be involved.

Remodeling the regulation of iron metabolism during erythroid differentiation to ensure efficient heme biosynthesis

Terminal erythropoiesis is accompanied by extreme demand for iron to ensure proper hemoglobinization. Thus, erythroblasts must modify the "standard" post-transcriptional feedback regulation, balancing expression of ferritin (Fer; iron storage) versus transferrin receptor (TfR1; iron uptake) via specific mRNA binding of iron regulatory proteins (IRPs). Although erythroid differentiation involves high levels of incoming iron, TfR1 mRNA stability must be sustained and Fer mRNA translation must not be activated because iron storage would counteract hemoglobinization. Furthermore, translation of the erythroid-specific form of aminolevulinic acid synthase (ALAS-E) mRNA, catalyzing the first step of heme biosynthesis and regulated similarly as Fer mRNA by IRPs, must be ensured. We addressed these questions using mass cultures of primary murine erythroid progenitors from fetal liver, either undergoing sustained proliferation or highly synchronous differentiation. We indeed observed strong inhibition of Fer mRNA translation and efficient ALAS-E mRNA translation in differentiating erythroblasts. Moreover, in contrast to self-renewing cells, TfR1 stability and IRP mRNA binding were no longer modulated by iron supply. These and additional data stemming from inhibition of heme synthesis with succinylacetone or from iron overload suggest that highly efficient utilization of iron in mitochondrial heme synthesis during normal erythropoiesis alters the regulation of iron metabolism via the IRE/IRP system.

Development of a large-scale chemogenomics database to improve drug candidate selection and to understand mechanisms of chemical toxicity and action

Successful drug discovery requires accurate decision making in order to advance the best candidates from initial lead identification to final approval. Chemogenomics, the use of genomic tools in pharmacology and toxicology, offers a promising enhancement to traditional methods of target identification/validation, lead identification, efficacy evaluation, and toxicity assessment. To realize the value of chemogenomics information, a contextual database is needed to relate the physiological outcomes induced by diverse compounds to the gene expression patterns measured in the same animals. Massively parallel gene expression characterization coupled with traditional assessments of drug candidates provides additional, important mechanistic information, and therefore a means to increase the accuracy of critical decisions. A large-scale chemogenomics database developed from in vivo treated rats provides the context and supporting data to enhance and accelerate accurate interpretation of mechanisms of toxicity and pharmacology of chemicals and drugs. To date, approximately 600 different compounds, including more than 400 FDA approved drugs, 60 drugs approved in Europe and Japan, 25 withdrawn drugs, and 100 toxicants, have been profiled in up to 7 different tissues of rats (representing over 3200 different drug-dose-time-tissue combinations). Accomplishing this task required evaluating and improving a number of in vivo and microarray protocols, including over 80 rigorous quality control steps. The utility of pairing clinical pathology assessments with gene expression data is illustrated using three anti-neoplastic drugs: carmustine, methotrexate, and thioguanine, which had similar effects on the blood compartment, but diverse effects on hepatotoxicity. We will demonstrate that gene expression events monitored in the liver can be used to predict pathological events occurring in that tissue as well as in hematopoietic tissues.

Examination of mitochondrial protein targeting of haem synthetic enzymes: in vivo identification of three functional haem-responsive motifs in 5-aminolaevulinate synthase

The initial and the terminal three enzymes of the mammalian haem biosynthetic pathway are nuclear encoded, cytoplasmically synthesized and post-translationally translocated into the mitochondrion. The first enzyme, ALAS (5-aminolaevulinate synthase), occurs as an isoenzyme encoded on different chromosomes and is synthesized either as a housekeeping protein (ALAS-1) in all non-erythroid cell types, or only in differentiating erythroid precursor cells (ALAS-2). Both ALAS proteins possess mitochondrial targeting sequences that have putative haem-binding motifs. In the present study, evidence is presented demonstrating that two haem-binding motifs in the leader sequence, as well as one present in the N-terminus of the mature ALAS-1 function in vivo in the haem-regulated translocation of ALAS-1. Coproporphyrinogen oxidase, the antepenultimate pathway enzyme, possesses a leader sequence that is approx. 120 residues long. In contrast with an earlier report suggesting that only 30 residues were required for translocation of the coproporphyrinogen oxidase, we report that the complete leader is necessary for translocation and that this process is not haem-sensitive in vivo. PPO (protoporphyrinogen oxidase) lacks a typical mitochondrial targeting leader sequence and was found to be effectively targeted by just 17 N-terminal residues. Bacillus subtilis PPO, which is very similar to human PPO at its N-terminal end, is not targeted to the mitochondrion when expressed in mammalian cells, demonstrating that the translocation is highly specific with regard to both the length and spacing of charged residues in this targeting region. Ferrochelatase, the terminal enzyme, possesses a typical N-terminal leader sequence and no evidence of a role for the C-terminus was found in mitochondrial targeting.

Production and characterization of erythropoietic protoporphyric heterodimeric ferrochelatases

Mutations resulting in diminished activity of the dimeric enzyme ferrochelatase are a prerequisite for the inherited disorder erythropoietic protoporphyria (EPP). Patients with clinical EPP have only 10% to 30% of normal levels of ferrochelatase activity, and although many patients with EPP have one mutant allele and one "low-expression" normal allele, the possibility remains that, for some, low ferrochelatase activity may result from an EPP mutation that has an impact on both subunits of the wild-type/mutant heterodimer. Here we present data for 12 ferrochelatase wild-type/EPP mutant heterodimers showing that some mutations result in heterodimers with the residual activity anticipated from individual constituents, whereas others result in heterodimers with significantly lower activity than would be predicted. Although the data do not allow an a priori prediction of heterodimeric residual activity based solely on the in vitro activity of EPP homodimers or the position of the mutated residue within ferrochelatase, mutations that affect the dimer interface or [2Fe-2S] cluster have a significantly greater impact on residual activity than would be predicted. These data suggest that some EPP mutations may result in clinically overt EPP in the absence of a low-expression, wild-type allele; this is of potential significance for genetic counseling of patients with EPP.

The erythroid phenotype of EKLF-null mice: defects in hemoglobin metabolism and membrane stability

Development of red blood cells requires the correct regulation of cellular processes including changes in cell morphology, globin expression and heme synthesis. Transcription factors such as erythroid Kruppel-like factor EKLF (Klf1) play a critical role in erythropoiesis. Mice lacking EKLF die around embryonic day 14 because of defective definitive erythropoiesis, partly caused by a deficit in beta-globin expression. To identify additional target genes, we analyzed the phenotype and gene expression profiles of wild-type and EKLF null primary erythroid progenitors that were differentiated synchronously in vitro. We show that EKLF is dispensable for expansion of erythroid progenitors, but required for the last steps of erythroid differentiation. We identify EKLF-dependent genes involved in hemoglobin metabolism and membrane stability. Strikingly, expression of these genes is also EKLF-dependent in primitive, yolk sac-derived, blood cells. Consistent with lack of upregulation of these genes we find previously undetected morphological abnormalities in EKLF-null primitive cells. Our data provide an explanation for the hitherto unexplained severity of the EKLF null phenotype in erythropoiesis.

The effect of hypoxia and stem cell source on haemoglobin switching

This study investigated whether relative changes that accompany the naturally occurring shifts in haematopoietic sites during human development play a role in haemoglobin (Hb) switching or whether Hb switching is innately programmed into cells. CD34(+)/Lineage(-) haematopoietic stem/progenitor cells (HSCs) were isolated from human fetal liver (F-LVR), cord blood (CB), and adult bone marrow (ABM), and the Hb was characterized by flow cytometry on cultures that generated enucleated red cells. All feeder layers (stroma from F-LVR, ABM, and human fetal aorta) enhanced cell proliferation and erythropoiesis but did not affect Hb type. HSCs from CB and F-LVR generated the same Hb profile under normoxia and hypoxia. HSCs from ABM had single-positive HbA and double-positive HbA and HbF cells at normoxia and almost entirely double-positive cells at hypoxia. Further characterization of these ABM cultures was determined by following mRNA expression for the transcription factors erythroid Kruppel-like factor (EKLF) and fetal Kruppel-like factor (FKLF) as a function of time in cultures under hypoxia and normoxia. The erythroid-specific isoform of 5-amino-levulinate synthase (ALAS2) was also expressed under hypoxic conditions. We conclude that Hb switching is affected by the environment but not all HSCs are preprogrammed to respond.