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

Exploring current and emerging therapies for porphyrias

Porphyrias are rare, mostly inherited disorders resulting from altered activity of specific enzymes in the haem synthesis pathway that lead to accumulation of pathway intermediates. Photocutaneous symptoms occur when excess amounts of photoreactive porphyrins circulate in the blood to the skin, whereas increases in potentially neurotoxic porphyrin precursors are associated with neurovisceral symptoms. Current therapies are suboptimal and their mechanisms are not well established. As described here, emerging therapies address underlying disease mechanisms by introducing a gene, RNA or other specific molecule with the potential to cure or slow progression of the disease. Recent progress in nanotechnology and nanoscience, particularly regarding particle design and formulation, is expanding disease targets. More secure and efficient drug delivery systems have extended our toolbox for transferring specific molecules, especially into hepatocytes, and led to proof-of-concept studies in animal models. Repurposing existing drugs as molecular chaperones or haem synthesis inhibitors is also promising. This review summarizes key examples of these emerging therapeutic approaches and their application for hepatic and erythropoietic porphyrias.

An inside out journey: biogenesis, ultrastructure and proteomic characterisation of the ectoparasitic flatworm Sparicotyle chrysophrii extracellular vesicles

BACKGROUND

Helminth extracellular vesicles (EVs) are known to have a three-way communication function among parasitic helminths, their host and the host-associated microbiota. They are considered biological containers that may carry virulence factors, being therefore appealing as therapeutic and prophylactic target candidates. This study aims to describe and characterise EVs secreted by Sparicotyle chrysophrii (Polyopisthocotyla: Microcotylidae), a blood-feeding gill parasite of gilthead seabream (Sparus aurata), causing significant economic losses in Mediterranean aquaculture.

METHODS

To identify proteins involved in extracellular vesicle biogenesis, genomic datasets from S. chrysophrii were mined in silico using known protein sequences from Clonorchis spp., Echinococcus spp., Fasciola spp., Fasciolopsis spp., Opisthorchis spp., Paragonimus spp. and Schistosoma spp. The location and ultrastructure of EVs were visualised by transmission electron microscopy after fixing adult S. chrysophrii specimens by high-pressure freezing and freeze substitution. EVs were isolated and purified from adult S. chrysophrii (n = 200) using a newly developed ultracentrifugation-size-exclusion chromatography protocol for Polyopisthocotyla, and EVs were characterised via nanoparticle tracking analysis and tandem mass spectrometry.

RESULTS

Fifty-nine proteins involved in EV biogenesis were identified in S. chrysophrii, and EVs compatible with ectosomes were observed in the syncytial layer of the haptoral region lining the clamps. The isolated and purified nanoparticles had a mean size of 251.8 nm and yielded 1.71 × 108 particles · mL-1. The protein composition analysis identified proteins related to peptide hydrolases, GTPases, EF-hand domain proteins, aerobic energy metabolism, anticoagulant/lipid-binding, haem detoxification, iron transport, EV biogenesis-related, vesicle-trafficking and other cytoskeletal-related proteins. Several identified proteins, such as leucyl and alanyl aminopeptidases, calpain, ferritin, dynein light chain, 14-3-3, heat shock protein 70, annexin, tubulin, glutathione S-transferase, superoxide dismutase, enolase and fructose-bisphosphate aldolase, have already been proposed as target candidates for therapeutic or prophylactic purposes.

CONCLUSIONS

We have unambiguously demonstrated for the first time to our knowledge the secretion of EVs by an ectoparasitic flatworm, inferring their biogenesis machinery at a genomic and transcriptomic level, and by identifying their location and protein composition. The identification of multiple therapeutic targets among EVs' protein repertoire provides opportunities for target-based drug discovery and vaccine development for the first time in Polyopisthocotyla (sensu Monogenea), and in a fish-ectoparasite model.

Transcription factor BACH1 in cancer: roles, mechanisms, and prospects for targeted therapy

Transcription factor BTB domain and CNC homology 1 (BACH1) belongs to the Cap 'n' Collar and basic region Leucine Zipper (CNC-bZIP) family. BACH1 is widely expressed in mammalian tissues, where it regulates epigenetic modifications, heme homeostasis, and oxidative stress. Additionally, it is involved in immune system development. More importantly, BACH1 is highly expressed in and plays a key role in numerous malignant tumors, affecting cellular metabolism, tumor invasion and metastasis, proliferation, different cell death pathways, drug resistance, and the tumor microenvironment. However, few articles systematically summarized the roles of BACH1 in cancer. This review aims to highlight the research status of BACH1 in malignant tumor behaviors, and summarize its role in immune regulation in cancer. Moreover, this review focuses on the potential of BACH1 as a novel therapeutic target and prognostic biomarker. Notably, the mechanisms underlying the roles of BACH1 in ferroptosis, oxidative stress and tumor microenvironment remain to be explored. BACH1 has a dual impact on cancer, which affects the accuracy and efficiency of targeted drug delivery. Finally, the promising directions of future BACH1 research are prospected. A systematical and clear understanding of BACH1 would undoubtedly take us one step closer to facilitating its translation from basic research into the clinic.

Translation Fidelity and Respiration Deficits in CLPP-Deficient Tissues: Mechanistic Insights from Mitochondrial Complexome Profiling

The mitochondrial matrix peptidase CLPP is crucial during cell stress. Its loss causes Perrault syndrome type 3 (PRLTS3) with infertility, neurodegeneration, and a growth deficit. Its target proteins are disaggregated by CLPX, which also regulates heme biosynthesis via unfolding ALAS enzymes, providing access for pyridoxal-5'-phosphate (PLP). Despite efforts in diverse organisms with multiple techniques, CLPXP substrates remain controversial. Here, avoiding recombinant overexpression, we employed complexomics in mitochondria from three mouse tissues to identify endogenous targets. A CLPP absence caused the accumulation and dispersion of CLPX-VWA8 as AAA+ unfoldases, and of PLPBP. Similar changes and CLPX-VWA8 co-migration were evident for mitoribosomal central protuberance clusters, translation factors like GFM1-HARS2, the RNA granule components LRPPRC-SLIRP, and enzymes OAT-ALDH18A1. Mitochondrially translated proteins in testes showed reductions to <30% for MTCO1-3, the mis-assembly of the complex IV supercomplex, and accumulated metal-binding assembly factors COX15-SFXN4. Indeed, heavy metal levels were increased for iron, molybdenum, cobalt, and manganese. RT-qPCR showed compensatory downregulation only for Clpx mRNA; most accumulated proteins appeared transcriptionally upregulated. Immunoblots validated VWA8, MRPL38, MRPL18, GFM1, and OAT accumulation. Co-immunoprecipitation confirmed CLPX binding to MRPL38, GFM1, and OAT, so excess CLPX and PLP may affect their activity. Our data mechanistically elucidate the mitochondrial translation fidelity deficits which underlie progressive hearing impairment in PRLTS3.

The Hepatic Porphyrias: Revealing the Complexities of a Rare Disease

The porphyrias are a group of metabolic disorders that are caused by defects in heme biosynthesis pathway enzymes. The result is accumulation of heme precursors, which can cause neurovisceral and/or cutaneous photosensitivity. Liver is commonly either a source or target of excess porphyrins, and porphyria-associated hepatic dysfunction ranges from minor abnormalities to liver failure. In this review, the first of a three-part series, we describe the defects commonly found in each of the eight enzymes involved in heme biosynthesis. We also discuss the pathophysiology of the hepatic porphyrias in detail, covering epidemiology, histopathology, diagnosis, and complications. Cellular consequences of porphyrin accumulation are discussed, with an emphasis on oxidative stress, protein aggregation, hepatocellular cancer, and endothelial dysfunction. Finally, we review current therapies to treat and manage symptoms of hepatic porphyria.

Inhibition of ALA dehydratase activity in heme biosynthesis reduces cytoglobin expression which is related to the proliferation and viability of keloid fibroblasts

The aim of this study was to analyze the effect of heme synthesis inhibition on cytoglobin expression and its correlation with keloid fibroblast viability and proliferation. The study was conducted on primary culture of keloid fibroblasts. Heme synthesis in keloid fibroblasts was inhibited using succinyl acetone. We measured amino levulinic acid dehydratase (ALAD) enzyme activity using a colorimetric method; cytoglobin mRNA expression using qRT-PCR, cytoglobin protein expression using ELISA and immunocytochemistry, fibroblast viability using the MTT test; and fibroblast proliferation using BrdU test. The results showed that the ALAD enzyme activity level was lower in the keloid fibroblasts treated with succinyl-acetone (SA, 1, 2.5, and 5 mM) than in the control. The cytoglobin mRNA and protein expressions level were significantly lower in the keloid fibroblasts cultured with 2.5 mM and 5 mM SA than in the control and 1 mM SA. The viability and proliferation of the keloid fibroblasts decreased when the SA concentration was increased. In conclusion, the use of succinyl acetone at a concentration of 1; 2.5; and 5 mM caused decrease ALAD enzyme activity which indicated the inhibition of the heme synthesis. Inhibition of heme synthesis can affect cytoglobin expression, which correlates with the viability and proliferation of keloid fibroblasts.

NO-ferroheme is a signaling entity in the vasculature

Despite wide appreciation of the biological role of nitric oxide (NO) synthase (NOS) signaling, questions remain about the chemical nature of NOS-derived bioactivity. Here we show that NO-like bioactivity can be efficiently transduced by mobile NO-ferroheme species, which can transfer between proteins, partition into a hydrophobic phase and directly activate the sGC-cGMP-PKG pathway without intermediacy of free NO. The NO-ferroheme species (with or without a protein carrier) efficiently relax isolated blood vessels and induce hypotension in rodents, which is greatly potentiated after the blockade of NOS activity. While free NO-induced relaxations are abolished by an NO scavenger and in the presence of red blood cells or blood plasma, a model compound, NO-ferroheme-myoglobin preserves its vasoactivity suggesting the physiological relevance of NO-ferroheme species. We conclude that NO-ferroheme behaves as a signaling entity in the vasculature.

Active Estrogen-Succinate Metabolism Promotes Heme Accumulation and Increases the Proliferative and Invasive Potential of Endometrial Cancer Cells

Uterine endometrial cancer (UEC) is an estrogen-related tumor. Succinate and heme metabolism play important roles in the progression of multiple tumors. However, the relationship between estrogen, succinate, and heme metabolism and related regulatory mechanisms remain largely unknown. In this study, we observed that the expression of aminolevulinate delta synthase 1 (ALAS1) and solute carrier family member 38 (SLC25A38) in UEC tissues is significantly higher than that in normal tissues. Further analysis showed that estrogen and succinate increased the expression of ALAS1 and SLC25A38 in uterine endometrial cancer cells (UECC), and the administration of succinate upregulated the level of the estrogen receptor (ER). Silencing nuclear receptor coactivator 1 (NCOA1) reversed the effects of estrogen and succinate via downregulation of ALAS1 expression. Additionally, exposure of UECC to heme increased cell viability and invasiveness, while silencing the NCOA1 gene weakened this effect. These findings revealed that estrogen and succinate can synergistically increase the expression of ALAS1 and SLC25A38 via the ERβ/NCOA1 axis, promoting heme accumulation and increasing the proliferative and invasive potential of UECC.

Heme Interactions as Regulators of the Alternative Pathway Complement Responses and Implications for Heme-Associated Pathologies

Heme (Fe²⁺-protoporphyrin IX) is a pigment of life, and as a prosthetic group in several hemoproteins, it contributes to diverse critical cellular processes. While its intracellular levels are tightly regulated by networks of heme-binding proteins (HeBPs), labile heme can be hazardous through oxidative processes. In blood plasma, heme is scavenged by hemopexin (HPX), albumin and several other proteins, while it also interacts directly with complement components C1q, C3 and factor I. These direct interactions block the classical pathway (CP) and distort the alternative pathway (AP). Errors or flaws in heme metabolism, causing uncontrolled intracellular oxidative stress, can lead to several severe hematological disorders. Direct interactions of extracellular heme with alternative pathway complement components (APCCs) may be implicated molecularly in diverse conditions at sites of abnormal cell damage and vascular injury. In such disorders, a deregulated AP could be associated with the heme-mediated disruption of the physiological heparan sulphate-CFH coat of stressed cells and the induction of local hemostatic responses. Within this conceptual frame, a computational evaluation of HBMs (heme-binding motifs) aimed to determine how heme interacts with APCCs and whether these interactions are affected by genetic variation within putative HBMs. Combined computational analysis and database mining identified putative HBMs in all of the 16 APCCs examined, with 10 exhibiting disease-associated genetic (SNPs) and/or epigenetic variation (PTMs). Overall, this article indicates that among the pleiotropic roles of heme reviewed, the interactions of heme with APCCs could induce differential AP-mediated hemostasis-driven pathologies in certain individuals.

Hemoglobin uptake and utilization by human protozoan parasites: a review

The protozoan disease is a major global health concern. Amoebiasis, leishmaniasis, Chagas disease, and African sleeping sickness affect several million people worldwide, leading to millions of deaths annually and immense social and economic problems. Iron is an essential nutrient for nearly all microbes, including invading pathogens. The majority of iron in mammalian hosts is stored intracellularly in proteins, such as ferritin and hemoglobin (Hb). Hb, present in blood erythrocytes, is a very important source of iron and amino acids for pathogenic microorganisms ranging from bacteria to eukaryotic pathogens, such as worms, protozoa, yeast, and fungi. These organisms have developed adequate mechanisms to obtain Hb or its byproducts (heme and globin) from the host. One of the major virulence factors identified in parasites is parasite-derived proteases, essential for host tissue degradation, immune evasion, and nutrient acquisition. The production of Hb-degrading proteases is a Hb uptake mechanism that degrades globin in amino acids and facilitates heme release. This review aims to provide an overview of the Hb and heme-uptake mechanisms utilized by human pathogenic protozoa to survive inside the host.

METTL3-modified lncRNA-SNHG8 binds to PTBP1 to regulate ALAS2 expression to increase oxidative stress and promote myocardial infarction

Myocardial infarction (MI) is one of the important factors leading to death in today's society. Therefore, to study the related mechanism of MI and reduce myocardial ischemia-reperfusion injury is an important link to reduce MI injury. MI mice in vivo and cell model in vitro were constructed. The cardiac function and MI area of mice were detected, and myocardial tissue injury was detected by HE staining. ALAS2 expression in mice myocardial tissue was detected by IHC. The expressions of lncRNA-SNHG8, METTL3, PTBP1 and ALAS2 in myocardial tissue or cardiomyocytes were detected by qRT-PCR assay. MTT assay was used to measured viability of cardiomyocytes. The oxidative stress level in myocardial tissue or cardiomyocytes was detected by ELISA assay and ROS assay. RIP-qPCR and RNA pulldown assays determined the interaction between METTL3 and lncRNA-SNHG8, as well as PTBP1 and ALAS2. lncRNA-SNHG8 knockdown in MI mice was reduced myocardial infarction size, alleviated myocardial tissue injury and oxidative stress, and inhibited ALAS2 expression in myocardial tissue. RNA pulldown and RIP assays showed that lncRNA-SNHG8 binged with PTBP1 and PTBP1 interacted with ALAS2 mRNA. Knockdown of lncRNA-SNHG8, METTL3 or PTBP1 in MI cells enhanced viability of myocardial cells, attenuated ROS release and MDA level, increased SOD level, alleviated oxidative stress. ALAS overexpression attenuated the corresponding effect of knockdown of lncRNA-SNHG8 and/or PTBP1 on MI cells. In sum, our paper is demonstrated for the first time that METTL3 can promote lncRNA-SNHG8 through m6A modification, thereby regulating ALAS2 to induce oxidative stress and aggravate myocardial injury.

The tumor/normal tissue ratio of Keap1 protein is a predictor for lymphovascular invasion in colorectal cancer: A correlation study between the Nrf2 and KRas pathways

PURPOSE: Oxidative stress has impacts on the KRas and Nrf2/Keap1 pathways, which have multiple interactions with each other and play important roles in colorectal cancer (CRC). This study investigated the expressions of proteins in the KRas and Nrf2/Keap1 pathways and their associations with clinicopathological features in CRC.METHODS: The protein levels of Nrf2, Keap1, Bach1, p62, HO1, KRas, Erk, Raf1 and PI3K in both the tumor and normal tissues of 60 CRC subjects were determined by Western blot and their T/N (tumor/normal tissue) ratios were correlated with clinicopathological features.RESULTS: The T/N ratios of proteins in the KRas and Nrf2/Keap1 pathways had correlation patterns and proximity profiles in cluster dendrograms different in CRC with different status of lymphovascular invasion (LVI) or lymph node/distant metastases. The Keap1 protein T/N ratio was a significant predictor (odd ratio: 2.24; 95% confidence interval: 1.26 - 4.38) of LVI, which in turn predicted metastases (11.0; 3.49 - 39.8).CONCLUSION: The interactions between the KRas and Nrf2/Keap1 pathways may be affected differently by LVI and metastases, and the protein T/N ratio of Keap1 may be helpful for predicting LVI in CRC.

5-Aminolevulinic Acid-Induced Protoporphyrin IX Fluorescence Imaging for Tumor Detection: Recent Advances and Challenges

5-Aminolevulinic acid (5-ALA) is a natural amino acid and a precursor of heme and chlorophyll. Exogenously administered 5-ALA is metabolized into protoporphyrin IX (PpIX). PpIX accumulates in cancer cells because of the low activity of ferrochelatase, an enzyme that metabolizes PpIX to heme. High expression of 5-ALA influx transporters, such as peptide transporters 1/2, in cancer cells also enhances PpIX production. Because PpIX radiates red fluorescence when excited with blue/violet light, 5-ALA has been used for the visualization of various tumors. 5-ALA photodynamic diagnosis (PDD) has been shown to improve the tumor removal rate in high-grade gliomas and non-muscular invasive bladder cancers. However, 5-ALA PDD remains a challenge as a diagnostic method because tissue autofluorescence interferes with PpIX signals in cases where tumors emit only weak signals, and non-tumorous lesions, such as inflammatory sites, tend to emit PpIX fluorescence. Here, we review the current outline of 5-ALA PDD and strategies for improving its diagnostic applicability for tumor detection, focusing on optical techniques and 5-ALA metabolic pathways in both viable and necrotic tumor tissues.

Congenital sideroblastic anemia model due to ALAS2 mutation is susceptible to ferroptosis

X-linked sideroblastic anemia (XLSA), the most common form of congenital sideroblastic anemia, is caused by a germline mutation in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene. In XLSA, defective heme biosynthesis leads to ring sideroblast formation because of excess mitochondrial iron accumulation. In this study, we introduced ALAS2 missense mutations on human umbilical cord blood-derived erythroblasts; hereafter, we refer to them as XLSA clones. XLSA clones that differentiated into mature erythroblasts showed an increased frequency of ring sideroblast formation with impaired hemoglobin biosynthesis. The expression profiling revealed significant enrichment of genes involved in ferroptosis, which is a form of regulated cell death induced by iron accumulation and lipid peroxidation. Notably, treatment with erastin, a ferroptosis inducer, caused a higher proportion of cell death in XLSA clones. XLSA clones exhibited significantly higher levels of intracellular lipid peroxides and enhanced expression of BACH1, a regulator of iron metabolism and potential accelerator of ferroptosis. In XLSA clones, BACH1 repressed genes involved in iron metabolism and glutathione synthesis. Collectively, defective heme biosynthesis in XLSA clones could confer enhanced BACH1 expression, leading to increased susceptibility to ferroptosis. The results of our study provide important information for the development of novel therapeutic targets for XLSA.

Regulation of protein function and degradation by heme, heme responsive motifs, and CO

Heme is an essential biomolecule and cofactor involved in a myriad of biological processes. In this review, we focus on how heme binding to heme regulatory motifs (HRMs), catalytic sites, and gas signaling molecules as well as how changes in the heme redox state regulate protein structure, function, and degradation. We also relate these heme-dependent changes to the affected metabolic processes. We center our discussion on two HRM-containing proteins: human heme oxygenase-2, a protein that binds and degrades heme (releasing Fe²⁺ and CO) in its catalytic core and binds Fe³⁺-heme at HRMs located within an unstructured region of the enzyme, and the transcriptional regulator Rev-erbβ, a protein that binds Fe³⁺-heme at an HRM and is involved in CO sensing. We will discuss these and other proteins as they relate to cellular heme composition, homeostasis, and trafficking. In addition, we will discuss the HRM-containing family of proteins and how the stability and activity of these proteins are regulated in a dependent manner through the HRMs. Then, after reviewing CO-mediated protein regulation of heme proteins, we turn our attention to the involvement of heme, HRMs, and CO in circadian rhythms. In sum, we stress the importance of understanding the various roles of heme and the distribution of the different heme pools as they relate to the heme redox state, CO, and heme binding affinities.

Role of Iron Metabolism-Related Genes in Prenatal Development: Insights from Mouse Transgenic Models

Iron is an essential nutrient during all stages of mammalian development. Studies carried out over the last 20 years have provided important insights into cellular and systemic iron metabolism in adult organisms and led to the deciphering of many molecular details of its regulation. However, our knowledge of iron handling in prenatal development has remained remarkably under-appreciated, even though it is critical for the health of both the embryo/fetus and its mother, and has a far-reaching impact in postnatal life. Prenatal development requires a continuous, albeit quantitatively matched with the stage of development, supply of iron to support rapid cell division during embryogenesis in order to meet iron needs for erythropoiesis and to build up hepatic iron stores, (which are the major source of this microelement for the neonate). Here, we provide a concise overview of current knowledge of the role of iron metabolism-related genes in the maintenance of iron homeostasis in pre- and post-implantation development based on studies on transgenic (mainly knock-out) mouse models. Most studies on mice with globally deleted genes do not conclude whether underlying in utero iron disorders or lethality is due to defective placental iron transport or iron misregulation in the embryo/fetus proper (or due to both). Therefore, there is a need of animal models with tissue specific targeted deletion of genes to advance the understanding of prenatal iron metabolism.

Dysregulation of homocysteine homeostasis in acute intermittent porphyria patients receiving heme arginate or givosiran

Acute intermittent porphyria (AIP) is a rare metabolic disease caused by mutations within the hydroxymethylbilane synthase gene. Previous studies have reported increased levels of plasma total homocysteine (tHcy) in symptomatic AIP patients. In this study, we present long-term data for tHcy and related parameters for an AIP patient cohort (n = 37) in different clinical disease-states. In total, 25 patients (68%) presented with hyperhomocysteinemia (HHcy; tHcy > 15 μmol/L) during the observation period. HHcy was more frequent in AIP patients with recurrent disease receiving heme arginate, than in nonrecurrent (median tHcy: 21.6 μmol/L; range: 10-129 vs median tHcy: 14.5 μmol/L; range 6-77). Long-term serial analyses showed a high within-person tHcy variation, especially among the recurrent patients (coefficient of variation: 16.4%-78.8%). HHcy was frequently associated with low blood concentrations of pyridoxal-5'-phosphate and folate, while cobalamin concentration and the allele distribution of the methylene-tetrahydrofolate-reductase gene were normal. Strikingly, 6 out of the 9 recurrent patients who were later included in a regime of givosiran, a small-interfering RNA that effectively reduced recurrent attacks, showed further increased tHcy (median tHcy in 9 patients: 105 μmol/L; range 16-212). Screening of amino acids in plasma by liquid-chromatography showed co-increased levels of methionine (median 71 μmol/L; range 23-616; normal <40), suggestive of acquired deficiency of cystathionine-β-synthase. The kynunerine/tryptophan ratio in plasma was, however, normal, indicating a regular metabolism of tryptophan by heme-dependent enzymes. In conclusion, even if HHcy was observed in AIP patients receiving heme arginate, givosiran induced an aggravation of the dysregulation, causing a co-increase of tHcy and methionine resembling classic homocystinuria.

Cytochrome P450-mediated estrogen catabolism therapeutic avenues in epilepsy

Epilepsy is a neuropsychiatric disorder, which does not have any identifiable cause. However, experimental and clinical results have asserted that the sex hormone estrogen level and endocrine system function influence the seizure and epileptic episodes. There are available drugs to control epilepsy, which passes through the metabolism process. Cytochrome P-450 family 1 (CYP1A1) is a heme-containing mono-oxygenase that are induced several folds in most of the tissues and cells contributing to their differential expression, which regulates various metabolic processes upon administration of therapeutics. CYP1A1 gene family has been found to metabolize estrogen, a female sex hormone, which plays a central role in maintaining the health of brain altering the level of estrogen active neuropsychiatric disorder like epilepsy. Hence, in this article, we endeavor to provide an opinion of estrogen, its effects on epilepsy and catamenial epilepsy, their metabolism by CYP1A1 and new way forward to differential diagnosis and clinical management of epilepsy in future.

Scavenging of Labile Heme by Hemopexin Is a Key Checkpoint in Cancer Growth and Metastases

Hemopexin (Hx) is a scavenger of labile heme. Herein, we present data defining the role of tumor stroma-expressed Hx in suppressing cancer progression. Labile heme and Hx levels are inversely correlated in the plasma of patients with prostate cancer (PCa). Further, low expression of Hx in PCa biopsies characterizes poorly differentiated tumors and correlates with earlier time to relapse. Significantly, heme promotes tumor growth and metastases in an orthotopic murine model of PCa, with the most aggressive phenotype detected in mice lacking Hx. Mechanistically, labile heme accumulates in the nucleus and modulates specific gene expression via interacting with guanine quadruplex (G4) DNA structures to promote PCa growth. We identify c-MYC as a heme:G4-regulated gene and a major player in heme-driven cancer progression. Collectively, these results reveal that sequestration of labile heme by Hx may block heme-driven tumor growth and metastases, suggesting a potential strategy to prevent and/or arrest cancer dissemination.

Canesin et al. describe a role and mechanism for labile heme as a key player in regulating gene expression to promote carcinogenesis via binding to G-quadruplex in the c-MYC promoter. Hemopexin, a heme scavenger, may be used as a strategy to block progression of cancer.

Iron deficiency plays essential roles in the trigger, treatment, and prognosis of autosomal dominant hypophosphatemic rickets

By analyzing iron status of 14 ADHR patients, we found that iron deficiency was an important trigger of ADHR. Correcting iron deficiency significantly improved patients' symptoms. Meanwhile, patients' serum phosphate showed positive correlations with iron metabolism parameters and hemoglobin-related parameters, suggesting the necessity of monitoring and correcting the iron status in ADHR.

INTRODUCTION

Autosomal dominant hypophosphatemic rickets (ADHR) is unique for its incomplete penetrance, variety of disease onsets, and waxing and waning phenotypes. Iron deficiency is a trigger of ADHR. This study aimed to clarify the role of iron deficiency in ADHR.

METHODS

Data of clinical manifestations and laboratory examinations were collected from patients among eight kindreds with ADHR. Multiple regression and Pearson's correlation tests were performed to test the relationships of serum phosphate levels and other laboratory variables during the patients' follow-ups.

RESULTS

Among 23 ADHR patients with fibroblast growth factor 23 (FGF23) mutations, 14 patients presented with obvious symptoms. Ten patients had iron deficiency at the onset of ADHR, coinciding with menarche, menorrhagia, pregnancy, and chronic gastrointestinal bleeding. Two patients who did not have their iron status tested presented with symptoms after abortion and pregnancy in one patient each, which suggested that they also had iron deficiency at onset. Patients were treated with ferrous succinate tablets, vitamin C, and neutral phosphate and calcitriol. With correction of the iron status, the patients' symptoms showed notable improvement, with increased serum phosphate levels. Two patients' FGF23 levels also declined to the normal range. There were strong correlations between serum phosphate and serum iron levels (r = 0.7689, p < 0.0001), serum ferritin levels (r = 0.5312, p = 0.002), iron saturation (r = 0.7907, p < 0.0001), and transferrin saturation (r = 0.7875, p < 0.001). We also examined the relationships between serum phosphate levels and hemoglobin-related indices, which were significant (hemoglobin: r = 0.71, p < 0.0001; MCV: r = 0.7589, p < 0.0001; MCH: r = 0.8218, p < 0.0001; and MCHC: r = 0.7751, p < 0.0001). Longitudinal data of six patients' follow-up also showed synchronous changes in serum phosphate with serum iron levels.

CONCLUSIONS

Iron deficiency plays an important role in triggering ADHR. Monitoring and correcting the iron status are helpful for diagnosing and treating ADHR. Iron metabolism parameters and hemoglobin-related parameters are positively correlated with serum phosphate levels in patients with ADHR and iron deficiency, and these might serve as good indicators of prognosis of ADHR.

Altered gene expression profile of Wolbachia pipientis wAlbB strain following transinfection from its native host Aedes albopictus to Aedes aegypti cells

Wolbachia is an obligate intracellular bacterial symbiont prevalent among arthropods and nematodes. To survive and reproduce, Wolbachia interacts with and modifies host subcellular structures, while sensing and responding to changes within the cellular environment. In mutualistic associations, Wolbachia may provision the host with metabolites, or help to maintain the chemical homeostasis of the host cell. Some strains can rapidly invade insect populations by manipulating host reproductive biology, while also preventing viral replication, allowing their use in vector control of arthropod-borne viruses. The Aedes albopictus-derived strain wAlbB is promising in this regard. When transinfected into the Yellow fever mosquito, Aedes aegypti, wAlbB reaches high frequencies within wild populations, and strongly inhibits viral transmission. Despite its obvious potential, much is still unknown about the molecular interactions between Wolbachia and host that enable its use in vector control. Furthermore, most Wolbachia transinfection research to date has focused on host effects. In the current study, we used a cell line model to explore the effect of transinfection of wAlbB from Ae. albopictus to Ae. aegypti. Using RNA sequencing, we show that several genes associated with host-symbiont interactions were downregulated by transinfection, with the greatest downregulation exhibited by prophage-associated genes.

Systematic Surveys of Iron Homeostasis Mechanisms Reveal Ferritin Superfamily and Nucleotide Surveillance Regulation to be Modified by PINK1 Absence

Iron deprivation activates mitophagy and extends lifespan in nematodes. In patients suffering from Parkinson’s disease (PD), PINK1-PRKN mutations via deficient mitophagy trigger iron accumulation and reduce lifespan. To evaluate molecular effects of iron chelator drugs as a potential PD therapy, we assessed fibroblasts by global proteome profiles and targeted transcript analyses. In mouse cells, iron shortage decreased protein abundance for iron-binding nucleotide metabolism enzymes (prominently XDH and ferritin homolog RRM2). It also decreased the expression of factors with a role for nucleotide surveillance, which associate with iron-sulfur-clusters (ISC), and are important for growth and survival. This widespread effect included prominently Nthl1-Ppat-Bdh2, but also mitochondrial Glrx5-Nfu1-Bola1, cytosolic Aco1-Abce1-Tyw5, and nuclear Dna2-Elp3-Pold1-Prim2. Incidentally, upregulated Pink1-Prkn levels explained mitophagy induction, the downregulated expression of Slc25a28 suggested it to function in iron export. The impact of PINK1 mutations in mouse and patient cells was pronounced only after iron overload, causing hyperreactive expression of ribosomal surveillance factor Abce1 and of ferritin, despite ferritin translation being repressed by IRP1. This misregulation might be explained by the deficiency of the ISC-biogenesis factor GLRX5. Our systematic survey suggests mitochondrial ISC-biogenesis and post-transcriptional iron regulation to be important in the decision, whether organisms undergo PD pathogenesis or healthy aging.

Pathways of heme utilization in fungi

Iron acquisition is challenging in most environments. As an alternative to elemental iron, organisms can take up iron-protoporphyrin IX, or heme. Heme can be found in decaying organic matter and is particularly prevalent in animal hosts. Fungi have evolved at least three distinct endocytosis-mediated heme uptake systems, which have been studied in detail in the organisms Candida albicans, Cryptococcus neoformans and Schizosaccharomyces pombe. Here we summarize the known molecular details of these three uptake systems that enable parasitic and saprophytic fungi to take advantage of external heme as either cellular iron or heme sources.

Trace Elements, PPARs, and Metabolic Syndrome

Metabolic syndrome (MetS) is a constellation of metabolic derangements, including central obesity, insulin resistance, hypertension, glucose intolerance, and dyslipidemia. The pathogenesis of MetS has been intensively studied, and now many factors are recognized to contribute to the development of MetS. Among these, trace elements influence the structure of proteins, enzymes, and complex carbohydrates, and thus an imbalance in trace elements is an independent risk factor for MetS. The molecular link between trace elements and metabolic homeostasis has been established, and peroxisome proliferator-activated receptors (PPARs) have appeared as key regulators bridging these two elements. This is because on one hand, PPARs are actively involved in various metabolic processes, such as abdominal adiposity and insulin sensitivity, and on the other hand, PPARs sensitively respond to changes in trace elements. For example, an iron overload attenuates hepatic mRNA expression of Ppar-α; zinc supplementation is considered to recover the DNA-binding activity of PPAR-α, which is impaired in steatotic mouse liver; selenium administration downregulates mRNA expression of Ppar-γ, thereby improving lipid metabolism and oxidative status in the liver of high-fat diet (HFD)-fed mice. More importantly, PPARs' expression and activity are under the control of the circadian clock and show a robust 24 h rhythmicity, which might be the reasons for the side effects and the clinical limitations of trace elements targeting PPARs. Taken together, understanding the casual relationships among trace elements, PPARs' actions, and the pathogenesis of MetS is of great importance. Further studies are required to explore the chronopharmacological effects of trace elements on the diurnal oscillation of PPARs and the consequent development of MetS.

Formation of novel N-acetylcysteine-hemin adducts abrogates hemin-induced cytotoxicity and suppresses the NRF2-driven stress response in human pro-erythroid K562 cells

Heme (iron protoporphyrin IX), as the prosthetic group in hemoproteins, regulates vital cellular functions in human tissues. However, free heme released during hemolysis events promotes severe complications to millions of people worldwide. Over the years, thiols like glutathione (GSH) were known to antagonize heme toxicity. In this study, we have uncovered the underlying molecular mechanism by which N-acetylcysteine (NAC), a well-known thiol prevents hemin-induced cytotoxicity (HIC). Hemin-responsive human pro-erythroid K562 cells were employed to assess hemin intracellular accumulation and cytotoxicity at concentrations ≥50 μΜ, in cultures exposed only to hemin and/or both hemin and NAC. NAC inhibited the intracellular accumulation of hemin and prevented hemin-induced cell growth inhibition, cell death, oxidative stress, and accumulation of ubiquitinated proteins. Meanwhile, the activation of the NF-E2-related factor-2 (NRF2)-driven stress gene activation, a key element involved in HIC, was suppressed by NAC. A refined mechanism of the chemical reaction between NAC and hemin leading to adduct formation via a nucleophilic attack on hemin was uncovered for the first time by tandem mass spectrometry analysis (LC-MS/MS). Such thiol-hemin adducts acted as intermediates to mitigate HIC and to suppress hemin-induced NRF2-driven gene activation. Our findings support the concept that NAC-hemin adduct formation is the major novel molecular mechanism rather than the reactive oxygen species-scavenging capacity of thiols to protect cells from HIC. Our results imply that thiols and their derivatives can be of potential therapeutic value in hemolytic disorders.

From Synthesis to Utilization: The Ins and Outs of Mitochondrial Heme

Heme is a ubiquitous and essential iron containing metallo-organic cofactor required for virtually all aerobic life. Heme synthesis is initiated and completed in mitochondria, followed by certain covalent modifications and/or its delivery to apo-hemoproteins residing throughout the cell. While the biochemical aspects of heme biosynthetic reactions are well understood, the trafficking of newly synthesized heme—a highly reactive and inherently toxic compound—and its subsequent delivery to target proteins remain far from clear. In this review, we summarize current knowledge about heme biosynthesis and trafficking within and outside of the mitochondria.

Occupational Lead Poisoning in a Patient with Acute Abdomen and Normocytic Anemia

We herein report a 24-year-old male construction worker with occupational lead poisoning who presented with acute abdomen and normocytic anemia. The levels of urinary delta-aminolevulinic acid and free erythrocyte protoporphyrin were elevated without any increase in the level of urine porphobilinogen. Detection of an elevated blood lead level of 100 μg/dL confirmed a diagnosis of lead poisoning. Chelation therapy with calcium disodium ethylenediaminetetraacetate resulted in prompt improvement of the clinical symptoms and the blood lead level. Clinicians should be aware that lead poisoning caused by occupational exposure can still occur sporadically in construction workers in Japan.

GLRX5 mutations impair heme biosynthetic enzymes ALA synthase 2 and ferrochelatase in Human congenital sideroblastic anemia

Non-syndromic microcytic congenital sideroblastic anemia (cSA) is predominantly caused by defective genes encoding for either ALAS2, the first enzyme of heme biosynthesis pathway or SLC25A38, the mitochondrial importer of glycine, an ALAS2 substrate. Herein we explored a new case of cSA with two mutations in GLRX5, a gene for which only two patients have been reported so far. The patient was a young female with biallelic compound heterozygous mutations in GLRX5 (p.Cys67Tyr and p.Met128Lys). Three-D structure analysis confirmed the involvement of Cys67 in the coordination of the [2Fe2S] cluster and suggested a potential role of Met128 in partner interactions. The protein-level of ferrochelatase, the terminal-enzyme of heme process, was increased both in patient-derived lymphoblastoid and CD34+ cells, however, its activity was drastically decreased. The activity of ALAS2 was found altered and possibly related to a defect in the biogenesis of its co-substrate, the succinyl-CoA. Thus, the patient exhibits both a very low ferrochelatase activity without any accumulation of porphyrins precursors in contrast to what is reported in erythropoietic protoporphyria with solely impaired ferrochelatase activity. A significant oxidative stress was evidenced by decreased reduced glutathione and aconitase activity, and increased MnSOD protein expression. This oxidative stress depleted and damaged mtDNA, decreased complex I and IV activities and depleted ATP content. Collectively, our study demonstrates the key role of GLRX5 in modulating ALAS2 and ferrochelatase activities and in maintaining mitochondrial function.

G-quadruplexes Sequester Free Heme in Living Cells

Heme is an essential cofactor for many enzymes, but free heme is toxic and its levels are tightly regulated. G-quadruplexes bind heme avidly in vitro, raising the possibility that they may sequester heme in vivo. If so, then treatment that displaces heme from quadruplexes is predicted to induce expression of genes involved in iron and heme homeostasis. Here we show that PhenDC3, a G-quadruplex ligand structurally unrelated to heme, displaces quadruplex-bound heme in vitro and alters transcription in cultured human cells, upregulating genes that support heme degradation and iron homeostasis, and most strikingly causing a 30-fold induction of heme oxidase 1, the key enzyme in heme degradation. We propose that G-quadruplexes sequester heme to protect cells from the pathophysiological consequences of free heme.

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.

Proteomic analysis of saliva from partially and fully engorged adult female Rhipicephalus microplus (Acari: Ixodidae)

Rhipicephalus microplus salivary gland secretes a number of complex bioactive proteins during feeding. These components are important in feeding and affect anti-coagulation, anti-inflammation and also have anti-microbial effects. In this study, tick saliva was collected from partially engorged female (PEF) and fully engorged female (FEF) ticks. Liquid chromatography tandem-mass spectrometry (LC-MS/MS) and isobaric tags for relative and absolute quantification (iTRAQ) were used to identify and quantify R. microplus salivary proteins. A total of 322 unique peptides were detected and 151 proteins were characterized in both PEF and FEF. Of these, 41 proteins are considered as high-confidence proteins. Fifteen high-confidence proteins were upregulated and six high-confidence proteins were downregulated (p < 0.05; PEF:FEF ratio ≥ 1.2 or PEF:FEF ratio ≤ 0.83); 17 high-confidence proteins are slightly changed (PEF:FEF ratio > 0.83 and < 1.2). These high-confidence proteins are involved in several physiological roles, including egg development, transportation of proteins, immunity and anti-microorganism, anti-coagulant, and adhesion. In comparison with PEF, the number of upregulated proteins exceeded the number of proteins downregulated. Salivary protein may be induced by the blood-meal and these proteins contribute to successful feeding.

Molecular pathophysiology and genetic mutations in congenital sideroblastic anemia

Sideroblastic anemia is a heterogeneous congenital and acquired disorder characterized by anemia and the presence of ring sideroblasts in the bone marrow. Congenital sideroblastic anemia (CSA) is a rare disease caused by mutations in genes involved in the heme biosynthesis, iron-sulfur [Fe-S] cluster biosynthesis, and mitochondrial protein synthesis. The most prevalent form of CSA is X-linked sideroblastic anemia, caused by mutations in the erythroid-specific δ-aminolevulinate synthase (ALAS2), which is the first enzyme of the heme biosynthesis pathway in erythroid cells. To date, a remarkable number of genetically undefined CSA cases remain, but a recent application of the next-generation sequencing technology has recognized novel causative genes for CSA. However, in most instances, the detailed molecular mechanisms of how defects of each gene result in the abnormal mitochondrial iron accumulation remain unclear. This review aims to cover the current understanding of the molecular pathophysiology of CSA.

The Role of Iron in Friedreich's Ataxia: Insights From Studies in Human Tissues and Cellular and Animal Models

Friedreich’s ataxia (FRDA) is a rare early-onset degenerative disease that affects both the central and peripheral nervous systems, and other extraneural tissues, mainly the heart and endocrine pancreas. This disorder progresses as a mixed sensory and cerebellar ataxia, primarily disturbing the proprioceptive pathways in the spinal cord, peripheral nerves and nuclei of the cerebellum. FRDA is an inherited disease with an autosomal recessive pattern caused by an insufficient amount of the nuclear-encoded mitochondrial protein frataxin, which is an essential and highly evolutionary conserved protein whose deficit results in iron metabolism dysregulation and mitochondrial dysfunction. The first experimental evidence connecting frataxin with iron homeostasis came from Saccharomyces cerevisiae; iron accumulates in the mitochondria of yeast with deletion of the frataxin ortholog gene. This finding was soon linked to previous observations of iron deposits in the hearts of FRDA patients and was later reported in animal models of the disease. Despite advances made in the understanding of FRDA pathophysiology, the role of iron in this disease has not yet been completely clarified. Some of the questions still unresolved include the molecular mechanisms responsible for the iron accumulation and iron-mediated toxicity. Here, we review the contribution of the cellular and animal models of FRDA and relevance of the studies using FRDA patient samples to gain knowledge about these issues. Mechanisms of mitochondrial iron overload are discussed considering the potential roles of frataxin in the major mitochondrial metabolic pathways that use iron. We also analyzed the effect of iron toxicity on neuronal degeneration in FRDA by reactive oxygen species (ROS)-dependent and ROS-independent mechanisms. Finally, therapeutic strategies based on the control of iron toxicity are considered.

Regulation of globin-heme balance in Diamond-Blackfan anemia by HSP70/GATA1

Diamond-Blackfan anemia (DBA) is a congenital erythroblastopenia that is characterized by a blockade in erythroid differentiation related to impaired ribosome biogenesis. DBA phenotype and genotype are highly heterogeneous. We have previously identified 2 in vitro erythroid cell growth phenotypes for primary CD34⁺ cells from DBA patients and following short hairpin RNA knockdown of RPS19, RPL5, and RPL11 expression in normal human CD34⁺ cells. The haploinsufficient RPS19 in vitro phenotype is less severe than that of 2 other ribosomal protein (RP) mutant genes. We further documented that proteasomal degradation of HSP70, the chaperone of GATA1, is a major contributor to the defect in erythroid proliferation, delayed erythroid differentiation, increased apoptosis, and decreased globin expression, which are all features of the RPL5 or RPL11 DBA phenotype. In the present study, we explored the hypothesis that an imbalance between globin and heme synthesis may be involved in pure red cell aplasia of DBA. We identified disequilibrium between the globin chain and the heme synthesis in erythroid cells of DBA patients. This imbalance led to accumulation of excess free heme and increased reactive oxygen species production that was more pronounced in cells of the RPL5 or RPL11 phenotype. Strikingly, rescue experiments with wild-type HSP70 restored GATA1 expression levels, increased globin synthesis thereby reducing free heme excess and resulting in decreased apoptosis of DBA erythroid cells. These results demonstrate the involvement of heme in DBA pathophysiology and a major role of HSP70 in the control of balanced heme/globin synthesis.

Heme-dependent Inactivation of 5-Aminolevulinate Synthase from Caulobacter crescentus

The biosynthesis of heme is strictly regulated, probably because of the toxic effects of excess heme and its biosynthetic precursors. In many organisms, heme biosynthesis starts with the production of 5-aminolevulinic acid (ALA) from glycine and succinyl-coenzyme A, a process catalyzed by a homodimeric enzyme, pyridoxal 5′-phosphate (PLP)-dependent 5-aminolevulinate synthase (ALAS). ALAS activity is negatively regulated by heme in various ways, such as the repression of ALAS gene expression, degradation of ALAS mRNA, and inhibition of mitochondrial translocation of the mammalian precursor protein. There has been no clear evidence, however, that heme directly binds to ALAS to negatively regulate its activity. We found that recombinant ALAS from Caulobacter crescentus was inactivated via a heme-mediated feedback manner, in which the essential coenzyme PLP was rel eased to form the inactive heme-bound enzyme. The spectroscopic properties of the heme-bound ALAS showed that a histidine-thiolate hexa-coordinated ferric heme bound to each subunit with a one-to-one stoichiometry. His340 and Cys398 were identified as the axial ligands of heme, and mutant ALASs lacking either of these ligands became resistant to heme-mediated inhibition. ALAS expressed in C. crescentus was also found to bind heme, suggesting that heme-mediated feedback inhibition of ALAS is physiologically relevant in C. crescentus.

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.

Rare anemias due to genetic iron metabolism defects

Anemia is defined by a deficiency of hemoglobin, an iron-rich protein that binds oxygen in the blood. It can be due to multiple causes, either acquired or genetic. Alterations of genes involved in iron metabolism may be responsible, usually at a young age, for rare forms of chronic and often severe congenital anemia. These diseases encompass a variety of sideroblastic anemias, characterized by the presence of ring sideroblasts in the bone marrow. Clinical expression of congenital sideroblastic anemia is either monosyndromic (restricted to hematological lineages) or polysyndromic (with systemic expression), depending on whether iron metabolism, and especially heme synthesis, is directly or indirectly affected. Beside sideroblastic anemias, a number of other anemias can develop due to mutations of key proteins acting either on cellular iron transport (such as the DMT1 transporter), plasma iron transport (transferrin), and iron recycling (ceruloplasmin). Contrasting with the aforementioned entities which involve compartmental, and sometimes, systemic iron excess, the iron refractory iron deficiency anemia (IRIDA) corresponds to a usually severe anemia with whole body iron deficiency related to chronic increase of plasma hepcidin, the systemic negative regulator of plasma iron. Once clinically suggested, these diseases are confirmed by genetic testing in specialized laboratories.

Heme Binding to Porphobilinogen Deaminase from Vibrio cholerae Decelerates the Formation of 1-Hydroxymethylbilane

Porphobilinogen deaminase (PBGD) is an enzyme that catalyzes the formation of hydroxymethylbilane, a tetrapyrrole intermediate, during heme biosynthesis through the stepwise polymerization of four molecules of porphobilinogen. PBGD from Vibrio cholerae was expressed in Escherichia coli and characterized in this study. Unexpectedly, spectroscopic measurements revealed that PBGD bound one equivalent of heme with a dissociation constant of 0.33 ± 0.01 μM. The absorption and resonance Raman spectra suggested that heme is a mixture of the 5-coordinate and 6-coordinate hemes. Mutational studies indicated that the 5-coordinate heme possessed Cys105 as a heme axial ligand, and His227 was coordinated to form the 6-coordinate heme. Upon heme binding, the deamination activity decreased by approximately 15%. The crystal structure of PBGD revealed that His227 was located near Cys105, but the side chain of His227 did not point toward Cys105. The addition of the cyanide ion to heme-PBGD abolished the effect of heme binding on the enzymatic activity. Therefore, coordination of His227 to heme appeared to induce reorientation of the domains containing Cys105, leading to a decrease in the enzymatic activity. This is the first report indicating that the PBGD activity is controlled by heme, the final product of heme biosynthesis. This finding improves our understanding of the mechanism by which heme biosynthesis is regulated.

Alzheimer's disease: as it was in the beginning

Since Alzheimer's disease was first described in 1907, many attempts have been made to reveal its main cause. Nowadays, two forms of the disease are known, and while the hereditary form of the disease is clearly caused by mutations in one of several genes, the etiology of the sporadic form remains a mystery. Both forms share similar sets of neuropathological and molecular manifestations, including extracellular deposition of amyloid-beta, intracellular accumulation of hyperphosphorylated tau protein, disturbances in both the structure and functions of mitochondria, oxidative stress, metal ion metabolism disorders, impairment of N-methyl-D-aspartate receptor-related signaling pathways, abnormalities of lipid metabolism, and aberrant cell cycle reentry in some neurons. Such a diversity of symptoms led to proposition of various hypotheses for explaining the development of Alzheimer's disease, the amyloid hypothesis, which postulates the key role of amyloid-beta in Alzheimer's disease development, being the most prominent. However, this hypothesis does not fully explain all of the molecular abnormalities and is therefore heavily criticized. In this review, we propose a hypothetical model of Alzheimer's disease progression, assuming a key role of age-related mitochondrial dysfunction, as was postulated in the mitochondrial cascade hypothesis. Our model explains the connections between all the symptoms of Alzheimer's disease, with particular attention to autophagy, metal metabolism disorders, and aberrant cell cycle re-entry in neurons. Progression of the Alzheimer's disease appears to be a complex process involving aging and too many protective mechanisms affecting one another, thereby leading to even greater deleterious effects.

Effects of volatile anaesthetics on heme metabolism in a murine genetic model of Acute Intermittent Porphyria. A comparative study with other porphyrinogenic drugs

BACKGROUND

Acute Intermittent Porphyria (AIP) is an inherited disease produced by a deficiency of Porphobilinogen deaminase (PBG-D). The aim of this work was to evaluate the effects of Isoflurane and Sevoflurane on heme metabolism in a mouse genetic model of AIP to further support our previous proposal for avoiding their use in porphyric patients. A comparative study was performed administering the porphyrinogenic drugs allylisopropylacetamide (AIA), barbital and ethanol, and also between sex and mutation using AIP (PBG-D activity 70% reduced) and T1 (PBG-D activity 50% diminished) mice.

METHODS

The activities of 5-Aminolevulinic synthetase (ALA-S), PBG-D, Heme oxygenase (HO) and CYP2E1; the expression of ALA-S and the levels of 5-aminolevulinic acid (ALA) were measured in different tissues of mice treated with the drugs mentioned.

RESULTS

Isoflurane increased liver, kidney and brain ALA-S activity of AIP females but only affected kidney AIP males. Sevoflurane induced ALA-S activity in kidney and brain of female AIP group. PBG-D activity was further reduced by Isoflurane in liver male T1; in AIP male mice activity remained in its low basal levels. Ethanol and barbital also caused biochemical alterations. Only AIA triggered neurological signs similar to those observed during human acute attacks in male AIP being the symptoms less pronounced in females although ALA-S induction was greater. Heme degradation was affected.

DISCUSSION

Biochemical alterations caused by the porphyrinogenic drugs assayed were different in male and female mice and also between T1 and AIP being more affected the females of AIP group.

GENERAL SIGNIFICANCE

This is the first study using volatile anaesthetics in an AIP genetic model confirming Isoflurane and Sevoflurane porphyrinogenicity.

Modulation of mitochondrial metabolism as a biochemical trait in blood feeding organisms: the redox vampire hypothesis redux

Hematophagous organisms undergo remarkable metabolic changes during the blood digestion process, increasing fermentative glucose metabolism, and reducing respiratory rates, both consequence of functional mitochondrial remodeling. Here, we review the pathways involved in energy metabolism and mitochondrial functionality in a comparative framework across different hematophagous species, and consider how these processes regulate redox homeostasis during blood digestion. The trend across distinct species indicate that a switch in energy metabolism might represent an important defensive mechanism to avoid the potential harmful interaction of oxidants generated from aerobic energy metabolism with products derived from blood digestion. Indeed, in insect vectors, blood feeding transiently reduces respiratory rates and oxidant production, irrespective of tissue and insect model. On the other hand, a different scenario is observed in several unrelated parasite species when exposed to blood digestion products, as respiratory rates reduce and mitochondrial oxidant production increase. The emerging picture indicates that re-wiring of energy metabolism, through reduced mitochondrial function, culminates in improved tolerance to redox insults and seems to represent a key step for hematophagous organisms to cope with the overwhelming and potentially toxic blood meal.

Iron deficiency beyond erythropoiesis: should we be concerned?

OBJECTIVE

To consider the key implications of iron deficiency for biochemical and physiological functions beyond erythropoiesis.

METHODS

PubMed was searched for relevant journal articles published up to August 2017.

RESULTS

Anemia is the most well-recognized consequence of persisting iron deficiency, but various other unfavorable consequences can develop either before or concurrently with anemia. Mitochondrial function can be profoundly disturbed since iron is a cofactor for heme-containing enzymes and non-heme iron-containing enzymes in the mitochondrial electron transport chain. Biosynthesis of heme and iron-sulfur clusters in the mitochondria is inhibited, disrupting synthesis of compounds such as hemoglobin, myoglobin, cytochromes and nitric oxide synthase. The physiological consequences include fatigue, lethargy, and dyspnea; conversely, iron repletion in iron-deficient individuals has been shown to improve exercise capacity. The myocardium, with its high energy demands, is particularly at risk from the effects of iron deficiency. Randomized trials have found striking improvements in disease severity in anemic but also non-anemic chronic heart failure patients with iron deficiency after iron therapy. In vitro and pre-clinical studies have demonstrated that iron is required by numerous enzymes involved in DNA replication and repair, and for normal cell cycle regulation. Iron is also critical for immune cell growth, proliferation, and differentiation, and for specific cell-mediated effector pathways. Observational studies have shown that iron-deficient individuals have defective immune function, particularly T-cell immunity, but more evidence is required. Pre-clinical models have demonstrated abnormal myelogenesis, brain cell metabolism, neurotransmission, and hippocampal formation in iron-deficient neonates and young animals. In humans, iron deficiency anemia is associated with poorer cognitive and motor skills. However, the impact of iron deficiency without anemia is less clear.

CONCLUSION

The widespread cellular and physiological effects of iron deficiency highlight the need for early detection and treatment of iron deficiency, both to ameliorate these non-erythropoietic effects, and to avoid progression to iron deficiency anemia.

Iron metabolism in erythroid cells and patients with congenital sideroblastic anemia

Sideroblastic anemias are anemic disorders characterized by the presence of ring sideroblasts in a patient's bone marrow. These disorders are typically divided into two types, congenital or acquired sideroblastic anemia. Recently, several genes were reported as responsible for congenital sideroblastic anemia; however, the relationship between the function of the gene products and ring sideroblasts is largely unclear. In this review article, we will focus on the iron metabolism in erythroid cells as well as in patients with congenital sideroblastic anemia.

The NO-heme signaling hypothesis

While the biological role of nitric oxide (NO) synthase (NOS) is appreciated, several fundamental aspects of the NOS/NO-related signaling pathway(s) remain incompletely understood. Canonically, the NOS-derived NO diffuses through the (inter)cellular milieu to bind the prosthetic ferro(Fe²⁺)-heme group of the soluble guanylyl cyclase (sGC). The formation of ternary NO-ferroheme-sGC complex results in the enzyme activation and accelerated production of the second messenger, cyclic GMP. This paper argues that cells dynamically generate mobile/exchangeable NO-ferroheme species, which activate sGC and regulate the function of some other biomolecules. In contrast to free NO, the mobile NO-ferroheme may ensure safe, efficient and coordinated delivery of the signal within and between cells. The NO-heme signaling may contribute to a number of NOS/NO-related phenomena (e.g. nitrite bioactivity, selective protein S-(N-)nitrosation, endothelium and erythrocyte-dependent vasodilation, some neural and immune NOS functions) and predicts new NO-related discoveries, diagnostics and therapeutics.

Physiological stress-induced corticosterone increases heme uptake via KLF4-HCP1 signaling pathway in hippocampus neurons

Iron overload has attracted much attention because of its adverse effect in increasing the risk of developing several neurodegenerative disorders. Under various pathologic conditions, a lot of heme are released. The aggregation of heme is more neurotoxic than that of iron released from the heme breakdown. Our previous studies demonstrated that psychological stress (PS) is a risk factor of cerebral iron metabolism disorders, thus causing iron accumulation in rat brains. In the present study, we found PS could increase heme uptake via heme carrier protein 1 (HCP1) in rat brains. We demonstrated that Glucocorticoid (GC), which is largely secreted under stress, could up-regulate HCP1 expression, thus promoting heme uptake in neurons. We also ascertained that HCP1 expression can be induced by GC through a transcription factor, Krüppel-like factor 4 (KLF4). These results may gain new insights into the etiology of heme uptake and iron accumulation in PS rats, and find new therapeutic targets of iron accumulation in Parkinson’s disease or Alzheimer’s disease.

Dynamics of absorption, metabolism, and excretion of 5-aminolevulinic acid in human intestinal Caco-2 cells

5-Aminolevulinic acid (ALA) is a precursor for the biosynthesis of porphyrins and heme. Although the oral administration of ALA has been widely applied in clinical settings, the dynamics of its absorption, metabolism, and excretion within enterocytes remain unknown. In this study, after enterocytic differentiation, Caco-2 cells were incubated with 200 µM ALA and/or 100 µM sodium ferrous citrate (SFC) for up to 72 h. Both ALA and the combination of ALA and SFC promoted the synthesis of heme, without affecting the expression of genes involved in intestinal iron transport, such as DMT1 and FPN. The enhanced heme synthesis in Caco-2 cells was more pronounced under the effect of the combination of ALA and SFC than under the effect of ALA alone, as reflected by the induced expression of heme oxygenase 1 (HO-1), as well as a reduced protein level of the transcriptional corepressor Bach1. Chromatin immunoprecipitation analysis confirmed Bach1 chromatin occupancy at the enhancer regions of HO-1, which were significantly decreased by the addition of ALA and SFC. Finally, Transwell culture of Caco-2 cells suggested that the administered ALA to the intestinal lumen was partially transported into vasolateral space. These findings enhance our understanding of the absorption and metabolism of ALA in enterocytes, which could aid in the development of a treatment strategy for various conditions such as anemia.

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Combination of ALA and SFC promotes heme synthesis than ALA alone in Caco-2 cells.

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Heme induces HO-1 by inhibiting transcriptional corepressor Bach1 in Caco-2 cells.

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Addition of ALA to intestinal lumen was partially transported to vasolateral space.

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Our data enhance the understanding of the dynamics of ALA in enterocytes.

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.