Heme as a magnificent molecule with multiple missions: heme determines its own fate and governs cellular homeostasis

Identification of Genes for Synthesis of the Blue Pigment, Biliverdin IXα, in the Blue Coral Heliopora coerulea

Heliopora coerulea is the only species in the subclass Octocorallia that has a crystalline aragonite skeleton. The skeleton has been reported to contain the blue pigment, biliverdin IXα, which is formed by heme oxygenase (HO) during heme decomposition. There is little information regarding gene expression in H. coerulea; therefore, the biosynthesis pathway for biliverdin IXα is poorly understood. To identify the genes related to heme synthesis and degradation, metatranscripts of H. coerulea and its symbiont Symbiodinium spp. were sequenced and separated from the host- and symbiont-derived sequences. From the metatranscriptome analyses, all genes for heme synthesis and three HOs were isolated from the host and symbiont. From our phylogenetic and amino acid analysis, we noted that one of the HO isoforms in the host coral was predicted to possess HO activity. However, biliverdin reductase, which reduces biliverdin to bilirubin, was not identified in the present study. Similarly, biliverdin reductase was not identified in the transcripts of the red coral Corallium rubrum, a species that also belongs to Octocorallia. However, genes related to heme synthesis and HO were found in C. rubrum. We speculate that Heliopora coerulea can produce biliverdin and accumulate it in the skeleton, while red corals and other Octocorallia species cannot. Further information from molecular studies of H. coerulea will provide insights into the synthesis of biliverdin IXα, the blue pigment in the hard crystalline aragonite skeleton, and will be fundamental to future ecological and physiological studies.

Activated Oncogenic Pathway Modifies Iron Network in Breast Epithelial Cells: A Dynamic Modeling Perspective

Dysregulation of iron metabolism in cancer is well documented and it has been suggested that there is interdependence between excess iron and increased cancer incidence and progression. In an effort to better understand the linkages between iron metabolism and breast cancer, a predictive mathematical model of an expanded iron homeostasis pathway was constructed that includes species involved in iron utilization, oxidative stress response and oncogenic pathways. The model leads to three predictions. The first is that overexpression of iron regulatory protein 2 (IRP2) recapitulates many aspects of the alterations in free iron and iron-related proteins in cancer cells without affecting the oxidative stress response or the oncogenic pathways included in the model. This prediction was validated by experimentation. The second prediction is that iron-related proteins are dramatically affected by mitochondrial ferritin overexpression. This prediction was validated by results in the pertinent literature not used for model construction. The third prediction is that oncogenic Ras pathways contribute to altered iron homeostasis in cancer cells. This prediction was validated by a combination of simulation experiments of Ras overexpression and catalase knockout in conjunction with the literature. The model successfully captures key aspects of iron metabolism in breast cancer cells and provides a framework upon which more detailed models can be built.

Iron is required for cellular metabolism and growth, but can be toxic due to its ability to cause high oxidative stress and consequently DNA damage. To prevent damage, all organisms that require iron have developed mechanisms to tightly control iron levels. Dysregulation of iron metabolism is detrimental and can contribute to a wide range of diseases, including cancer. This paper presents a predictive mathematical model of iron regulation linked to iron utilization, oxidative stress, and the oncogenic response specific to normal breast epithelial cells. The model uses a discrete modeling framework to generate novel biological hypotheses for an investigation of how normal breast cells become malignant cells, capturing a breast cancer phenotype of iron homeostasis through overexpression and knockout simulations. The new biology discovered is (1) IRP2 overexpression alters the iron homeostasis pathway in breast cells, without affecting the oxidative stress response or oncogenic pathways, (2) an activated oncogenic pathway disrupts iron regulation in breast cancer cells.

Impact of TET2 deficiency on iron metabolism in erythroblasts

Sideroblastic anemia is characterized by the presence of ring sideroblasts (RSs), which are caused by iron accumulation in the mitochondria of erythroblasts and are present in both the acquired and congenital forms of the disease. However, the mechanism leading to RS formation remains elusive. Acquired sideroblastic anemia is usually observed in myelodysplastic syndrome (MDS). Because a subset of MDS harbors a somatic mutation of TET2, it may be involved in iron metabolism and/or heme biosynthesis in erythroblasts. Tet2 knockdown (Tet2^trap) induced exhibited mild normocytic anemia and elevated serum ferritin levels in 4-month-old mice. Although typical RSs were not observed, increased mitochondrial ferritin (FTMT) amounts were observed in the erythroblasts of Tet2-knockdown mice. Quantitative real-time polymerase chain reaction demonstrated significant dysregulation of genes involved in iron and heme metabolism, including Hmox1, Fech, Abcb7, and Sf3b1 downregulation. After the identification of a cytosine-guanine island in the promoters of Fech, Abcb7, and Sf3b1, we evaluated DNA methylation status and found significantly higher methylation levels at the CpG sites in the erythroblasts of Tet2-knockdown mice. Furthermore, Tet2 knockdown in erythroblasts resulted in decreased heme concentration and accumulation of FTMT. Therefore, TET2 plays a role in the iron and heme metabolism in erythroblasts.

Molecular dynamics analysis of the structural and dynamic properties of the functionally enhanced hepta-variant of mouse 5-aminolevulinate synthase

Heme biosynthesis, a complex, multistage, and tightly controlled process, starts with 5-aminolevulinate (ALA) production, which, in metazoa and certain bacteria, is a reaction catalyzed by 5-aminolevulinate synthase (ALAS), a pyridoxal 5'-phosphate (PLP)-dependent enzyme. Functional aberrations in ALAS are associated with several human diseases. ALAS can adopt open and closed conformations, with segmental rearrangements of a C-terminal, 16-amino acid loop and an α-helix regulating accessibility to the ALAS active site. Of the murine erythroid ALAS (mALAS2) forms previously engineered to assess the role of the flexible C-terminal loop versus mALAS2 function one stood out due to its impressive gain in catalytic power. To elucidate how the simultaneously introduced seven mutations of this activity-enhanced variant affected structural and dynamic properties of mALAS2, we conducted extensive molecular dynamics simulation analysis of the dimeric forms of wild-type mALAS2, hepta-variant and Rhodobacter capsulatus ALAS (aka R. capsulatus HemA). This analysis revealed that the seven simultaneous mutations in the C-terminal loop, which extends over the active site of the enzyme, caused the bacterial and murine proteins to adopt different conformations. Specifically, a new β-strand in the mutated 'loop' led to interaction with two preexisting β-strands and formation of an anti-parallel three-stranded β-sheet, which likely endowed the murine hepta-variant a more 'stable' open conformation than that of wild-type mALAS2, consistent with a kinetic mechanism involving a faster closed-to-open conformation transition and product release for the mutated than wild-type enzyme. Further, the dynamic behavior of the mALAS2 protomers was strikingly different in the two dimeric forms.

Differential Gene Expression in Menstrual Endometrium From Women With Self-Reported Heavy Menstrual Bleeding

Heavy menstrual bleeding (HMB) is a significant social and public health issue for menstruating women. Development of targeted treatments has been limited by poor understanding of local mechanisms underlying HMB. We aimed to determine how gene expression differs in menstrual phase endometrium from women with HMB. Menstrual phase endometrial biopsies were collected from women with (n = 7) and without (n = 10) HMB (regular menstrual cycles, no known pelvic pathology), as well as women with uterine fibroids (n = 7, n = 4 had HMB). Biopsies were analyzed using Illumina Sentrix Human HT12 arrays and data analyzed using "Remove Unwanted Variation-inverse". Ingenuity Pathway Analysis and the Database for Annotation, Visualization and Integrated Discovery v6.7 were used to identify gene pathways, functional gene clusters, and upstream regulators specific to the clinical groupings. Individual genes of interest were examined using quantitative polymerase chain reaction. In total, 829 genes were differentially expressed in one or more comparisons. Significant canonical pathways and gene clusters enriched in controls relative to both HMB and fibroid groups suggest the mechanisms responsible for HMB include modifications of the endometrial inflammatory or infection response. In contrast, differentially expressed genes in women with fibroids suggest modifications of hemoglobin, antigen processing, and the major histocompatibility complex (class II, beta chain) activity. In conclusion, HMB associated with fibroids may be regulated by different endometrial mechanisms from HMB in women without fibroids and from normal menstrual bleeding. These novel data provide numerous testable hypotheses that will advance our understanding of the mechanisms responsible for HMB.

Novel Mechanisms for Heme-dependent Degradation of ALAS1 Protein as a Component of Negative Feedback Regulation of Heme Biosynthesis

In eukaryotic cells, heme production is tightly controlled by heme itself through negative feedback-mediated regulation of nonspecific 5-aminolevulinate synthase (ALAS1), which is a rate-limiting enzyme for heme biosynthesis. However, the mechanism driving the heme-dependent degradation of the ALAS1 protein in mitochondria is largely unknown. In the current study, we provide evidence that the mitochondrial ATP-dependent protease ClpXP, which is a heteromultimer of CLPX and CLPP, is involved in the heme-dependent degradation of ALAS1 in mitochondria. We found that ALAS1 forms a complex with ClpXP in a heme-dependent manner and that siRNA-mediated suppression of either CLPX or CLPP expression induced ALAS1 accumulation in the HepG2 human hepatic cell line. We also found that a specific heme-binding motif on ALAS1, located at the N-terminal end of the mature protein, is required for the heme-dependent formation of this protein complex. Moreover, hemin-mediated oxidative modification of ALAS1 resulted in the recruitment of LONP1, another ATP-dependent protease in the mitochondrial matrix, into the ALAS1 protein complex. Notably, the heme-binding site in the N-terminal region of the mature ALAS1 protein is also necessary for the heme-dependent oxidation of ALAS1. These results suggest that ALAS1 undergoes a conformational change following the association of heme to the heme-binding motif on this protein. This change in the structure of ALAS1 may enhance the formation of complexes between ALAS1 and ATP-dependent proteases in the mitochondria, thereby accelerating the degradation of ALAS1 protein to maintain appropriate intracellular heme levels.

Evolution of the SOUL Heme-Binding Protein Superfamily Across Eukarya

SOUL homologs constitute a heme-binding protein superfamily putatively involved in heme and tetrapyrrole metabolisms associated with a number of physiological processes. Despite their omnipresence across the tree of life and the biochemical characterization of many SOUL members, their functional role and the evolutionary events leading to such remarkable protein repertoire still remain cryptic. To explore SOUL evolution, we apply a computational phylogenetic approach, including a relevant number of SOUL homologs, to identify paralog forms and reconstruct their genealogy across the tree of life and within species. In animal lineages, multiple gene duplication or loss events and paralog functional specializations underlie SOUL evolution from the dawn of ancestral echinoderm and mollusc SOUL forms. In photosynthetic organisms, SOUL evolution is linked to the endosymbiosis events leading to plastid acquisition in eukaryotes. Derivative features, such as the F2L peptide and BH3 domain, evolved in vertebrates and provided innovative functionality to support immune response and apoptosis. The evolution of elements such as the N-terminal protein domain DUF2358, the His42 residue, or the tetrapyrrole heme-binding site is modern, and their functional implications still unresolved. This study represents the first in-depth analysis of SOUL protein evolution and provides novel insights in the understanding of their obscure physiological role.

Neural clocks and Neuropeptide F/Y regulate circadian gene expression in a peripheral metabolic tissue

Metabolic homeostasis requires coordination between circadian clocks in different tissues. Also, systemic signals appear to be required for some transcriptional rhythms in the mammalian liver and the Drosophila fat body. Here we show that free-running oscillations of the fat body clock require clock function in the PDF-positive cells of the fly brain. Interestingly, rhythmic expression of the cytochrome P450 transcripts, sex-specific enzyme 1 (sxe1) and Cyp6a21, which cycle in the fat body independently of the local clock, depends upon clocks in neurons expressing neuropeptide F (NPF). NPF signaling itself is required to drive cycling of sxe1 and Cyp6a21 in the fat body, and its mammalian ortholog, Npy, functions similarly to regulate cycling of cytochrome P450 genes in the mouse liver. These data highlight the importance of neuronal clocks for peripheral rhythms, particularly in a specific detoxification pathway, and identify a novel and conserved role for NPF/Npy in circadian rhythms.

DOI:http://dx.doi.org/10.7554/eLife.13552.001

Many processes in the body follow rhythms that repeat over 24 hours and are synchronized to the cycle of day and night. Our sleep pattern is a well-known example, but others include daily fluctuations in body temperature and the production of several hormones. Internal clocks located in the brain and other organs drive these rhythms by altering the activity of certain genes depending on the time of day.

Animals have specific organs that contain enzymes needed to break down toxic molecules in the body, and the levels of several of these enzymes rise and fall over each 24-hour period. In mammals, these enzymes are found in the liver, but in insects they are found in an organ called the fat body. Here, Erion, King et al. set out to determine the extent to which the internal clock in the brain influences the daily rhythms of these enzymes.

The experiments show that a hormone released by the nervous system is required for the levels of the detoxifying enzymes to change in 24-hour cycles. This hormone – termed Neuropeptide F in fruit flies and Neuropeptide Y in mice – is also known to stimulate both mice and fruit flies to eat. Since toxic molecules often enter the body during feeding, Erion, King et al. speculate that it may be beneficial to link the detoxification process to feeding by using the same mechanism to control both processes. The next step following on from this work would be to find out exactly how neuropeptide F drives the 24-hour rhythms in the fat body and other organs.

DOI:http://dx.doi.org/10.7554/eLife.13552.002

Identification of a novel putative mitochondrial protein FAM210B associated with erythroid differentiation

The transcription factor GATA-1 plays an essential role in erythroid differentiation. To identify novel GATA-1 target genes, we analyzed a merged ChIP-seq and expression profiling dataset. We identified FAM210B as a putative novel GATA-1 target gene. Study results demonstrated that GATA-1 directly regulates FAM210B expression, presumably by binding to an intronic enhancer region. Both human and murine FAM210B are abundantly expressed in the later stages of erythroblast development. Moreover, the deduced amino acid sequence predicted that FAM210B is a membrane protein, and Western blot analysis demonstrated its mitochondrial localization. Loss-of-function analysis in erythroid cells suggested that FAM210B may be involved in erythroid differentiation. The identification and characterization of FAM210B provides new insights in the study of erythropoiesis and hereditary anemias.

Acquisition of exogenous haem is essential for tick reproduction

Haem and iron homeostasis in most eukaryotic cells is based on a balanced flux between haem biosynthesis and haem oxygenase-mediated degradation. Unlike most eukaryotes, ticks possess an incomplete haem biosynthetic pathway and, together with other (non-haematophagous) mites, lack a gene encoding haem oxygenase. We demonstrated, by membrane feeding, that ticks do not acquire bioavailable iron from haemoglobin-derived haem. However, ticks require dietary haemoglobin as an exogenous source of haem since, feeding with haemoglobin-depleted serum led to aborted embryogenesis. Supplementation of serum with haemoglobin fully restored egg fertility. Surprisingly, haemoglobin could be completely substituted by serum proteins for the provision of amino-acids in vitellogenesis. Acquired haem is distributed by haemolymph carrier protein(s) and sequestered by vitellins in the developing oocytes. This work extends, substantially, current knowledge of haem auxotrophy in ticks and underscores the importance of haem and iron metabolism as rational targets for anti-tick interventions.

DOI:http://dx.doi.org/10.7554/eLife.12318.001

Ticks are small blood-feeding parasites that transmit a range of diseases through their bites, including Lyme disease and encephalitis in humans. Like other blood-feeders, ticks acquire essential nutrients from their host in order to develop and reproduce.

Iron and haem (the iron-containing part of haemoglobin) are essential for the metabolism of every breathing animal on Earth. Most organisms obtain iron by degrading haem and, reciprocally, most of the iron in cells is used to make haem. However, an initial search of existing genome databases revealed that ticks lack the genes required to make the proteins that make and degrade haem.

Perner et al. wanted to find out if ticks can steal haem from the host and use it for their own development. To achieve this, Perner et al. exploited a method of tick membrane feeding that simulates natural feeding on a host by using a silicone imitation of a skin and cow smell extracts (“l´odeur de vache”). Ticks were fed either a haemoglobin-rich (whole blood) or a haemoglobin-poor (serum) diet. This experiment revealed that ticks can develop normally without haemoglobin, but female ticks fed a haemoglobin-poor diet lay sterile eggs out of which no offspring can hatch.

Further investigation showed that haemoglobin is vitally important as a source of haem but not as a source of the amino acids needed to produce the vitellin proteins that nourish embryos. As ticks are not armed with the ability to degrade haem, they do not acquire iron from the host haem but rather from a serum transferrin, a major iron transporter protein found in mammalian blood. Further experiments revealed that ticks have evolved proteins that can transport and store haem and so make the obtained haem available across the whole tick body.

Overall, Perner et al.’s findings suggest that targeting the mechanisms by which ticks metabolise haem and iron could lead to the design of new “anti-tick” strategies.

DOI:http://dx.doi.org/10.7554/eLife.12318.002

ApoHRP-based assay to measure intracellular regulatory heme

The majority of the heme-binding proteins possess a "heme-pocket" that stably binds to heme. Usually known as housekeeping heme-proteins, they participate in a variety of metabolic reactions (e.g., catalase). Heme also binds with lower affinity to the "Heme-Regulatory Motifs" (HRM) in specific regulatory proteins. This type of heme binding is known as exchangeable or regulatory heme (RH). Heme binding to HRM proteins regulates their function (e.g., Bach1). Although there are well-established methods for assaying total cellular heme (e.g., heme-proteins plus RH), currently there is no method available for measuring RH independent of the total heme (TH). The current study describes and validates a new method to measure intracellular RH. This method is based on the reconstitution of apo-horseradish peroxidase (apoHRP) with heme to form holoHRP. The resulting holoHRP activity is then measured with a colorimetric substrate. The results show that apoHRP specifically binds RH but not with heme from housekeeping heme-proteins. The RH assay detects intracellular RH. Furthermore, using conditions that create positive (hemin) or negative (N-methyl protoporphyrin IX) controls for heme in normal human fibroblasts (IMR90), the RH assay shows that RH is dynamic and independent of TH. We also demonstrated that short-term exposure to subcytotoxic concentrations of lead (Pb), mercury (Hg), or amyloid-β (Aβ) significantly alters intracellular RH with little effect on TH. In conclusion the RH assay is an effective assay to investigate intracellular RH concentration and demonstrates that RH represents ∼6% of total heme in IMR90 cells.

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.

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.

Measurement of Heme Synthesis Levels in Mammalian Cells

Heme serves as the prosthetic group for a wide variety of proteins known as hemoproteins, such as hemoglobin, myoglobin and cytochromes. It is involved in various molecular and cellular processes such as gene transcription, translation, cell differentiation and cell proliferation. The biosynthesis levels of heme vary across different tissues and cell types and is altered in diseased conditions such as anemia, neuropathy and cancer. This technique uses [4-(14)C] 5-aminolevulinic acid ([(14)C] 5-ALA), one of the early precursors in the heme biosynthesis pathway to measure the levels of heme synthesis in mammalian cells. This assay involves incubation of cells with [(14)C] 5-ALA followed by extraction of heme and measurement of the radioactivity incorporated into heme. This procedure is accurate and quick. This method measures the relative levels of heme biosynthesis rather than the total heme content. To demonstrate the use of this technique the levels of heme biosynthesis were measured in several mammalian cell lines.

Iron Homeostasis and Trypanosoma brucei Associated Immunopathogenicity Development: A Battle/Quest for Iron

African trypanosomosis is a chronic debilitating disease affecting the health and economic well-being of developing countries. The immune response during African trypanosome infection consisting of a strong proinflammatory M1-type activation of the myeloid phagocyte system (MYPS) results in iron deprivation for these extracellular parasites. Yet, the persistence of M1-type MYPS activation causes the development of anemia (anemia of chronic disease, ACD) as a most prominent pathological parameter in the mammalian host, due to enhanced erythrophagocytosis and retention of iron within the MYPS thereby depriving iron for erythropoiesis. In this review we give an overview of how parasites acquire iron from the host and how iron modulation of the host MYPS affects trypanosomosis-associated anemia development. Finally, we also discuss different strategies at the level of both the host and the parasite that can/might be used to modulate iron availability during African trypanosome infections.

Global analysis of posttranscriptional gene expression in response to sodium arsenite

BACKGROUND

Inorganic arsenic species are potent environmental toxins and causes of numerous health problems. Most studies have assumed that arsenic-induced changes in mRNA levels result from effects on gene transcription.

OBJECTIVES

We evaluated the prevalence of changes in mRNA stability in response to sodium arsenite in human fibroblasts.

METHODS

We used microarray analyses to determine changes in steady-state mRNA levels and mRNA decay rates following 24-hr exposure to noncytotoxic concentrations of sodium arsenite, and we confirmed some of these changes using real-time reverse-transcription polymerase chain reaction (RT-PCR).

RESULTS

In arsenite-exposed cells, 186 probe set-identified transcripts were significantly increased and 167 were significantly decreased. When decay rates were analyzed after actinomycin D treatment, only 4,992 (9.1%) of probe set-identified transcripts decayed by > 25% after 4 hr. Of these, 70 were among the 353 whose steady-state levels were altered by arsenite, and of these, only 4 exhibited significantly different decay rates between arsenite and control treatment. Real-time RT-PCR confirmed a major, significant arsenite-induced stabilization of the mRNA encoding δ aminolevulinate synthase 1 (ALAS1), the rate-limiting enzyme in heme biosynthesis. This change presumably accounted for at least part of the 2.7-fold increase in steady-state ALAS1 mRNA levels seen after arsenite treatment. This could reflect decreases in cellular heme caused by the massive induction by arsenite of heme oxygenase mRNA (HMOX1; 68-fold increase), the rate-limiting enzyme in heme catabolism.

CONCLUSIONS

We conclude that arsenite modification of mRNA stability is relatively uncommon, but in some instances can result in significant changes in gene expression.

Heme exporter FLVCR is required for T cell development and peripheral survival

All aerobic cells and organisms must synthesize heme from the amino acid glycine and the tricarboxylic acid cycle intermediate succinyl CoA for incorporation into hemoproteins, such as the cytochromes needed for oxidative phosphorylation. Most studies on heme regulation have been done in erythroid cells or hepatocytes; however, much less is known about heme metabolism in other cell types. The feline leukemia virus subgroup C receptor (FLVCR) is a 12-transmembrane domain surface protein that exports heme from cells, and it was shown to be required for erythroid development. In this article, we show that deletion of Flvcr in murine hematopoietic precursors caused a complete block in αβ T cell development at the CD4(+)CD8(+) double-positive stage, although other lymphoid lineages were not affected. Moreover, FLVCR was required for the proliferation and survival of peripheral CD4(+) and CD8(+) T cells. These studies identify a novel and unexpected role for FLVCR, a major facilitator superfamily metabolite transporter, in T cell development and suggest that heme metabolism is particularly important in the T lineage.

Complicity of haem in some adverse drug-reactions

Genetic variants in haem metabolism enzymes can be predisposition factors for adverse reactions in some individuals. New areas of haem biology may also be associated with idiosyncratic effects which are yet to be identified.

Aminolevulinate synthase 2 mediates erythrocyte differentiation by regulating larval globin expression during Xenopus primary hematopoiesis

Hemoglobin synthesis by erythrocytes continues throughout a vertebrate's lifetime. The mechanism of mammalian heme synthesis has been studied for many years; aminolevulinate synthase 2 (ALAS2), a heme synthetase, is associated with X-linked dominant protoporphyria in humans. Amphibian and mammalian blood cells differ, but little is known about amphibian embryonic hemoglobin synthesis. We investigated the function of the Xenopus alas2 gene (Xalas2) in primitive amphibian erythrocytes and found that it is first expressed in primitive erythroid cells before hemoglobin alpha 3 subunit (hba3) during primary hematopoiesis and in the posterior ventral blood islands at the tailbud stage. Xalas2 is not expressed during secondary hematopoiesis in the dorsal lateral plate. Hemoglobin was barely detectable by o-dianisidine staining and hba3 transcript levels decreased in Xalas2-knockdown embryos. These results suggest that Xalas2 might be able to synthesize hemoglobin during hematopoiesis and mediate erythrocyte differentiation by regulating hba3 expression in Xenopus laevis.

Time-dependent effects of the flame retardant tris(1,3-dichloro-2-propyl) phosphate (TDCPP) on mRNA expression, in vitro and in ovo, reveal optimal sampling times for rapidly metabolized compounds

The flame retardant, tris(1,3-dichloro-2-propyl) phosphate (TDCPP), was previously shown to affect chicken embryo growth, gallbladder size, and lipid homeostasis. A microarray study, however, revealed only modest transcriptional alterations in liver tissue of pipping embryos (days 20-21), which was attributed to the rapid metabolism of TDCPP throughout incubation. To identify the most appropriate sampling time for rapidly metabolized compounds, the present study assessed the time-dependent effects of TDCPP on 27 genes, in ovo (50 µg [116 nmol] TDCPP/g egg) and in vitro (10 µM), using a chicken ToxChip polymerase chain reaction array. The greatest magnitude in dysregulation (up to 362-fold) occurred on day 8 of incubation (in ovo) with alterations of genes involved in phase I, II, and III metabolism, among others. Gallbladder hypotrophy was observed by embryonic day 12, corroborating the finding in pipping embryos from our previous study. From days 12 to 19, genes involved in lipid homeostasis, steroid hormone metabolism, and oxidative stress were affected. In chicken embryonic hepatoctyes (CEHs), TDCPP was completely metabolized to bis(1,3-dichloro-2-propyl) phosphate (BDCPP) within 36 h, but transcriptional changes remained significant up to 36 h. These changes were not attributed to BDCPP exposure as it only altered 1 gene (CYP1A4). An 18-h exposure in CEHs altered the greatest number of genes, making it an appropriate time point for high-throughput chemical screening; however, depending on the biological pathways of interest, shorter or longer incubation times may be more informative. Overall, TDCPP elicits the transcriptional and phenotypic alterations observed in vitro and in ovo, whereas its major metabolite, BDCPP, is far less biologically active.

Effect of 5-aminolevulinic acid on erythropoiesis: a preclinical in vitro characterization for the treatment of congenital sideroblastic anemia

Congenital sideroblastic anemia (CSA) is a hereditary disorder characterized by microcytic anemia and bone marrow sideroblasts. The most common form of CSA is attributed to mutations in the X-linked gene 5-aminolevulinic acid synthase 2 (ALAS2). ALAS2 is a mitochondrial enzyme, which utilizes glycine and succinyl-CoA to form 5-aminolevulinic acid (ALA), a crucial precursor in heme synthesis. Therefore, ALA supplementation could be an effective therapeutic strategy to restore heme synthesis in CSA caused by ALAS2 defects. In a preclinical study, we examined the effects of ALA in human erythroid cells, including K562 cells and human induced pluripotent stem cell-derived erythroid progenitor (HiDEP) cells. ALA treatment resulted in significant dose-dependent accumulation of heme in the K562 cell line. Concomitantly, the treatment substantially induced erythroid differentiation as assessed using benzidine staining. Quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis confirmed significant upregulation of heme-regulated genes, such as the globin genes [hemoglobin alpha (HBA) and hemoglobin gamma (HBG)] and the heme oxygenase 1 (HMOX1) gene, in K562 cells. Next, to investigate the mechanism by which ALA is transported into erythroid cells, quantitative RT-PCR analysis was performed on previously identified ALA transporters, including solute carrier family 15 (oligopeptide transporter), member (SLC15A) 1, SLC15A2, solute carrier family 36 (proton/amino acid symporter), member (SLC36A1), and solute carrier family 6 (neurotransmitter transporter), member 13 (SLC6A13). Our analysis revealed that SLC36A1 was abundantly expressed in erythroid cells. Thus, gamma-aminobutyric acid (GABA) was added to K562 cells to competitively inhibit SLC36A1-mediated transport. GABA treatment significantly impeded the ALA-mediated increase in the number of hemoglobinized cells as well as the induction of HBG, HBA, and HMOX1. Finally, small-interfering RNA-mediated knockdown of ALAS2 in HiDEP cells considerably decreased the expression of HBA, HBG, and HMOX1, and these expression levels were rescued with ALA treatment. In summary, ALA appears to be transported into erythroid cells mainly by SLC36A1 and is utilized to generate heme. ALA may represent a novel therapeutic option for CSA treatment, particularly for cases harboring ALAS2 mutations.

Protein/protein interactions in the mammalian heme degradation pathway: heme oxygenase-2, cytochrome P450 reductase, and biliverdin reductase

Heme oxygenase (HO) catalyzes the rate-limiting step in the O2-dependent degradation of heme to biliverdin, CO, and iron with electrons delivered from NADPH via cytochrome P450 reductase (CPR). Biliverdin reductase (BVR) then catalyzes conversion of biliverdin to bilirubin. We describe mutagenesis combined with kinetic, spectroscopic (fluorescence and NMR), surface plasmon resonance, cross-linking, gel filtration, and analytical ultracentrifugation studies aimed at evaluating interactions of HO-2 with CPR and BVR. Based on these results, we propose a model in which HO-2 and CPR form a dynamic ensemble of complex(es) that precede formation of the productive electron transfer complex. The (1)H-(15)N TROSY NMR spectrum of HO-2 reveals specific residues, including Leu-201, near the heme face of HO-2 that are affected by the addition of CPR, implicating these residues at the HO/CPR interface. Alanine substitutions at HO-2 residues Leu-201 and Lys-169 cause a respective 3- and 22-fold increase in K(m) values for CPR, consistent with a role for these residues in CPR binding. Sedimentation velocity experiments confirm the transient nature of the HO-2 · CPR complex (K(d) = 15.1 μM). Our results also indicate that HO-2 and BVR form a very weak complex that is only captured by cross-linking. For example, under conditions where CPR affects the (1)H-(15)N TROSY NMR spectrum of HO-2, BVR has no effect. Fluorescence quenching experiments also suggest that BVR binds HO-2 weakly, if at all, and that the previously reported high affinity of BVR for HO is artifactual, resulting from the effects of free heme (dissociated from HO) on BVR fluorescence.

Heme exporter FLVCR1a regulates heme synthesis and degradation and controls activity of cytochromes P450

BACKGROUND & AIMS

The liver has one of the highest rates of heme synthesis of any organ. More than 50% of the heme synthesized in the liver is used for synthesis of P450 enzymes, which metabolize exogenous and endogenous compounds that include natural products, hormones, drugs, and carcinogens. Feline leukemia virus subgroup C cellular receptor 1a (FLVCR1a) is plasma membrane heme exporter that is ubiquitously expressed and controls intracellular heme content in hematopoietic lineages. We investigated the role of Flvcr1a in liver function in mice.

METHODS

We created mice with conditional disruption of Mfsd7b, which encodes Flvcr1a, in hepatocytes (Flvcr1a(fl/fl);alb-cre mice). Mice were analyzed under basal conditions, after phenylhydrazine-induced hemolysis, and after induction of cytochromes P450 synthesis. Livers were collected and analyzed by histologic, quantitative real-time polymerase chain reaction, and immunoblot analyses. Hepatic P450 enzymatic activities were measured.

RESULTS

Flvcr1a(fl/fl);alb-cre mice accumulated heme and iron in liver despite up-regulation of heme oxygenase 1, ferroportin, and ferritins. Hepatic heme export activity of Flvcr1a was closely associated with heme biosynthesis, which is required to sustain cytochrome induction. Upon cytochromes P450 stimulation, Flvcr1a(fl/fl);alb-cre mice had reduced cytochrome activity, associated with accumulation of heme in hepatocytes. The expansion of the cytosolic heme pool in these mice was likely responsible for the early inhibition of heme synthesis and increased degradation of heme, which reduced expression and activity of cytochromes P450.

CONCLUSIONS

In livers of mice, Flvcr1a maintains a free heme pool that regulates heme synthesis and degradation as well as cytochromes P450 expression and activity. These findings have important implications for drug metabolism.

Common defects of mitochondria and iron in neurodegeneration and diabetes (MIND): a paradigm worth exploring

A popular, if not centric, approach to the study of an event is to first consider that of the simplest cause. When dissecting the underlying mechanisms governing idiopathic diseases, this generally takes the form of an ab initio genetic approach. To date, this genetic 'smoking gun' has remained elusive in diabetes mellitus and for many affected by neurodegenerative diseases. With no single gene, or even subset of genes, conclusively causative in all cases, other approaches to the etiology and treatment of these diseases seem reasonable, including the correlation of a systems' predisposed sensitivity to particular influence. In the cases of diabetes mellitus and neurodegenerative diseases, overlapping themes of mitochondrial influence or dysfunction and iron dyshomeostasis are apparent and relatively consistent. This mini-review discusses the influence of mitochondrial function and iron homeostasis on diabetes mellitus and neurodegenerative disease, namely Alzheimer's disease. Also discussed is the incidence of diabetes accompanied by neuropathy and neurodegeneration along with neurodegenerative disorders prone to development of diabetes. Mouse models containing multiple facets of this overlap are also described alongside current molecular trends attributed to both diseases. As a way of approaching the idiopathic and complex nature of these diseases we are proposing the consideration of a MIND (mitochondria, iron, neurodegeneration, and diabetes) paradigm in which systemic metabolic influence, iron homeostasis, and respective genetic backgrounds play a central role in the development of disease.

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.

Assessment of hematopoietic failure due to Rpl11 deficiency in a zebrafish model of Diamond-Blackfan anemia by deep sequencing

Background

Diamond–Blackfan anemia is a rare congenital red blood cell dysplasia that develops soon after birth. RPL11 mutations account for approximately 4.8% of human DBA cases with defective hematopoietic phenotypes. However, the mechanisms by which RPL11 regulates hematopoiesis in DBA remain elusive. In this study, we analyzed the transcriptome using deep sequencing data from an Rpl11-deficient zebrafish model to identify Rpl11-mediated hematopoietic failure and investigate the underlying mechanisms.

Results

We characterized hematological defects in Rpl11-deficient zebrafish embryos by identifying affected hematological genes, hematopoiesis-associated pathways, and regulatory networks. We found that hemoglobin biosynthetic and hematological defects in Rpl11-deficient zebrafish were related to dysregulation of iron metabolism-related genes, including tfa, tfr1b, alas2 and slc25a37, which are involved in heme and hemoglobin biosynthesis. In addition, we found reduced expression of the hematopoietic stem cells (HSC) marker cmyb and HSC transcription factors tal1 and hoxb4a in Rpl11-deficient zebrafish embryos, indicating that the hematopoietic defects may be related to impaired HSC formation, differentiation, and proliferation. However, Rpl11 deficiency did not affect the development of other blood cell lineages such as granulocytes and myelocytes.

Conclusion

We identified hematopoietic failure of Rpl11-deficient zebrafish embryos using transcriptome deep sequencing and elucidated potential underlying mechanisms. The present analyses demonstrate that Rpl11-deficient zebrafish may serve as a model of DBA and may provide insights into the pathogenesis of mutant RPL11-mediated human DBA disease.

Pathophysiology and genetic mutations in congenital sideroblastic anemia

Sideroblastic anemias are heterogeneous congenital and acquired disorders characterized by anemia and the presence of ringed sideroblasts in the bone marrow. Congenital sideroblastic anemia (CSA) is a rare disease caused by mutations of genes involved in heme biosynthesis, iron-sulfur [Fe-S] cluster biosynthesis, and mitochondrial protein synthesis. The most common form is X-linked sideroblastic anemia, due to mutations in the erythroid-specific δ-aminolevulinate synthase (ALAS2), which is the first enzyme of the heme biosynthesis pathway in erythroid cells. Other known etiologies include mutations in the erythroid specific mitochondrial transporter (SLC25A38), adenosine triphosphate (ATP) binding cassette B7 (ABCB7), glutaredoxin 5 (GLRX5), thiamine transporter SLC19A2, the RNA-modifying enzyme pseudouridine synthase (PUS1), and mitochondrial tyrosyl-tRNA synthase (YARS2), as well as mitochondrial DNA deletions. Due to its rarity, however, there have been few systematic pathophysiological and genetic investigations focusing on sideroblastic anemia. Therefore, a nationwide survey of sideroblastic anemia was conducted in Japan to investigate the epidemiology and pathogenesis of this disease. This review will cover the findings of this recent survey and summarize the current understanding of the pathophysiology and genetic mutations involved in CSA.

Chronopharmacology: new insights and therapeutic implications

Most facets of mammalian physiology and behavior vary according to time of day, thanks to endogenous circadian clocks. Therefore, it is not surprising that many aspects of pharmacology and toxicology also oscillate according to the same 24-h clocks. Daily oscillations in abundance of proteins necessary for either drug absorption or metabolism result in circadian pharmacokinetics, and oscillations in the physiological systems targeted by these drugs result in circadian pharmacodynamics. These clocks are present in most cells of the body, organized in a hierarchical fashion. Interestingly, some aspects of physiology and behavior are controlled directly via a "master clock" in the suprachiasmatic nuclei of the hypothalamus, whereas others are controlled by "slave" oscillators in separate brain regions or body tissues. Recent research shows that these clocks can respond to different cues and thereby show different phase relationships. Therefore, full prediction of chronopharmacology in pathological contexts will likely require a systems biology approach that considers chronointeractions among different clock-regulated systems.

FLVCR is necessary for erythroid maturation, may contribute to platelet maturation, but is dispensable for normal hematopoietic stem cell function

Heme is a pleiotropic molecule that is important for oxygen and oxidative metabolism, most notably as the prosthetic group of hemoglobin and cytochromes. Because excess free intracellular heme is toxic, organisms have developed mechanisms to tightly regulate its concentration. One mechanism is through active heme export by the group C feline leukemia virus receptor (FLVCR). Previously, we have shown that FLVCR is necessary for embryonic and postnatal erythropoiesis. However, FLVCR is also expressed in numerous other tissues, including hematopoietic stem cells (HSCs). To explore a possible role for FLVCR in HSC function, we performed serial, competitive repopulation transplant experiments using FLVCR-deleted and control bone marrow cells, along with wild-type competitor cells. Loss of FLVCR did not impact HSC function under steady-state or myelotoxic stress conditions (such as arsenic or radiation exposure), nor did FLVCR deletion result in alterations in the various progenitor compartments. However, even when 95% of the donor bone marrow cells lacked FLVCR, all red cells in recipient mice were wild type. This is due to the increased apoptosis of FLVCR-deleted proerythroblasts. Also, remarkably, loss of FLVCR increased megakaryocyte ploidy. Together, these findings show FLVCR is redundant in stem cells but has critical and contrasting stage-specific roles in discrete hematopoietic lineages.

Analysis of Fe(III) heme binding to cysteine-containing heme-regulatory motifs in proteins

Regulatory heme binds to specific motifs in proteins and controls a variety of biochemical processes. Several of these proteins were recently shown to form complexes with ferric and/or ferrous heme via a cysteine residue as axial ligand. The objective of this study was to examine the heme-binding properties of a series of cysteine-containing peptides with focus on CP motif sequences. The peptides displayed different binding behavior upon Fe(III) heme application with characteristic wavelength shifts of the Soret band to 370 nm or 420-430 nm and in some cases to both wavelengths. Whereas for most of the peptides containing a cysteine only a shift to 420-430 nm was observed, CP-containing peptides exhibited a preference for a shift to 370 nm. Detailed structural investigation using Raman and NMR spectroscopy on selected representatives revealed different binding modes with respect to iron ion coordination, which reflected the results of the UV-vis studies. A predicted short sequence stretch derived from dipeptidyl peptidase 8 was additionally examined with respect to CP motif binding to heme on the peptide as well as on the protein level. The heme association was confirmed with the first solution structure of a CP-peptide-heme complex and, moreover, an inhibitory effect of Fe(III) heme on the enzyme's activity. The relevance of both the use of model compounds to elucidate the molecular mechanism underlying regulatory heme binding and its potential for the investigation of regulatory heme control is discussed.

Coordinated expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 and heme oxygenase 2: evidence for a regulatory link between glycolysis and heme catabolism

Heme is an essential requirement for cell survival. Heme oxygenase (HO) is the rate-limiting enzyme in heme catabolism and consists of two isozymes, HO-1 and HO-2. To identify the protein that regulates the expression or function of HO-1 or HO-2, we searched for proteins that interact with both isozymes, using protein microarrays. We thus identified 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4) that synthesizes or degrades fructose-2,6-bisphosphate, a key activator of glycolysis, depending on cellular microenvironments. Importantly, HO-2 and PFKFB4 are predominantly expressed in haploid spermatids. Here, we show a drastic reduction in expression levels of PFKFB4 mRNA and protein and HO-2 mRNA in HepG2 human hepatoma cells in responses to glucose deprivation (≤ 2.5 mM), which occurred concurrently with remarkable induction of HO-1 mRNA and protein. Knockdown of HO-2 expression in HepG2 cells, using small interfering RNA, caused PFKFB4 mRNA levels to decrease with a concurrent increase in HO-1 expression. Thus, in HepG2 cells, HO-1 expression was increased, when expression levels of HO-2 and PFKFB4 mRNAs were decreased. Conversely, overexpression of HO-2 in HepG2 cells caused the level of co-expressed PFKFB4 protein to increase. These results suggest a potential regulatory role for HO-2 in ensuring PFKFB4 expression. Moreover, in D407 human retinal pigment epithelial cells, glucose deprivation decreased the expression levels of PFKFB4, HO-1, and HO-2 mRNAs. Thus, glucose deprivation consistently down-regulated the expression of PFKFB4 and HO-2 mRNAs in both HepG2 cells and RPE cells. We therefore postulate that PFKFB4 and HO-2 are expressed in a coordinated manner to maintain glucose homeostasis.

Identification of a bacteria-like ferrochelatase in Strongyloides venezuelensis, an animal parasitic nematode

Heme is an essential molecule for vast majority of organisms serving as a prosthetic group for various hemoproteins. Although most organisms synthesize heme from 5-aminolevulinic acid through a conserved heme biosynthetic pathway composed of seven consecutive enzymatic reactions, nematodes are known to be natural heme auxotrophs. The completely sequenced Caenorhabditis elegans genome, for example, lacks all seven genes for heme biosynthesis. However, genome/transcriptome sequencing of Strongyloides venezuelensis, an important model nematode species for studying human strongyloidiasis, indicated the presence of a gene for ferrochelatase (FeCH), which catalyzes the terminal step of heme biosynthesis, whereas the other six heme biosynthesis genes are apparently missing. Phylogenetic analyses indicated that nematode FeCH genes, including that of S. venezuelensis (SvFeCH) have a fundamentally different evolutionally origin from the FeCH genes of non-nematode metazoa. Although all non-nematode metazoan FeCH genes appear to be inherited vertically from an ancestral opisthokont, nematode FeCH may have been acquired from an alpha-proteobacterium, horizontally. The identified SvFeCH sequence was found to function as FeCH as expected based on both in vitro chelatase assays using recombinant SvFeCH and in vivo complementation experiments using an FeCH-deficient strain of Escherichia coli. Messenger RNA expression levels during the S. venezuelensis lifecycle were examined by real-time RT-PCR. SvFeCH mRNA was expressed at all the stages examined with a marked reduction at the infective third-stage larvae. Our study demonstrates the presence of a bacteria-like FeCH gene in the S. venezuelensis genome. It appeared that S. venezuelensis and some other animal parasitic nematodes reacquired the once-lost FeCH gene. Although the underlying evolutionary pressures that necessitated this reacquisition remain to be investigated, it is interesting that the presence of FeCH genes in the absence of other heme biosynthesis genes has been reported only for animal pathogens, and this finding may be related to nutritional availability in animal hosts.

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.

Heme transport and erythropoiesis

In humans, systemic heme homeostasis is achieved via coordinated regulation of heme synthesis, transport and degradation. Although the heme biosynthesis and degradation pathways have been well characterized, the pathways for heme trafficking and incorporation into hemoproteins remain poorly understood. In the past few years, researchers have exploited genetic, cellular and biochemical tools, to identify heme transporters and, in the process, reveal unexpected functions for this elusive group of proteins. However, given the complexity of heme trafficking pathways, current knowledge of heme transporters is fragmented and sometimes contradictory. This review seeks to focus on recent studies on heme transporters with specific emphasis on their functions during erythropoiesis.

Emerging roles of cadmium and heme oxygenase in type-2 diabetes and cancer susceptibility

Many decades after an outbreak of severe cadmium poisoning, known as Itai-itai disease, cadmium continues to pose a significant threat to human health worldwide. This review provides an update on the effects of this environmental toxicant cadmium, observed in numerous populations despite modest exposure levels. In addition, it describes the current knowledge on the link between heme catabolism and glycolysis. It examines novel functions of heme oxygenase-2 (HO-2) that protect against type 2-diabetes and obesity, which have emerged from diabetic/obese phenotypes of the HO-2 knockout mouse model. Increased cancer susceptibility in type-2 diabetes has been noted in several large cohorts. This is a cause for concern, given the high prevalence of type-2 diabetes worldwide. A lifetime exposure to cadmium is associated with pre-diabetes, diabetes, and overall cancer mortality with sex-related differences in specific types of cancer. Liver and kidney are target organs for the toxic effects of cadmium. These two organs are central to the maintenance of blood glucose levels. Further, inhibition of gluconeogenesis is a known effect of heme, while cadmium has the propensity to alter heme catabolism. This raises the possibility that cadmium may mimic certain HO-2 deficiency conditions, resulting in diabetic symptoms. Intriguingly, evidence has emerged from a recent study to suggest the potential interaction and co-regulation of HO-2 with the key regulator of glycolysis: 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4). HO-2 could thus be critical to a metabolic switch to cancer-prone cells because the enzyme PFKFB and glycolysis are metabolic requirements for cell proliferation and resistance to apoptosis.

The mitochondrial heme exporter FLVCR1b mediates erythroid differentiation

Feline leukemia virus subgroup C receptor 1 (FLVCR1) is a cell membrane heme exporter that maintains the balance between heme levels and globin synthesis in erythroid precursors. It was previously shown that Flvcr1-null mice died in utero due to a failure of erythropoiesis. Here, we identify Flvcr1b, a mitochondrial Flvcr1 isoform that promotes heme efflux into the cytoplasm. Flvcr1b overexpression promoted heme synthesis and in vitro erythroid differentiation, whereas silencing of Flvcr1b caused mitochondrial heme accumulation and termination of erythroid differentiation. Furthermore, mice lacking the plasma membrane isoform (Flvcr1a) but expressing Flvcr1b had normal erythropoiesis, but exhibited hemorrhages, edema, and skeletal abnormalities. Thus, FLVCR1b regulates erythropoiesis by controlling mitochondrial heme efflux, whereas FLVCR1a expression is required to prevent hemorrhages and edema. The aberrant expression of Flvcr1 isoforms may play a role in the pathogenesis of disorders characterized by an imbalance between heme and globin synthesis.

Spectroscopic detection and quantification of heme and heme degradation products

Heme and heme degradation products play critical roles in numerous biological phenomena which until now have only been partially understood. One reason for this is the very low concentrations at which free heme, its complexes and the partly unstable degradation products occur in living cells. Therefore, powerful and specific detection methods are needed. In this contribution, the potential of nondestructive Raman spectroscopy for the detection, quantification and discrimination of heme and heme degradation products is investigated. Resonance Raman spectroscopy using different excitation wavelengths (413, 476, 532, and 752 nm) is employed to estimate the limit of detection for hemin, myoglobin, biliverdin, and bilirubin. Concentrations in the low micromolar range (down to 3 μmol/L) could be reliably detected when utilizing the resonance enhancement effect. Furthermore, a systematic study on the surface-enhanced Raman spectroscopy (SERS) detection of hemin in the presence of other cellular components, such as the highly similar cytochrome c, DNA, and the important antioxidant glutathione, is presented. A microfluidic device was used to reproducibly create a segmented flow of aqueous droplets and oil compartments. Those aqueous droplets acted as model chambers where the analytes have to compete for the colloid. With the help of statistical analysis, it was possible to detect and differentiate the pure substances as well as the binary mixtures and gain insights into their interaction.

Heme arginate potentiates latent HIV-1 reactivation while inhibiting the acute infection

Human immunodeficiency virus-1 (HIV-1) successfully escapes from host immune surveillance, vaccines and antiretroviral agents. The available antiretroviral compounds can only control viremia, but it is impossible to eliminate the virus from the organism, namely because HIV-1 provirus persists in the reservoir cells from which the virus repeatedly disseminates into new cells. Current therapeutic approaches, however, do not specifically address the stage of virus reactivation. Heme has been demonstrated as very efficient in inhibiting HIV-1 reverse transcription, while its derivative hemin ameliorated HIV-1 infection via induction of heme oxygenase-1. Normosang (heme arginate; HA) is a human hemin-containing compound used to treat acute porphyria. In this work, we studied the effects of HA in HIV-1-acutely infected T-cell lines, and in cell lines harboring either a complete HIV-1 provirus (ACH-2 cells) or an HIV-1 "mini-virus" (Jurkat clones expressing EGFP under control of HIV LTR). We demonstrate that HA inhibited HIV-1 replication during the acute infection, which was accompanied by the inhibition of reverse transcription. On the other hand, HA alone stimulated the reactivation of HIV-1 "mini-virus" and synergized with phorbol ester or TNF-α in the reactivation of HIV-1 provirus. The stimulatory effects of HA were inhibited by N-acetyl cysteine, suggesting an increased redox stress and activation of NF-κB. Further, HA induced expression of heme oxygenase-1 (HO-1) in ACH-2 cells, while HO-1 was found expressed in untreated Jurkat clones. Inhibitor of HO-1 activity, tin protoporphyrin IX, further increased HA-mediated reactivation of HIV-1 "mini-virus" in Jurkat clones, and this effect was also inhibited by N-acetyl cysteine. The stimulatory effects of HA on HIV-1 reactivation thus seem to involve HO-1 and generation of free radicals. Additionally, the effective concentrations of HA did neither affect normal T-cell activation with PMA nor induce activation of the unstimulated cells. In conclusion, HA appears to possess a combination of unique properties that could help to decrease the pool of latently infected reservoir cells, while simultaneously inhibiting HIV-1 replication in newly infected cells. Our results thus suggest a new direction to explore in treatment of HIV/AIDS disease.

Roles of porphyrin and iron metabolisms in the δ-aminolevulinic acid (ALA)-induced accumulation of protoporphyrin and photodamage of tumor cells

δ-Aminolevulinic acid (ALA)-induced porphyrin accumulation is widely used in the treatment of cancer, as photodynamic therapy. To clarify the mechanisms of the tumor-preferential accumulation of protoporphyrin, we examined the effect of the expression of heme-biosynthetic and -degradative enzymes on the ALA-induced accumulation of protoporphyrin as well as photodamage. The transient expression of heme-biosynthetic enzymes in HeLa cells caused variations of the ALA-induced accumulation of protoporphyrin. When ALA-treated cells were exposed to white light, the extent of photodamage of the cells was dependent on the accumulation of protoporphyrin. The decrease of the accumulation of protoporphyrin was observed in the cells treated with inducers of heme oxygenase (HO)-1. The ALA-dependent accumulation of protoporphyrin was decreased in HeLa cells by transfection with HO-1 and HO-2 cDNA. Conversely, knockdown of HO-1/-2 with siRNAs enhanced the ALA-induced protoporphyrin accumulation and photodamage. The ALA effect was decreased with HeLa cells expressing mitoferrin-2, a mitochondrial iron transporter, whereas it was enhanced by the mitoferrin-2 siRNA transfection. These results indicated that not only the production of porphyrin intermediates but also the reuse of iron from heme and mitochondrial iron utilization control the ALA-induced accumulation of protoporphyrin in cancerous cells.

Integrative survival response evoked by heme oxygenase-1 and heme metabolites

Heme oxygenase (HO) catalyzes the rate-limiting step in heme degradation to produce carbon monoxide (CO), iron, and biliverdin. Biliverdin is subsequently converted to bilirubin by its reductase, and iron is recycled for heme synthesis. The inducible HO isoform, HO-1, is involved in the protection of multiple tissues and organs. The mechanism of protective actions of HO-1 has not been completely elucidated, but recent evidence suggests that one or more of heme metabolites can mediate the protective effects of HO-1. Particularly, CO mimics the antioxidant, anti-inflammatory, anti-apoptotic and antiproliferative actions of HO-1. Many of these effects of CO depend on the production of cyclic guanosine monophosphate (cGMP), and the modulation of mitogen-activated protein kinase (MAPK) pathways. The transcription factors, including nuclear factor E2-related factor-2 (Nrf2), and their upstream kinases, including MAPK pathway, play an important regulatory role in HO-1 expression by dietary antioxidants and drugs. This review attempts to concisely summarize the molecular and biochemical characteristics of HO-1, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by dietary antioxidants and drugs. In addition, the cytoprotective roles of HO-1 shall be discussed from the perspective of each of the metabolic by-products.

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.