ATP-binding cassette B10 regulates early steps of heme synthesis

Heme metabolism in nonerythroid cells

Heme is an iron-containing prosthetic group necessary for the function of several proteins termed "hemoproteins." Erythrocytes contain most of the body's heme in the form of hemoglobin and contain high concentrations of free heme. In nonerythroid cells, where cytosolic heme concentrations are 2 to 3 orders of magnitude lower, heme plays an essential and often overlooked role in a variety of cellular processes. Indeed, hemoproteins are found in almost every subcellular compartment and are integral in cellular operations such as oxidative phosphorylation, amino acid metabolism, xenobiotic metabolism, and transcriptional regulation. Growing evidence reveals the participation of heme in dynamic processes such as circadian rhythms, NO signaling, and the modulation of enzyme activity. This dynamic view of heme biology uncovers exciting possibilities as to how hemoproteins may participate in a range of physiologic systems. Here, we discuss how heme is regulated at the level of its synthesis, availability, redox state, transport, and degradation and highlight the implications for cellular function and whole organism physiology.

Cardiac PET Imaging of ATP Binding Cassette (ABC) Transporters: Opportunities and Challenges

Adenosine triphosphate binding cassette (ABC) transporters are a broad family of membrane protein complexes that use energy to transport molecules across cells and/or intracellular organelle lipid membranes. Many drugs used to treat cardiac diseases have an affinity for these transporters. Among others, P-glycoprotein (P-gp) plays an essential role in regulating drug concentrations that reach cardiac tissue and therefore contribute to cardiotoxicity. As a molecular imaging modality, positron emission tomography (PET) has emerged as a viable technique to investigate the function of P-gp in organs and tissues. Using PET imaging to evaluate cardiac P-gp function provides new insights for drug development and improves the precise use of medications. Nevertheless, information in this field is limited. In this review, we aim to examine the current applications of ABC transporter PET imaging and its tracers in the heart, with a specific emphasis on P-gp. Furthermore, the opportunities and challenges in this novel field will be discussed.

Ironing out the details: ferroptosis and its relevance to diabetic cardiomyopathy

Ferroptosis is a newly identified myocardial cell death mechanism driven by iron-dependent lipid peroxidation. The presence of elevated intramyocardial lipid levels and excessive iron in patients with diabetes suggest a predominant role of ferroptosis in diabetic cardiomyopathy. As myocardial cell death is a precursor of heart failure, and intensive glycemic control cannot abate the increased risk of heart failure in patients with diabetes, targeting myocardial cell death via ferroptosis is a promising therapeutic avenue to prevent and/or treat diabetic cardiomyopathy. This review provides updated and comprehensive molecular mechanisms underpinning ferroptosis, clarifies several misconceptions about ferroptosis, emphasizes the importance of ferroptosis in diabetes-induced myocardial cell death, and offers valuable approaches to evaluate and target ferroptosis in the diabetic heart. Furthermore, basic concepts and ideas presented in this review, including glutathione peroxidase-4-independent and mitochondrial mechanisms of ferroptosis, are also important for investigating ferroptosis in other diabetic organs, as well as nondiabetic and metabolically compromised hearts.

Unleashing the potential of 5-Aminolevulinic acid: Unveiling a promising target for cancer diagnosis and treatment beyond photodynamic therapy

The therapeutic properties of 5-aminolevulinic acid (5-ALA) have been extensively studied for cancer detection and treatment using photodynamic therapy (PDT). When administered externally, 5-ALA is converted to protoporphyrin IX (PpIX) in cancer cells, which generates reactive oxygen species (ROS) upon exposure to light. This process enables targeted cell death induction and cancer detection. Given the highly conserved nature of heme biosynthesis over billions of years, we hypothesized that natural mechanisms might exist to prevent excessive accumulation of PpIX or heme resulting from 5-ALA overload. Therefore, we anticipated alterations in protein expression profiles upon exogenous administration of 5-ALA. To understand cellular responses to 5-ALA, we investigated protein expression changes and identified OR1B1 as a promising target in bladder, prostate, lung, and cervical cancer cells. OR1B1 expression was observed only with 5-ALA and ferrous chloride, highlighting the central role of heme in this discovery. Immunofluorescence and electron microscopy confirmed OR1B1's sub-cellular localization. These findings suggest that 5-ALA transformation in cancer cells and OR1B1 expression have potential for enhancing cancer detection and developing alternative treatments, including immunotherapy. This approach overcomes the limitations of PDT and opens new avenues for effective and targeted cancer interventions.

Loss of the mitochondrial protein Abcb10 results in altered arginine metabolism in MEL and K562 cells and nutrient stress signaling through ATF4

Abcb10 is a mitochondrial membrane protein involved in hemoglobinization of red cells. Abcb10 topology and ATPase domain localization suggest it exports a substrate, likely biliverdin, out of mitochondria that is necessary for hemoglobinization. In this study we generated Abcb10 deletion cell lines in both mouse murine erythroleukemia (MEL) and human erythroid precursor human myelogenous leukemia (K562) cells to better understand the consequences of Abcb10 loss. Loss of Abcb10 resulted in an inability to hemoglobinize upon differentiation in both K562 and MEL cells with reduced heme and intermediate porphyrins and decreased levels of aminolevulinic acid synthase 2 activity. Metabolomic and transcriptional analyses revealed that Abcb10 loss gave rise to decreased cellular arginine levels, increased transcripts for cationic and neutral amino acid transporters with reduced levels of the citrulline to arginine converting enzymes argininosuccinate synthetase and argininosuccinate lyase. The reduced arginine levels in Abcb10 null cells gave rise to decreased proliferative capacity. Arginine supplementation improved both Abcb10 null proliferation and hemoglobinization upon differentiation. Abcb10 null cells showed increased phosphorylation of Eukaryotic Translation Initiation Factor 2 Subunit Alpha (eIF2A), increased expression of nutrient sensing transcription factor ATF4 and downstream targets DNA damage inducible transcript 3 (Chop), ChaC glutathione specific gamma-glutamylcyclotransferase 1 (Chac1) and arginyl-tRNA synthetase 1 (Rars). These results suggest that when the Abcb10 substrate is trapped in the mitochondria, the nutrient sensing machinery is turned on remodeling transcription to block protein synthesis necessary for proliferation and hemoglobin biosynthesis in erythroid models.

Iron homeostasis in the heart: Molecular mechanisms and pharmacological implications

Iron is necessary for the life of practically all living things, yet it may also harm people toxically. Accordingly, humans and other mammals have evolved an effective and tightly regulatory system to maintain iron homeostasis in healthy tissues, including the heart. Iron deficiency is common in patients with heart failure, and is associated with worse prognosis in this population; while the prevalence of iron overload-related cardiovascular disorders is also increasing. Therefore, enhancing the therapy of patients with cardiovascular disorders requires a thorough understanding of iron homeostasis. Here, we give readers an overview of the fundamental mechanisms governing systemic iron homeostasis as well as the most recent knowledge about the intake, storage, use, and export of iron from the heart. Genetic mouse models used for investigation of iron metabolism in various in vivo scenarios are summarized and highlighted. We also go through different clinical conditions and therapeutic approaches that target cardiac iron dyshomeostasis. Finally, we conclude the review by outlining the present knowledge gaps and important open questions in this field in order to guide future research on cardiac iron metabolism.

The molecular and metabolic landscape of iron and ferroptosis in cardiovascular disease

The maintenance of iron homeostasis is essential for proper cardiac function. A growing body of evidence suggests that iron imbalance is the common denominator in many subtypes of cardiovascular disease. In the past 10 years, ferroptosis, an iron-dependent form of regulated cell death, has become increasingly recognized as an important process that mediates the pathogenesis and progression of numerous cardiovascular diseases, including atherosclerosis, drug-induced heart failure, myocardial ischaemia-reperfusion injury, sepsis-induced cardiomyopathy, arrhythmia and diabetic cardiomyopathy. Therefore, a thorough understanding of the mechanisms involved in the regulation of iron metabolism and ferroptosis in cardiomyocytes might lead to improvements in disease management. In this Review, we summarize the relationship between the metabolic and molecular pathways of iron signalling and ferroptosis in the context of cardiovascular disease. We also discuss the potential targets of ferroptosis in the treatment of cardiovascular disease and describe the current limitations and future directions of these novel treatment targets.

Genomic adaptation of Ethiopian indigenous cattle to high altitude

The mountainous areas of Ethiopia represent one of the most extreme environmental challenges in Africa faced by humans and other inhabitants. Selection for high-altitude adaptation is expected to have imprinted the genomes of livestock living in these areas. Here we assess the genomic signatures of positive selection for high altitude adaptation in three cattle populations from the Ethiopian mountainous areas (Semien, Choke, and Bale mountains) compared to three Ethiopian lowland cattle populations (Afar, Ogaden, and Boran), using whole-genome resequencing and three genome scan approaches for signature of selection (iHS, XP-CLR, and PBS). We identified several candidate selection signature regions and several high-altitude adaptation genes. These include genes such as ITPR2, MB, and ARNT previously reported in the human population inhabiting the Ethiopian highlands. Furthermore, we present evidence of strong selection and high divergence between Ethiopian high- and low-altitude cattle populations at three new candidate genes (CLCA2, SLC26A2, and CBFA2T3), putatively linked to high-altitude adaptation in cattle. Our findings provide possible examples of convergent selection between cattle and humans as well as unique African cattle signature to the challenges of living in the Ethiopian mountainous regions.

A Thiosemicarbazone Derivative as a Booster in Photodynamic Therapy-A Way to Improve the Therapeutic Effect

Photodynamic therapy is one of the most patient friendly and promising anticancer therapies. The active ingredient is irradiated protoporphyrin IX, which is produced in the body that transfers energy to the oxygen-triggering phototoxic reaction. This effect could be enhanced by using iron chelators, which inhibit the final step of heme biosynthesis, thereby increasing the protoporphyrin IX concentration. In the presented work, we studied thiosemicarbazone derivative, which is a universal enhancer of the phototoxic effect. We examined several genes that are involved in the transport of the heme substrates and heme itself. The results indicate that despite an elevated level of ABCG2, which is responsible for the PpIX efflux, its concentration in a cell is sufficient to trigger a photodynamic reaction. This effect was not observed for 5-ALA alone. The analyzed cell lines differed in the scale of the effect and a correlation with the PpIX accumulation was observed. Additionally, an increased activation of the iron transporter MFNR1 was also detected, which indicated that the regulation of iron transport is essential in PDT.

Identification of crucial genes and metabolites regulating the eggshell brownness in chicken

BACKGROUND

Protoporphyrin IX (Pp IX) is the primary pigment for brown eggshells. However, the regulatory mechanisms directing Pp IX synthesis, transport, and genetic regulation during eggshell calcification in chickens remain obscure. In this study, we investigated the mechanism of brown eggshell formation at different times following oviposition, using White Leghorn hens (WS group), Rhode Island Red light brown eggshell line hens (LBS group) and Rhode Island Red dark brown eggshell line hens (DBS group).

RESULTS

At 4, 16 and 22 h following oviposition, Pp IX concentrations in LBS and DBS groups were significantly higher in shell glands than in liver (P < 0.05). Pp IX concentrations in shell glands of LBS and DBS groups at 16 and 22 h following oviposition were significantly higher than WS group (P < 0.05). In comparative transcriptome analysis, δ-aminolevulinate synthase 1 (ALAS1), solute carrier family 25 member 38 (SLC25A38), ATP binding cassette subfamily G member 2 (ABCG2) and feline leukemia virus subgroup C cellular receptor 1 (FLVCR1), which were associated with Pp IX synthesis, were identified as differentially expressed genes (DEGs). RT-qPCR results showed that the expression level of ALAS1 in shell glands was significantly higher in DBS group than in WS group at 16 and 22 h following oviposition (P < 0.05). In addition, four single nucleotide polymorphisms (SNPs) in ALAS1 gene that were significantly associated with eggshell brownness were identified. By identifying the differential metabolites in LBS and DBS groups, we found 11-hydroxy-E4-neuroprostane in shell glands and 15-dehydro-prostaglandin E1(1-) and prostaglandin G2 2-glyceryl ester in uterine fluid were related to eggshell pigment secretion.

CONCLUSIONS

In this study, the regulatory mechanisms of eggshell brownness were studied comprehensively by different eggshell color and time following oviposition. Results show that Pp IX is synthesized de novo and stored in shell gland, and ALAS1 is a key gene regulating Pp IX synthesis in the shell gland. We found three transporters in Pp IX pathway and three metabolites in shell glands and uterine fluid that may influence eggshell browning.

A Recap of Heme Metabolism towards Understanding Protoporphyrin IX Selectivity in Cancer Cells

Mitochondria are essential organelles of mammalian cells, often emphasized for their function in energy production, iron metabolism and apoptosis as well as heme synthesis. The heme is an iron-loaded porphyrin behaving as a prosthetic group by its interactions with a wide variety of proteins. These complexes are termed hemoproteins and are usually vital to the whole cell comportment, such as the proteins hemoglobin, myoglobin or cytochromes, but also enzymes such as catalase and peroxidases. The building block of porphyrins is the 5-aminolevulinic acid, whose exogenous administration is able to stimulate the entire heme biosynthesis route. In neoplastic cells, this methodology repeatedly demonstrated an accumulation of the ultimate heme precursor, the fluorescent protoporphyrin IX photosensitizer, rather than in healthy tissues. While manifold players have been proposed, numerous discrepancies between research studies still dispute the mechanisms underlying this selective phenomenon that yet requires intensive investigations. In particular, we wonder what are the respective involvements of enzymes and transporters in protoporphyrin IX accretion. Is this mainly due to a boost in protoporphyrin IX anabolism along with a drop of its catabolism, or are its transporters deregulated? Additionally, can we truly expect to find a universal model to explain this selectivity? In this report, we aim to provide our peers with an overview of the currently known mitochondrial heme metabolism and approaches that could explain, at least partly, the mechanism of protoporphyrin IX selectivity towards cancer cells.

Regulation of Heme Synthesis by Mitochondrial Homeostasis Proteins

Heme plays a central role in diverse, life-essential processes that range from ubiquitous, housekeeping pathways such as respiration, to highly cell-specific ones such as oxygen transport by hemoglobin. The regulation of heme synthesis and its utilization is highly regulated and cell-specific. In this review, we have attempted to describe how the heme synthesis machinery is regulated by mitochondrial homeostasis as a means of coupling heme synthesis to its utilization and to the metabolic requirements of the cell. We have focused on discussing the regulation of mitochondrial heme synthesis enzymes by housekeeping proteins, transport of heme intermediates, and regulation of heme synthesis by macromolecular complex formation and mitochondrial metabolism. Recently discovered mechanisms are discussed in the context of the model organisms in which they were identified, while more established work is discussed in light of technological advancements.

Intracellular IL-32 regulates mitochondrial metabolism, proliferation, and differentiation of malignant plasma cells

Interleukin-32 (IL-32) is a nonclassical cytokine expressed in cancers, inflammatory diseases, and infections. Its expression is regulated by two different oxygen sensing systems; HIF1α and cysteamine dioxygenase (ADO), indicating that IL-32 may be involved in the response to hypoxia. We here demonstrate that endogenously expressed, intracellular IL-32 interacts with components of the mitochondrial respiratory chain and promotes oxidative phosphorylation. Knocking out IL-32 in three myeloma cell lines reduced cell survival and proliferation in vitro and in vivo. High-throughput transcriptomic and MS-metabolomic profiling of IL-32 KO cells revealed that cells depleted of IL-32 had perturbations in metabolic pathways, with accumulation of lipids, pyruvate precursors, and citrate. IL-32 was expressed in a subgroup of myeloma patients with inferior survival, and primary myeloma cells expressing IL-32 had a gene signature associated with immaturity, proliferation, and oxidative phosphorylation. In conclusion, we demonstrate a previously unrecognized role of IL-32 in the regulation of plasma cell metabolism.

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Intracellular IL-32 is an endogenous growth factor for malignant plasma cells

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IL-32 interacts with components of the electron transport chain

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IL-32 promotes oxidative phosphorylation

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IL-32 is expressed by immature, CD45 + highly proliferating malignant plasma cells

Triad role of hepcidin, ferroportin, and Nrf2 in cardiac iron metabolism: From health to disease

Iron is an essential trace element required for several vital physiological and developmental processes, including erythropoiesis, bone, and neuronal development. Iron metabolism and oxygen homeostasis are interlinked to perform a vital role in the functionality of the heart. The metabolic machinery of the heart utilizes almost 90 % of oxygen through the electron transport chain. To handle this tremendous level of oxygen, the iron metabolism in the heart is utmost crucial. Iron availability to the heart is therefore tightly regulated by (i) the hepcidin/ferroportin axis, which controls dietary iron absorption, storage, and recycling, and (ii) iron regulatory proteins 1 and 2 (IRP1/2) via hypoxia inducible factor 1 (HIF1) pathway. Despite iron being vital to the heart, recent investigations have demonstrated that iron imbalance is a common manifestation in conditions of heart failure (HF), since free iron readily transforms between Fe²⁺ and Fe³⁺via the Fenton reaction, leading to reactive oxygen species (ROS) production and oxidative damage. Therefore, to combat iron-mediated oxidative stress, targeting Nrf2/ARE antioxidant signaling is rational. The involvement of Nrf2 in regulating several genes engaged in heme synthesis, iron storage, and iron export is beginning to be uncovered. Consequently, it is possible that Nrf2/hepcidin/ferroportin might act as an epicenter connecting iron metabolism to redox alterations. However, the mechanism bridging the two remains obscure. In this review, we tried to summarize the contemporary insight of how cardiomyocytes regulate intracellular iron levels and discussed the mechanisms linking cardiac dysfunction with iron imbalance. Further, we emphasized the impact of Nrf2 on the interplay between systemic/cardiac iron control in the context of heart disease, particularly in myocardial ischemia and HF.

ATP-binding cassette transporters mediate differential biosynthesis of glycosphingolipid species

The cytosolic-oriented glucosylceramide (GlcCer) synthase is enigmatic, requiring nascent GlcCer translocation to the luminal Golgi membrane to access glycosphingolipid (GSL) anabolic glycosyltransferases. The mechanism by which GlcCer is flipped remains unclear. To investigate the role of GlcCer-binding partners in this process, we previously made cleavable, biotinylated, photoreactive GlcCer analogs in which the reactive nitrene was closely apposed to the GlcCer head group, while maintaining a C16-acyl chain. GlcCer-binding protein specificity was validated for both photoprobes. Using one probe, XLB, here we identified ATP-binding cassette (ABC) transporters ABCA3, ABCB4, and ABCB10 as unfractionated microsomal GlcCer-binding proteins in DU-145 prostate tumor cells. siRNA knockdown (KD) of these transporters differentially blocked GSL synthesis assessed in toto and via metabolic labeling. KD of ABCA3 reduced acid/neutral GSL levels, but increased those of LacCer, while KD of ABCB4 preferentially reduced neutral GSL levels, and KD of ABCB10 reduced levels of both neutral and acidic GSLs. Depletion of ABCA12, implicated in GlcCer transport, preferentially decreased neutral GSL levels, while ABCB1 KD preferentially reduced gangliosides, but increased neutral GSL Gb3. These results imply that multiple ABC transporters may provide distinct but overlapping GlcCer and LacCer pools within the Golgi lumen for anabolism of different GSL series by metabolic channeling. Differential ABC family member usage may fine-tune GSL biosynthesis depending on cell/tissue type. We conclude that ABC transporters provide a new tool for the regulation of GSL biosynthesis and serve as potential targets to reduce selected GSL species/subsets in diseases in which GSLs are dysregulated.

The convoluted history of haem biosynthesis

The capacity of haem to transfer electrons, bind diatomic gases, and catalyse various biochemical reactions makes it one of the essential biomolecules on Earth and one that was likely used by the earliest forms of cellular life. Since the description of haem biosynthesis, our understanding of this multi-step pathway has been almost exclusively derived from a handful of model organisms from narrow taxonomic contexts. Recent advances in genome sequencing and functional studies of diverse and previously neglected groups have led to discoveries of alternative routes of haem biosynthesis that deviate from the 'classical' pathway. In this review, we take an evolutionarily broad approach to illuminate the remarkable diversity and adaptability of haem synthesis, from prokaryotes to eukaryotes, showing the range of strategies that organisms employ to obtain and utilise haem. In particular, the complex evolutionary histories of eukaryotes that involve multiple endosymbioses and horizontal gene transfers are reflected in the mosaic origin of numerous metabolic pathways with haem biosynthesis being a striking case. We show how different evolutionary trajectories and distinct life strategies resulted in pronounced tensions and differences in the spatial organisation of the haem biosynthesis pathway, in some cases leading to a complete loss of a haem-synthesis capacity and, rarely, even loss of a requirement for haem altogether.

Novel pharmacological approach for the prevention of multidrug resistance (MDR) in a human leukemia cell line

BACKGROUND

Drug resistance mechanisms are the regulatory factors associated with drug metabolism and drug transport to inward and outward of the target cells. Maybridge fragment (MBF) library is a collection of pharmacophore rich compounds having affinity with membrane transporters. This study has been designed to evaluate the efficacy of MBFs in overcoming the leukemic cells' resistance to imatinib.

METHODS

Imatinib resistant cells (K562-R) were prepared using myelogenous leukemia cell line (K562) by titration method. The four MBFs were prioritized for determining their effect on imatinib resistance. The cells were treated with imatinib and MBFs and the MTT assay was performed to evaluate the efficacy of MBFs in enhancing the imatinib mediated cell death. The transcript levels of Bcr-Abl1 gene and efflux transporter genes were determined by RT-qPCR analysis.

RESULTS

The MBFs enhanced the imatinib mediated cell death of K562-R cells. There was also a significant decrease in the mRNA levels of the major drug efflux genes (ABCB1, ABCB10, ABCC1 and ABCG2) when treated with a combination of imatinib and MBF in comparison to imatinib treatment alone.

CONCLUSION

The drug efflux is one of the mechanisms of multidrug resistance in cancer cells and the MBFs used in this study were all found to significantly overcome the imatinib resistance by limiting the expression of efflux genes. This study, therefore, highlights the potential of Maybridge compounds in treating the drug resistant leukemia.

Ironing out mechanisms of iron homeostasis and disorders of iron deficiency

Iron plays an important role in mammalian physiological processes. It is a critical component for the function of many proteins, including enzymes that require heme and iron-sulfur clusters. However, excess iron is also detrimental because of its ability to catalyze the formation of reactive oxygen species. As a result, cellular and systemic iron levels are tightly regulated to prevent oxidative damage. Iron deficiency can lead to a number of pathological conditions, the most prominent being anemia. Iron deficiency should be corrected to improve adult patients' symptoms and to facilitate normal growth during fetal development and childhood. However, inappropriate use of intravenous iron in chronic conditions, such as cancer and heart failure, in the absence of clear iron deficiency can lead to unwanted side effects. Thus, this form of therapy should be reserved for certain patients who cannot tolerate oral iron and need rapid iron replenishment. Here, we will review cellular and systemic iron homeostasis and will discuss complications of iron deficiency.

Whole genome re-sequencing identifies unique adaption of single nucleotide polymorphism, insertion/deletion and structure variation related to hypoxia in Tibetan chickens

BACKGROUND

The adaptation to hypoxia in high altitude areas has great research value in the field of biological sciences. Tibetan chicken has unique adaptability to high-altitude, low pressure and anoxic conditions, and served as a biological model to search for genetic diversity of hypoxia adaption.

METHODS

The whole genome re-sequencing technology was conducted to investigate the genetic diversity.

RESULTS

In this study, we obtained quantity genetic resource, contained 5164926 single nucleotide polymorphisms (SNPs), 237504 Insertion/Deletion (InDel), 55606 structural variation types in all chromosomes of Tibetan chicken. Moreover, 17154 non-synonymous mutations, 45763 synonymous mutations, 258 InDel mutations and 9468 structural mutations were detected in coding sequencing (CDS) region. Furthermore, SNPs occur in 591 genes, including HIF1A, VEGF, MAPK 8/9/10/11, PPARA/D/G, NOTCH2, and ABCs, which were involved in 14 hypoxia-related pathways, such as VEGF signaling pathway, MAPK signaling pathway, PPAR signaling pathway and Notch signaling pathway. Among them, 19 genes with non-synonymous SNP variation in CDS were identified. Moreover, structure variation in CDS also occurred in the mentioned above genes with SNPs.

CONCLUSIONS

This study provides useful targets for clarifying the hypoxia adaptability of the domestication of chickens in Tibetan and may help breeding efforts to develop improved breeds for the highlands.

Stimulation of the human mitochondrial transporter ABCB10 by zinc-mesoporphrin

Heme biosynthesis occurs through a series of reactions that take place within the cytoplasm and mitochondria, so intermediates need to move across these cellular compartments. However, the specific membrane transport mechanisms involved in the process are not yet identified. The ATP-binding cassette protein ABCB10 is essential for normal heme production, as knocking down this transporter in mice is embryonically lethal and accompanied by severe anemia plus oxidative damage. The role of ABCB10 is unknown, but given its location in the inner mitochondrial membrane, it has been proposed as a candidate to export either an early heme precursor or heme. Alternatively, ABCB10 might transport a molecule important for protection against oxidative damage. To help discern between these possibilities, we decided to study the effect of heme analogs, precursors, and antioxidant peptides on purified human ABCB10. Since substrate binding increases the ATP hydrolysis rate of ABC transporters, we have determined the ability of these molecules to activate purified ABCB10 reconstituted in lipid nanodiscs using ATPase measurements. Under our experimental conditions, we found that the only heme analog increasing ABCB10 ATPase activity was Zinc-mesoporphyrin. This activation of almost seventy percent was specific for ABCB10, as the ATPase activity of a negative control bacterial ABC transporter was not affected. The activation was also observed in cysteine-less ABCB10, suggesting that Zinc-mesoporphyrin's effect did not require binding to typical heme regulatory motifs. Furthermore, our data indicate that ABCB10 was not directly activated by neither the early heme precursor delta-aminolevulinic acid nor glutathione, downsizing their relevance as putative substrates for this transporter. Although additional studies are needed to determine the physiological substrate of ABCB10, our findings reveal Zinc-mesoporphyrin as the first tool compound to directly modulate ABCB10 activity and raise the possibility that some actions of Zinc-mesoporphyrin in cellular and animal studies could be mediated by ABCB10.

Genetic screens reveal CCDC115 as a modulator of erythroid iron and heme trafficking

Transferrin-bound iron (TBI), the physiological circulating iron form, is acquired by cells through the transferrin receptor (TfR1) by endocytosis. In erythroid cells, most of the acquired iron is incorporated into heme in the mitochondria. Cellular trafficking of heme is indispensable for erythropoiesis and many other essential biological processes. Comprehensive elucidation of molecular pathways governing and regulating cellular iron acquisition and heme trafficking is required to better understand physiological and pathological processes affecting erythropoiesis. Here, we report the first genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) screens in human erythroid cells to identify determinants of iron and heme uptake, as well as heme-mediated erythroid differentiation. We identified several candidate modulators of TBI acquisition including TfR1, indicating that our approach effectively revealed players mechanistically relevant to the process. Interestingly, components of the endocytic pathway were also revealed as potential determinants of transferrin acquisition. We deciphered a role for the vacuolar-type H+ - ATPase (V- ATPase) assembly factor coiled-coil domain containing 115 (CCDC115) in TBI uptake and validated this role in CCDC115 deficient K562 cells. Our screen in hemin-treated cells revealed perturbations leading to cellular adaptation to heme, including those corresponding to trafficking mechanisms and transcription factors potentiating erythroid differentiation. Pathway analysis indicated that endocytosis and vesicle acidification are key processes for heme trafficking in erythroid precursors. Furthermore, we provided evidence that CCDC115, which we identified as required for TBI uptake, is also involved in cellular heme distribution. This work demonstrates a previously unappreciated common intersection in trafficking of transferrin iron and heme in the endocytic pathway of erythroid cells.

Symbiont replacements reset the co-evolutionary relationship between insects and their heritable bacteria

Auchenorrhynchan insects (Hemiptera) generally depend on two bacterial symbionts for nutrition. These bacteria experience extreme genome reduction and loss of essential cell functions that require direct host support, or the replacement of failing symbionts with more capable ones. However, it remains unclear how hosts adapt to integrate symbionts into their systems, particularly when they are replaced. Here, we comparatively investigated the evolution of host-support mechanisms in the glassy-winged sharpshooter, Homalodisca vitripennis (GWSS), and the aster leafhopper, Macrosteles quadrilineatus (ALF). ALF harbors the ancestral co-symbionts of the Auchenorrhyncha that have tiny genomes, Sulcia (190 kb) and Nasuia (112 kb). In GWSS, Sulcia retains an expanded genome (245 kb), but Nasuia was replaced by the more capable Baumannia (686 kb). To support their symbionts, GWSS and ALF have evolved novel mechanisms via horizontal gene transfer, gene duplication, and co-option of mitochondrial support genes. However, GWSS has fewer support systems targeting essential bacterial processes. In particular, although both hosts use ancestral mechanisms to support Sulcia, GWSS does not encode all of the same support genes required to sustain Sulcia-ALF or Nasuia. Moreover, GWSS support of Baumannia is far more limited and tailored to its expanded capabilities. Our results demonstrate how symbiont replacements shape host genomes and the co-evolutionary process.

Regulation of mitochondrial iron homeostasis by sideroflexin 2

Mitochondrial iron is indispensable for heme biosynthesis and iron–sulfur cluster assembly. Several mitochondrial transmembrane proteins have been implicated to function in the biosynthesis of heme and iron–sulfur clusters by transporting reaction intermediates. However, several mitochondrial proteins related to iron metabolism remain uncharacterized. Here, we show that human sideroflexin 2 (SFXN2), a member of the SFXN protein family, is involved in mitochondrial iron metabolism. SFXN2 is an evolutionarily conserved protein that localized to mitochondria via its transmembrane domain. SFXN2-knockout (KO) cells had an increased mitochondrial iron content, which was associated with decreases in the heme content and heme-dependent enzyme activities. By contrast, the activities of iron–sulfur cluster-dependent enzymes were unchanged in SFXN2-KO cells. Moreover, abnormal iron metabolism impaired mitochondrial respiration in SFXN2-KO cells and accelerated iron-mediated death of these cells. Our findings demonstrate that SFXN2 functions in mitochondrial iron metabolism by regulating heme biosynthesis.

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.

Identification of potential genes for human ischemic cardiomyopathy based on RNA-Seq data

Ischemic cardiomyopathy (ICM) is an important cause of heart failure, yet no ICM disease genes were stored in any public databases. Mutations of genes provided by RNA-Seq data could set a foundation for a variety of biological processes. This also made it possible to elucidate the mechanism and identify potential genes for ICM. In this paper, an integrated co-expression network was constructed using univariate and bivariate canonical correlation analysis for RNA-Seq data of human ICM samples. Three ICM-related modules were recognized after comparing between Pearson correlation coefficients of ICM samples and normal controls. Furthermore, 32 ICM potential genes were identified from ICM-related modules considering protein-protein interactions. Most of these genes were verified to be involved in ICM and diseases caused it by OMIM and literature. Our study could provide a novel perspective for potential gene identification and the pathogenesis for ICM and other complex diseases.

Reductions in the mitochondrial ABC transporter Abcb10 affect the transcriptional profile of heme biosynthesis genes

ATP-binding cassette subfamily B member 10 (Abcb10) is a mitochondrial ATP-binding cassette (ABC) transporter that complexes with mitoferrin1 and ferrochelatase to enhance heme biosynthesis in developing red blood cells. Reductions in Abcb10 levels have been shown to reduce mitoferrin1 protein levels and iron import into mitochondria, resulting in reduced heme biosynthesis. As an ABC transporter, Abcb10 binds and hydrolyzes ATP, but its transported substrate is unknown. Here, we determined that decreases in Abcb10 did not result in protoporphyrin IX accumulation in morphant-treated zebrafish embryos or in differentiated Abcb10-specific shRNA murine Friend erythroleukemia (MEL) cells in which Abcb10 was specifically silenced with shRNA. We also found that the ATPase activity of Abcb10 is necessary for hemoglobinization in MEL cells, suggesting that the substrate transported by Abcb10 is important in mediating increased heme biosynthesis during erythroid development. Inhibition of 5-aminolevulinic acid dehydratase (EC 4.2.1.24) with succinylacetone resulted in both 5-aminolevulinic acid (ALA) accumulation in control and Abcb10-specific shRNA MEL cells, demonstrating that reductions in Abcb10 do not affect ALA export from mitochondria and indicating that Abcb10 does not transport ALA. Abcb10 silencing resulted in an alteration in the heme biosynthesis transcriptional profile due to repression by the transcriptional regulator Bach1, which could be partially rescued by overexpression of Alas2 or Gata1, providing a mechanistic explanation for why Abcb10 shRNA MEL cells exhibit reduced hemoglobinization. In conclusion, our findings rule out that Abcb10 transports ALA and indicate that Abcb10's ATP-hydrolysis activity is critical for hemoglobinization and that the substrate transported by Abcb10 provides a signal that optimizes hemoglobinization.

Structures and functions of mitochondrial ABC transporters

A small number of physiologically important ATP-binding cassette (ABC) transporters are found in mitochondria. Most are half transporters of the B group forming homodimers and their topology suggests they function as exporters. The results of mutant studies point towards involvement in iron cofactor biosynthesis. In particular, ABC subfamily B member 7 (ABCB7) and its homologues in yeast and plants are required for iron-sulfur (Fe-S) cluster biosynthesis outside of the mitochondria, whereas ABCB10 is involved in haem biosynthesis. They also play a role in preventing oxidative stress. Mutations in ABCB6 and ABCB7 have been linked to human disease. Recent crystal structures of yeast Atm1 and human ABCB10 have been key to identifying substrate-binding sites and transport mechanisms. Combined with in vitro and in vivo studies, progress is being made to find the physiological substrates of the different mitochondrial ABC transporters.

KMT2D regulates specific programs in heart development via histone H3 lysine 4 di-methylation

KMT2D, which encodes a histone H3K4 methyltransferase, has been implicated in human congenital heart disease in the context of Kabuki syndrome. However, its role in heart development is not understood. Here, we demonstrate a requirement for KMT2D in cardiac precursors and cardiomyocytes during cardiogenesis in mice. Gene expression analysis revealed downregulation of ion transport and cell cycle genes, leading to altered calcium handling and cell cycle defects. We further determined that myocardial Kmt2d deletion led to decreased H3K4me1 and H3K4me2 at enhancers and promoters. Finally, we identified KMT2D-bound regions in cardiomyocytes, of which a subset was associated with decreased gene expression and decreased H3K4me2 in mutant hearts. This subset included genes related to ion transport, hypoxia-reoxygenation and cell cycle regulation, suggesting that KMT2D is important for these processes. Our findings indicate that KMT2D is essential for regulating cardiac gene expression during heart development primarily via H3K4 di-methylation.

Highlighted article: Cardiac-specific depletion of the H3K4 methyltransferase KMT2D causes dysregulation of genes associated with cell cycle regulation, ion homeostasis and hypoxia signaling.

Mitochondrial iron overload: causes and consequences

Pathological overload of iron in the mitochondrial matrix has been observed in numerous diseases, including sideroblastic anemias, which have many causes, and in genetic diseases that affect iron-sulfur cluster biogenesis, heme synthesis, and mitochondrial protein translation and its products. Although high expression of the mitochondrial iron importer, mitoferrin, appears to be an underlying common feature, it is unclear what drives high mitoferrin expression and what other proteins are involved in trapping excess toxic iron in the mitochondrial matrix. Numerous examples of human diseases and model systems suggest that mitochondrial iron homeostasis is coordinated through transcriptional remodeling. A cytosolic/nuclear molecule may affect a transcriptional factor to coordinate the events that lead to iron accumulation, but no candidates for this role have yet been identified.

A heme-binding domain controls regulation of ATP-dependent potassium channels

Heme iron has many and varied roles in biology. Most commonly it binds as a prosthetic group to proteins, and it has been widely supposed and amply demonstrated that subtle variations in the protein structure around the heme, including the heme ligands, are used to control the reactivity of the metal ion. However, the role of heme in biology now appears to also include a regulatory responsibility in the cell; this includes regulation of ion channel function. In this work, we show that cardiac KATP channels are regulated by heme. We identify a cytoplasmic heme-binding CXXHX16H motif on the sulphonylurea receptor subunit of the channel, and mutagenesis together with quantitative and spectroscopic analyses of heme-binding and single channel experiments identified Cys628 and His648 as important for heme binding. We discuss the wider implications of these findings and we use the information to present hypotheses for mechanisms of heme-dependent regulation across other ion channels.

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

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

Identification of the Mitochondrial Heme Metabolism Complex

Heme is an essential cofactor for most organisms and all metazoans. While the individual enzymes involved in synthesis and utilization of heme are fairly well known, less is known about the intracellular trafficking of porphyrins and heme, or regulation of heme biosynthesis via protein complexes. To better understand this process we have undertaken a study of macromolecular assemblies associated with heme synthesis. Herein we have utilized mass spectrometry with coimmunoprecipitation of tagged enzymes of the heme biosynthetic pathway in a developing erythroid cell culture model to identify putative protein partners. The validity of these data obtained in the tagged protein system is confirmed by normal porphyrin/heme production by the engineered cells. Data obtained are consistent with the presence of a mitochondrial heme metabolism complex which minimally consists of ferrochelatase, protoporphyrinogen oxidase and aminolevulinic acid synthase-2. Additional proteins involved in iron and intermediary metabolism as well as mitochondrial transporters were identified as potential partners in this complex. The data are consistent with the known location of protein components and support a model of transient protein-protein interactions within a dynamic protein complex.

The Effect of 5-Aminolevulinic Acid on Cytochrome P450-Mediated Prodrug Activation

Of late, numerous prodrugs are widely used for therapy. The hemeprotein cytochrome P450 (CYP) catalyzes the activation of prodrugs to form active metabolites. Therefore, the activation of CYP function might allow the use of lower doses of prodrugs and decrease toxicity. We hypothesized that the addition of 5-aminolevulinic acid (ALA), a precursor in the porphyrin biosynthetic pathway, enhances the synthesis of heme, leading to the up-regulation of CYP activity. To test this hypothesis, we treated a human gastric cancer cell line with ALA and determined the effect on CYP-dependent prodrug activation. For this purpose, we focused on the anticancer prodrug tegafur, which is converted to its active metabolite 5-fluorouracil (5-FU) mainly by CYP2A6. We show here that ALA increased CYP2A6-dependent tegafur activation, suggesting that ALA elevated CYP activity and potentiated the activation of the prodrug.

Regulation of heme biosynthesis and transport in metazoa

Heme is an iron-containing tetrapyrrole that plays a critical role in regulating a variety of biological processes including oxygen and electron transport, gas sensing, signal transduction, biological clock, and microRNA processing. Most metazoan cells synthesize heme via a conserved pathway comprised of eight enzyme-catalyzed reactions. Heme can also be acquired from food or extracellular environment. Cellular heme homeostasis is maintained through the coordinated regulation of synthesis, transport, and degradation. This review presents the current knowledge of the synthesis and transport of heme in metazoans and highlights recent advances in the regulation of these pathways.

ATP Binding and Hydrolysis Properties of ABCB10 and Their Regulation by Glutathione

ABCB10 (ATP binding cassette sub-family B10) is a mitochondrial inner-membrane ABC transporter. ABCB10 has been shown to protect the heart from the impact of ROS during ischemia-reperfusion and to allow for proper hemoglobin synthesis during erythroid development. ABC transporters are proteins that increase ATP binding and hydrolysis activity in the presence of the transported substrate. However, molecular entities transported by ABCB10 and its regulatory mechanisms are currently unknown. Here we characterized ATP binding and hydrolysis properties of ABCB10 by using the 8-azido-ATP photolabeling technique. This technique can identify potential ABCB10 regulators, transported substrates and amino-acidic residues required for ATP binding and hydrolysis. We confirmed that Gly497 and Lys498 in the Walker A motif, Glu624 in the Walker B motif and Gly602 in the C-Loop motif of ABCB10 are required for proper ATP binding and hydrolysis activity, as their mutation changed ABCB10 8-Azido-ATP photo-labeling. In addition, we show that the potential ABCB10 transported entity and heme precursor delta-aminolevulinic acid (dALA) does not alter 8-azido-ATP photo-labeling. In contrast, oxidized glutathione (GSSG) stimulates ATP hydrolysis without affecting ATP binding, whereas reduced glutathione (GSH) inhibits ATP binding and hydrolysis. Indeed, we detectABCB10 glutathionylation in Cys547 and show that it is one of the exposed cysteine residues within ABCB10 structure. In all, we characterize essential residues for ABCB10 ATPase activity and we provide evidence that supports the exclusion of dALA as a potential substrate directly transported by ABCB10. Last, we show the first molecular mechanism by which mitochondrial oxidative status, through GSH/GSSG, can regulate ABCB10.

Heme and erythropoieis: more than a structural role

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

Regulation of the human ABCB10 gene by E2F transcription factors

Here, we report for the first time a functional study of transcriptional regulation of the human ABCB10 gene. We cloned a functional promoter sequence, and then showed that E2F2, E2F3 and E2F4 can activate transcription from the ABCB10 promoter. We identified sites responsible for this activation and confirmed direct binding of E2F4 to these sites in EMSA and ChIP assays. Finally, by silencing the expression of E2F factors we demonstrated their importance in maintenance of the basal ABCB10 expression. This study provides important atypical examples of E2F4 being a transcriptional activator rather than repressor as well as directly binding to a promoter and regulating it through an alternative and classical DNA consensus response element sequences. It also provides a mechanistic link between E2F4 and ABCB10, both of which are involved in the same physiological phenomena: erythroid lineage differentiation and maturation as well as protection against cardiomyocyte cell death.

Tissue-specific transcript profiling for ABC transporters in the sequestering larvae of the phytophagous leaf beetle Chrysomela populi

BACKGROUND

Insects evolved ingenious adaptations to use extraordinary food sources. Particularly, the diet of herbivores enriched with noxious plant secondary metabolites requires detoxification mechanisms. Sequestration, which involves the uptake, transfer, and concentration of occasionally modified phytochemicals into specialized tissues or hemolymph, is one of the most successful detoxification strategies found in most insect orders. Due to the ability of ATP-binding cassette (ABC) carriers to transport a wide range of molecules including phytochemicals and xenobiotics, it is highly likely that they play a role in this sequestration process. To shed light on the role of ABC proteins in sequestration, we describe an inventory of putative ABC transporters in various tissues in the sequestering juvenile poplar leaf beetle, Chrysomela populi.

RESULTS

In the transcriptome of C. populi, we predicted 65 ABC transporters. To link the proteins with a possible function, we performed comparative phylogenetic analyses with ABC transporters of other insects and of humans. While tissue-specific profiling of each ABC transporter subfamily suggests that ABCB, C and G influence the plant metabolite absorption in the gut, ABCC with 14 members is the preferred subfamily responsible for the excretion of these metabolites via Malpighian tubules. Moreover, salicin, which is sequestered from poplar plants, is translocated into the defensive glands for further deterrent production. In these glands and among all identified ABC transporters, an exceptionally high transcript level was observed only for Cpabc35 (Cpmrp). RNAi revealed the deficiency of other ABC pumps to compensate the function of CpABC35, demonstrating its key role during sequestration.

CONCLUSION

We provide the first comprehensive phylogenetic study of the ABC family in a phytophagous beetle species. RNA-seq data from different larval tissues propose the importance of ABC pumps to achieve a homeostasis of plant-derived compounds and offer a basis for future analyses of their physiological function in sequestration processes.

Heme in pathophysiology: a matter of scavenging, metabolism and trafficking across cell membranes

Heme (iron-protoporphyrin IX) is an essential co-factor involved in multiple biological processes: oxygen transport and storage, electron transfer, drug and steroid metabolism, signal transduction, and micro RNA processing. However, excess free-heme is highly toxic due to its ability to promote oxidative stress and lipid peroxidation, thus leading to membrane injury and, ultimately, apoptosis. Thus, heme metabolism needs to be finely regulated. Intracellular heme amount is controlled at multiple levels: synthesis, utilization by hemoproteins, degradation and both intracellular and intercellular trafficking. This review focuses on recent findings highlighting the importance of controlling intracellular heme levels to counteract heme-induced oxidative stress. The contributions of heme scavenging from the extracellular environment, heme synthesis and incorporation into hemoproteins, heme catabolism and heme transport in maintaining adequate intracellular heme content are discussed. Particular attention is put on the recently described mechanisms of heme trafficking through the plasma membrane mediated by specific heme importers and exporters. Finally, the involvement of genes orchestrating heme metabolism in several pathological conditions is illustrated and new therapeutic approaches aimed at controlling heme metabolism are discussed.