Iron, Heme Synthesis and Erythropoietic Porphyrias: A Complex Interplay

The regulation and function of Nrf2 signaling in ferroptosis-activated cancer therapy

Ferroptosis is an iron-dependent programmed cell death process that involves lipid oxidation via the Fenton reaction to produce lipid peroxides, causing disruption of the lipid bilayer, which is essential for cellular survival. Ferroptosis has been implicated in the occurrence and treatment response of various types of cancer, and targeting ferroptosis has emerged as a promising strategy for cancer therapy. However, cancer cells can escape cellular ferroptosis by activating or remodeling various signaling pathways, including oxidative stress pathways, thereby limiting the efficacy of ferroptosis-activating targeted therapy. The key anti-oxidative transcription factor, nuclear factor E2 related factor 2 (Nrf2 or NFE2L2), plays a dominant role in defense machinery by reprogramming the iron, intermediate, and glutathione peroxidase 4 (GPX4)-related network and the antioxidant system to attenuate ferroptosis. In this review, we summarize the recent advances in the regulation and function of Nrf2 signaling in ferroptosis-activated cancer therapy and explore the prospect of combining Nrf2 inhibitors and ferroptosis inducers as a promising cancer treatment strategy.

Erythropoietic protoporphyrias: updates and advances

Protoporphyrias are caused by pathogenic variants in genes encoding enzymes involved in heme biosynthesis. They induce the accumulation of a hydrophobic phototoxic compound, protoporphyrin (PPIX), in red blood cells (RBCs). PPIX is responsible for painful cutaneous photosensitivity, which severely impairs quality of life. Hepatic elimination of PPIX increases the risk of cholestatic liver disease, requiring lifelong monitoring. Treatment options are scarce and mainly limited to supportive care such as protection from visible light. Here, we review the pathophysiology of protoporphyrias, their diagnosis, and current recommendations for medical care. We discuss new therapeutic strategies, some of which are currently undergoing clinical trials and are likely to radically alter the severity of the disease in the years to come.

ALAS2 overexpression alleviates oxidative stress-induced ferroptosis in aortic aneurysms via GATA1 activation

Background

Aortic aneurysm, characterized by abnormal dilation of the aorta, poses significant health risks. This study aims to investigate the interaction between 5-aminolevulinate synthase 2 (ALAS2) and GATA-binding protein 1 (GATA1) in ferroptosis and oxidative stress responses in aortic aneurysm.

Methods

A weighted gene co-expression network analysis (WGCNA) was performed on the differentially expressed genes (DEGs) within the GSE9106 dataset to identify the key module. Subsequently, protein-protein interaction (PPI) network analysis was performed on the key module. Mouse aortic vascular smooth muscle cells (MOVAS) were treated with hydrogen peroxide (H2O2) to induce oxidative stress, and ferroptosis inducers and inhibitors were added to evaluate their effects on iron content and oxidative stress markers. Through a series of in vitro cellular experiments, we assessed cell viability, expression levels of GATA1 and iron mutation-associated proteins, as well as cellular phenotypes such as inflammatory responses and apoptosis rates.

Results

Three candidate genes (ALAS2, GYPA, and GYPB) were upregulated in the thoracic aortic aneurysm (TAA) samples of the GSE9106 dataset. The H2O2 treatment increased the MOVAS cells' iron content and oxidative stress, upregulated ALAS2 protein levels, and decreased the ferroptosis-related protein levels. ALAS2 overexpression reversed H2O2-induced apoptosis and increased the inflammatory cytokine levels. Additionally, the knockdown of GATA1 partially reversed the protective mechanism of overexpressed ALAS2 on H2O2-induced ferroptosis.

Conclusions

ALAS2 overexpression reduced H2O2-induced oxidative damage and iron-induced apoptosis in MOVAS cells, and GATA1 knockdown partially reversed this protective effect. These findings suggested that the ALAS2 and GATA1 regulatory pathways may be potential therapeutic targets in aortic aneurysms.

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Management of erythropoietic protoporphyria with cholestatic liver disease: A case report

Erythropoietic protoporphyria (EPP) is a rare metabolic disease of the heme biosynthetic pathway where an enzymatic dysfunction results in protoporphyrin IX (PPIX) accumulation in erythroid cells. The porphyrins are photo-reactive and are responsible for severe photosensitivity in patients, thus drastically decreasing their quality of life. The liver eliminates PPIX and as such, the main and rare complication of EPP is progressive cholestatic liver disease, which can lead to liver failure. The management of this complication is challenging, as it often requires a combination of approaches to promote PPIX elimination and suppress the patient's erythropoiesis. Here we described a 3-year follow-up of an EPP patient, with three episodes of liver involvement, aggravated by the coexistence of a factor VII deficiency. It covers all the different types of intervention available for the management of liver disease, right through to successful allogeneic hematopoietic stem cell transplantation.

How I treat erythropoietic protoporphyria and X-linked protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited cutaneous porphyria caused by reduced expression of ferrochelatase, the enzyme that catalyzes the final step in heme biosynthesis. The resultant accumulation of protoporphyrin IX leads to severe, painful cutaneous photosensitivity, as well as potentially life-threatening liver disease in a small percentage of patients. X-linked protoporphyria (XLP) is clinically similar to EPP but results from increased activity of δ-aminolevulinic acid synthase 2, the first step in heme biosynthesis in the bone marrow, and also causes protoporphyrin accumulation. Although historically the management of EPP and XLP (collectively termed protoporphyria) centered around avoidance of sunlight, novel therapies have recently been approved or are in development, which will alter the therapeutic landscape for these conditions. We present 3 patient cases, highlighting key treatment considerations in patients with protoporphyria, including (1) approach to photosensitivity, (2) managing iron deficiency in protoporphyria, and (3) understanding hepatic failure in protoporphyria.

Enniatin B and beauvericin affect intestinal cell function and hematological processes in Atlantic salmon (Salmo salar) after acute exposure

Unintentional use of mold-infested plant-based feed ingredients are sources of mycotoxins in fish feeds. The presence of the emerging mycotoxins ENNB and BEA in Norwegian commercial fish feeds and plant-based feed ingredients has raised concerns regarding the health effects on farmed Atlantic salmon (Salmon salar). Atlantic salmon pre-smolts were exposed to non-lethal doses of BEA and ENNB (ctrl, 50 and 500 μg/kg feed for 12 h), after which total RNA sequencing of the intestine and liver was carried out to evaluate gut health and identify possible hepatological changes after acute dietary exposure. ENNB and BEA did not trigger acute toxicity, however ENNB caused the onset of pathways linked to acute intestinal inflammation and BEA exposures caused the onset of hepatic hematological disruption. The prevalence and concentration of ENNB found in today's commercial feed could affect the fish health if consumed over a longer time-period.

Protoporphyrin IX Binds to Iron(II)-Loaded and to Zinc-Loaded Human Frataxin

(1) Background: Human frataxin is an iron binding protein that participates in the biogenesis of iron sulfur clusters and enhances ferrochelatase activity. While frataxin association to other proteins has been extensively characterized up to the structural level, much less is known about the putative capacity of frataxin to interact with functionally related metabolites. In turn, current knowledge about frataxin's capacity to coordinate metal ions is limited to iron (II and III); (2) Methods: here, we used NMR spectroscopy, Molecular Dynamics, and Docking approaches to demonstrate new roles of frataxin; (3) Results: We demonstrate that frataxin also binds Zn²⁺ in a structurally similar way to Fe²⁺, but with lower affinity. In turn, both Fe²⁺-loaded and Zn²⁺-loaded frataxins specifically associate to protoporphyrin IX with micromolar affinity, while apo-frataxin does not bind to the porphyrin. Protoporphyrin IX association to metal-loaded frataxin shares the binding epitope with ferrochelatase; and (4) Conclusions: these findings expand the plethora of relevant molecular targets for frataxin and may help to elucidate the yet unknown different roles that this protein exerts in iron regulation and metabolism.

Iron Metabolism in the Disorders of Heme Biosynthesis

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

A primer on heme biosynthesis

Heme (protoheme IX) is an essential cofactor for a large variety of proteins whose functions vary from one electron reactions to binding gases. While not ubiquitous, heme is found in the great majority of known life forms. Unlike most cofactors that are acquired from dietary sources, the vast majority of organisms that utilize heme possess a complete pathway to synthesize the compound. Indeed, dietary heme is most frequently utilized as an iron source and not as a source of heme. In Nature there are now known to exist three pathways to synthesize heme. These are the siroheme dependent (SHD) pathway which is the most ancient, but least common of the three; the coproporphyrin dependent (CPD) pathway which with one known exception is found only in gram positive bacteria; and the protoporphyrin dependent (PPD) pathway which is found in gram negative bacteria and all eukaryotes. All three pathways share a core set of enzymes to convert the first committed intermediate, 5-aminolevulinate (ALA) into uroporphyrinogen III. In the current review all three pathways are reviewed as well as the two known pathways to synthesize ALA. In addition, interesting features of some heme biosynthesis enzymes are discussed as are the regulation and disorders of heme biosynthesis.

Heme Biosynthetic Gene Expression Analysis With dPCR in Erythropoietic Protoporphyria Patients

Background: The heme biosynthesis (HB) involves eight subsequent enzymatic steps. Erythropoietic protoporphyria (EPP) is caused by loss-of-function mutations in the ferrochelatase (FECH) gene, which in the last HB step inserts ferrous iron into protoporphyrin IX (PPIX) to form heme.

Aim and method: The aim of this work was to for the first time analyze the mRNA expression of all HB genes in peripheral blood samples of patients with EPP having the same genotype FECH c.[215dupT]; [315-48T > C] as compared to healthy controls by highly sensitive and specific digital PCR assays (dPCR).

Results: We confirmed a decreased FECH mRNA expression in patients with EPP. Further, we found increased ALAS2 and decreased ALAS1, CPOX, PPOX and HMBS mRNA expression in patients with EPP compared to healthy controls. ALAS2 correlated with FECH mRNA expression (EPP: r = 0.63, p = 0.03 and controls: r = 0.68, p = 0.02) and blood parameters like PPIX (EPP: r = 0.58 p = 0.06).

Conclusion: Our method is the first that accurately quantifies HB mRNA from blood samples with potential applications in the monitoring of treatment effects of mRNA modifying therapies in vivo, or investigation of the HB pathway and its regulation. However, our findings should be studied in separated blood cell fractions and on the enzymatic level.

An Extended C-Terminus, the Possible Culprit for Differential Regulation of 5-Aminolevulinate Synthase Isoforms

5-Aminolevulinate synthase (ALAS; E.C. 2.3.1.37) is a pyridoxal 5′-phosphate (PLP)-dependent enzyme that catalyzes the key regulatory step of porphyrin biosynthesis in metazoa, fungi, and α-proteobacteria. ALAS is evolutionarily related to transaminases and is therefore classified as a fold type I PLP-dependent enzyme. As an enzyme controlling the key committed and rate-determining step of a crucial biochemical pathway ALAS is ideally positioned to be subject to allosteric feedback inhibition. Extensive kinetic and mutational studies demonstrated that the overall enzyme reaction is limited by subtle conformational changes of a hairpin loop gating the active site. These findings, coupled with structural information, facilitated early prediction of allosteric regulation of activity via an extended C-terminal tail unique to eukaryotic forms of the enzyme. This prediction was subsequently supported by the discoveries that mutations in the extended C-terminus of the erythroid ALAS isoform (ALAS2) cause a metabolic disorder known as X-linked protoporphyria not by diminishing activity, but by enhancing it. Furthermore, kinetic, structural, and molecular modeling studies demonstrated that the extended C-terminal tail controls the catalytic rate by modulating conformational flexibility of the active site loop. However, the precise identity of any such molecule remains to be defined. Here we discuss the most plausible allosteric regulators of ALAS activity based on divergences in AlphaFold-predicted ALAS structures and suggest how the mystery of the mechanism whereby the extended C-terminus of mammalian ALASs allosterically controls the rate of porphyrin biosynthesis might be unraveled.

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.

Microcytosis in Erythropoietic Protoporphyria

Partial deficiency of the last enzyme of the heme biosynthetic pathway, namely, ferrochelatase (FECH), is responsible for erythropoietic protoporphyria (EPP) in humans. This disorder is characterized by painful skin photosensitivity, due to excessive protoporphyrin IX (PPIX) production in erythrocytes. Although several papers report the presence of iron deficiency anemia in about 50% of EPP patients, there is still no a conclusive explanation of the why this occurs. In the present work, we explored hematological indices and iron status in 20 unrelated Italian EPP patients in order to propose a new hypothesis. Our data show that microcytosis is present in EPP patients also in the absence of anemia and iron deficiency with a link between PPIX accumulation and reduced MCV, probably indicating an indirect condition of heme deficiency. Patients studied had a downward shift of iron parameters due to increased hepcidin concentrations only in a state of repleted iron stores. Interestingly, hemoglobin synthesis was not limited by iron supply except in cases with further iron loss, in which concomitantly increased soluble transferrin (Tf) receptor (sTfR) levels were detected. The mechanisms involved in the iron uptake downregulation in EPP remain unclear, and the role of PPIX accumulation in microcytosis.