Regulation of hepcidin expression by inflammation-induced activin B

Inhibin subunit beta B (INHBB): an emerging role in tumor progression

The gene inhibin subunit beta B (INHBB) encodes the inhibin βB subunit, which is involved in forming protein members of the transforming growth factor-β (TGF-β) superfamily. The TGF-β superfamily is extensively involved in cell proliferation, differentiation, adhesion, movement, metabolism, communication, and death. Activins and inhibins, which belong to the TGF-β superfamily, were first discovered in ovarian follicular fluid. They were initially described as regulators of pituitary follicle-stimulating hormone (FSH) secretion both in vivo and in vitro. Later studies found that INHBB is expressed not only in reproductive organs such as the ovary, uterus, and testis but also in numerous other organs, including the brain, spinal cord, liver, kidneys, and adrenal glands. This wide distribution implies its involvement in the normal physiological functions of various organs; however, the mechanisms underlying these functions have not yet been fully elucidated. Recent studies suggest that INHBB plays a significant, yet complex role in tumorigenesis. It appears to have dual effects, promoting tumor progression in some contexts while inhibiting it in others, although these roles are not yet fully understood. In this paper, we review the different expression patterns, functions, and mechanisms of INHBB in normal and tumor tissues to illustrate the research prospects of INHBB in tumor progression.

Activin E upregulates uncoupling protein 1 and fibroblast growth factor 21 in brown adipocytes

Activin E activates brown and beige adipocytes and has been controversially implicated as a factor that induces obesity and fatty liver. Here, we sought to address this controversial issue by producing recombinant human activin E to evaluate its effects on HB2 brown adipocytes in vitro. Activin E increased uncoupling protein 1 (Ucp1) and fibroblast growth factor 21 (Fgf21) mRNA expression in the adipocytes. This upregulation was suppressed by SB431542, an inhibitor of activin receptor-like kinase (Alk) TGF-β type I receptors. SB431542 also inhibited the activin E-induced phosphorylation of Smad2/3. A promoter assay using a CAGA-Luc reporter and Alk expression vectors revealed that activin E activated the TGF-β/activin pathway via Alk7. The upregulation of Ucp1 and Fgf21 mRNA might be mediated through Alk7 and Smad2/3 phosphorylation. Activin E is a potential stimulator of energy expenditure by activating brown adipocytes and highlights its potential as a therapeutic target for treating obesity.

Hypoferremia of inflammation: Innate host defense against infections

Iron is an essential nutrient for microbes, plants and animals. Multicellular organisms have evolved multiple strategies to control invading microbes by restricting microbial access to iron. Hypoferremia of inflammation is a rapidly-acting organismal response that prevents the formation of iron species that would be readily accessible to microbes. This review takes an evolutionary perspective to explore the mechanisms and host defense function of hypoferremia of inflammation and its clinical implications.

Gastrointestinal pharmacology activins in liver health and disease

Activins are a subgroup of the TGFβ superfamily of growth and differentiation factors, dimeric in nature and consisting of two inhibin beta subunits linked via a disulfide bridge. Canonical activin signaling occurs through Smad2/3, with negative feedback initiated by Smad6/7 following signal transduction, which binds activin type I receptor preventing phosphorylation of Smad2/3 and activation of downstream signaling. In addition to Smad6/7, other inhibitors of activin signaling have been identified as well, including inhibins (dimers of an inhibin alpha and beta subunit), BAMBI, Cripto, follistatin, and follistatin-like 3 (fstl3). To date, activins A, B, AB, C, and E have been identified and isolated in mammals, with activin A and B having the most characterization of biological activity. Activin A has been implicated as a regulator of several important functions of liver biology, including hepatocyte proliferation and apoptosis, ECM production, and liver regeneration; the role of other subunits of activin in liver physiology are less understood. There is mounting data to suggest a link between dysregulation of activins contributing to various hepatic diseases such as inflammation, fibrosis, and hepatocellular carcinoma, and emerging studies demonstrating the protective and regenerative effects of inhibiting activins in mouse models of liver disease. Due to their importance in liver biology, activins demonstrate utility as a therapeutic target for the treatment of hepatic diseases such as cirrhosis, NASH, NAFLD, and HCC; further research regarding activins may provide diagnostic or therapeutic opportunity for those suffering from various liver diseases.

An emerging class of new therapeutics targeting TGF, Activin, and BMP ligands in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an often fatal condition, the primary pathology of which involves loss of pulmonary vascular perfusion due to progressive aberrant vessel remodeling. The reduced capacity of the pulmonary circulation places increasing strain on the right ventricle of the heart, leading to death by heart failure. Currently, licensed therapies are primarily vasodilators, which have increased the median post-diagnosis life expectancy from 2.8 to 7 years. Although this represents a substantial improvement, the search continues for transformative therapeutics that reverse established disease. The genetics of human PAH heavily implicates reduced endothelial bone morphogenetic protein (BMP) signaling as a causal role for the disease pathobiology. Recent approaches have focused on directly enhancing BMP signaling or removing the inhibitory influence of pathways that repress BMP signaling. In this critical commentary, we review the evidence underpinning the development of two approaches: BMP-based agonists and inhibition of activin/GDF signaling. We also address the key considerations and questions that remain regarding these approaches.

Inactivating the Uninhibited: The Tale of Activins and Inhibins in Pulmonary Arterial Hypertension

Advances in technology and biomedical knowledge have led to the effective diagnosis and treatment of an increasing number of rare diseases. Pulmonary arterial hypertension (PAH) is a rare disorder of the pulmonary vasculature that is associated with high mortality and morbidity rates. Although significant progress has been made in understanding PAH and its diagnosis and treatment, numerous unanswered questions remain regarding pulmonary vascular remodeling, a major factor contributing to the increase in pulmonary arterial pressure. Here, we discuss the role of activins and inhibins, both of which belong to the TGF-β superfamily, in PAH development. We examine how these relate to signaling pathways implicated in PAH pathogenesis. Furthermore, we discuss how activin/inhibin-targeting drugs, particularly sotatercep, affect pathophysiology, as these target the afore-mentioned specific pathway. We highlight activin/inhibin signaling as a critical mediator of PAH development that is to be targeted for therapeutic gain, potentially improving patient outcomes in the future.

Transcriptional activation of hepcidin by the microphthalmia/transcription factor E family

Hepcidin negatively regulates the circulating iron levels by inhibiting the intestinal absorption of iron as well as iron release from macrophages. Hepcidin activity is largely determined by its expression, which is regulated at the transcriptional level. Hepcidin transcription is induced not only by the iron status-related bone morphogenetic protein (BMP)-2/6, but also by inflammatory cytokines, such as interleukin (IL)-1β and IL-6. The present study reveals that the microphthalmia (MiT)/transcription factor E (TFE) family members are novel regulators of hepcidin transcription. Melanocyte-inducing transcription factor (MITF)-A, a member of the MiT/TFE family, was identified as a positive regulator of hepcidin transcription via stimulus screening for transcription regulators. An E-box (5'-CATGTG-3') spanning nt-645 to nt-640 of the murine hepcidin promoter was identified as an MITF-A-responsive element. Responsiveness to MITF-A on hepcidin transcription decreased when the cells were stimulated with BMP2 or IL-1β. These results suggest a functional interaction between the MITF pathway and BMP- or IL-1β-mediated signaling. TFEB and TFE3, members of the MiT/TFE family, also stimulated hepcidin transcription, but the main region responsible for hepcidin transcription was distinct from that induced by MITF-A. The region spanning nt-581 to nt-526 was involved in TFEB/TFE3-mediated hepcidin transcription. Considering that members of the MiT/TFE family act as regulators of starvation-induced lysosomal biogenesis, hepcidin expression may be controlled by additional pathways apart from those identified so far.

Activin B promotes the initiation and progression of liver fibrosis

The role of activin B, a transforming growth factor β (TGFβ) superfamily cytokine, in liver health and disease is largely unknown. We aimed to investigate whether activin B modulates liver fibrogenesis. Liver and serum activin B, along with its analog activin A, were analyzed in patients with liver fibrosis from different etiologies and in mouse acute and chronic liver injury models. Activin B, activin A, or both was immunologically neutralized in mice with progressive or established carbon tetrachloride (CCl₄ )-induced liver fibrosis. Hepatic and circulating activin B was increased in human patients with liver fibrosis caused by several liver diseases. In mice, hepatic and circulating activin B exhibited persistent elevation following the onset of several types of liver injury, whereas activin A displayed transient increases. The results revealed a close correlation of activin B with liver injury regardless of etiology and species. Injured hepatocytes produced excessive activin B. Neutralizing activin B largely prevented, as well as improved, CCl₄ -induced liver fibrosis, which was augmented by co-neutralizing activin A. Mechanistically, activin B mediated the activation of c-Jun-N-terminal kinase (JNK), the induction of inducible nitric oxide synthase (iNOS) expression, and the maintenance of poly (ADP-ribose) polymerase 1 (PARP1) expression in injured livers. Moreover, activin B directly induced a profibrotic expression profile in hepatic stellate cells (HSCs) and stimulated these cells to form a septa structure. Conclusions: We demonstrate that activin B, cooperating with activin A, mediates the activation or expression of JNK, iNOS, and PARP1 and the activation of HSCs, driving the initiation and progression of liver fibrosis.

Factors affecting the induction of uncoupling protein 1 in C2C12 myogenic cells

Brown/beige adipocytes, which are derived from skeletal muscle/smooth muscle-lineage cells, consume excess energy as heat through the expression of mitochondrial uncoupling protein 1 (UCP1). Previous studies have shown that forced expression of PR/SET domain (PRDM)-16 or early B-cell factor (EBF)-2 induced UCP1-positive adipocytes in C2C12 myogenic cells. Here, we explored the culture conditions to induce Ucp1 expression in C2C12 cells without introducing exogenous genes. Treatment with rosiglitazone (a peroxisome proliferator-activated receptor (PPAR)-γ agonist), GW501516 (a PPARδ agonist), and bone morphogenetic protein (BMP)-7 for 8 days efficiently increased Ucp1 expression in response to treatment with forskolin, an activator of the protein kinase A pathway. BMP7 dose-dependently increased forskolin-induced Ucp1 expression in the presence of rosiglitazone and GW501516; however, GW501516 was not required for Ucp1 induction. Additionally, the structurally related proteins, BMP6 and BMP9, efficiently increased forskolin-induced Ucp1 expression in rosiglitazone-treated cells. UCP1 protein was localized in cells with lipid droplets, but adipocytes were not always positive for UCP1. Continuous treatment with BMP7 was needed for the efficient induction of Ucp1 by forskolin treatment. Significant expression of Prdm16 was not detected, irrespective of the treatment, and treatment with rosiglitazone, GW501516, and BMP7 did not affect the expression levels of Ebf2. Fibroblast growth factor receptor (Fgfr)-3 expression levels were increased by BMP9 in rosiglitazone-treated cells, and molecules that upregulate Fgfr3 transcription partly overlapped with those that stimulate Ucp1 transcription. The present results provide basic information on the practical differentiation of myogenic cells to brown adipocytes.

Tubule-derived INHBB promotes interstitial fibroblast activation and renal fibrosis

Upstream stimuli for myofibroblast activation are of considerable interest for understanding the mechanisms underlying renal fibrosis. Activin B, a member of the TGF-β family, exists as a homodimer of inhibin subunit beta B (INHBB), but its role in renal fibrosis remains unknown. We found that INHBB expression was significantly increased in various renal fibrosis models and human chronic kidney disease specimens with renal fibrosis. Notably, the increase of INHBB occurred mainly in the tubular epithelial cells (TECs). In vivo, inhibiting INHBB blocked the activation of interstitial fibroblasts and ameliorated the renal fibrosis induced by unilateral ureteral obstruction or ischemia-reperfusion injury, while ectopic expression of INHBB in the TECs was able to activate interstitial fibroblasts and initiate interstitial fibrosis. In vitro, overexpression of INHBB in TECs led to the secretion of activin B, thereby promoting the proliferation and activation of interstitial fibroblasts through activin B/Smad signaling. Furthermore, inhibition of activin B/Smad signaling attenuated the fibrotic response caused by tubular INHBB. Mechanistically, the upregulation of INHBB depended on the transcription factor Sox9 in the injured TECs. Clinical analyses also identified a positive correlation between Sox9 and INHBB expression in human specimens, suggesting the Sox9/INHBB axis as a positive regulator of renal fibrosis. In conclusion, tubule-derived INHBB is implicated in the pathogenesis of renal fibrosis by activating the surrounding fibroblasts in a paracrine manner, thereby exhibiting as a potential therapeutic target. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Biochemical, Biophysical and Functional Characterization of an Insoluble Iron Containing Hepcidin–Ferritin Chimeric Monomer Assembled Together with Human Ferritin H/L Chains at Different Molar Ratios

Hepcidin and ferritin are key proteins of iron homeostasis in mammals. In this study, we characterize a chimera by fusing camel hepcidin to a human ferritin H-chain to verify if it retained the properties of the two proteins. The construct (HepcH) is expressed in E. coli in an insoluble and iron-containing form. To characterize it, the product was incubated with ascorbic acid and TCEP to reduce and solubilize the iron, which was quantified with ferrozine. HepcH bound approximately five times more iron than the wild type human ferritin, due to the presence of the hepcidin moiety. To obtain a soluble and stable product, the chimera was denatured and renatured together with different amounts of L-ferritin of the H-chain in order to produce 24-shell heteropolymers with different subunit proportions. They were analyzed by denaturing and non-denaturing PAGE and by mass spectroscopy. At the 1:5 ratio of HepcH to H- or L-ferritin, a stable and soluble molecule was obtained. Its biological activity was verified by its ability to both bind specifically cell lines that express ferroportin and to promote ferroportin degradation. This chimeric molecule showed the ability to bind both mouse J774 macrophage cells, as well as human HepG2 cells, via the hepcidin–ferroportin axis. We conclude that the chimera retains the properties of both hepcidin and ferritin and might be exploited for drug delivery.

Iron, Copper, and Zinc Homeostasis: Physiology, Physiopathology, and Nanomediated Applications

Understanding of how the human organism functions has preoccupied researchers in medicine for a very long time. While most of the mechanisms are well understood and detailed thoroughly, medicine has yet much to discover. Iron (Fe), Copper (Cu), and Zinc (Zn) are elements on which organisms, ranging from simple bacteria all the way to complex ones such as mammals, rely on these divalent ions. Compounded by the continuously evolving biotechnologies, these ions are still relevant today. This review article aims at recapping the mechanisms involved in Fe, Cu, and Zn homeostasis. By applying the knowledge and expanding on future research areas, this article aims to shine new light of existing illness. Thanks to the expanding field of nanotechnology, genetic disorders such as hemochromatosis and thalassemia can be managed today. Nanoparticles (NPs) improve delivery of ions and confer targeting capabilities, with the potential for use in treatment and diagnosis. Iron deficiency, cancer, and sepsis are persisting major issues. While targeted delivery using Fe NPs can be used as food fortifiers, chemotherapeutic agents against cancer cells and microbes have been developed using both Fe and Cu NPs. A fast and accurate means of diagnosis is a major impacting factor on outcome of patients, especially when critically ill. Good quality imaging and bed side diagnostic tools are possible using NPs, which may positively impact outcome.

Physiology and Inflammation Driven Pathophysiology of Iron Homeostasis-Mechanistic Insights into Anemia of Inflammation and Its Treatment

Anemia is very common in patients with inflammatory disorders. Its prevalence is associated with severity of the underlying disease, and it negatively affects quality of life and cardio-vascular performance of patients. Anemia of inflammation (AI) is caused by disturbances of iron metabolism resulting in iron retention within macrophages, a reduced erythrocyte half-life, and cytokine mediated inhibition of erythropoietin function and erythroid progenitor cell differentiation. AI is mostly mild to moderate, normochromic and normocytic, and characterized by low circulating iron, but normal and increased levels of the storage protein ferritin and the iron hormone hepcidin. The primary therapeutic approach for AI is treatment of the underlying inflammatory disease which mostly results in normalization of hemoglobin levels over time unless other pathologies such as vitamin deficiencies, true iron deficiency on the basis of bleeding episodes, or renal insufficiency are present. If the underlying disease and/or anemia are not resolved, iron supplementation therapy and/or treatment with erythropoietin stimulating agents may be considered whereas blood transfusions are an emergency treatment for life-threatening anemia. New treatments with hepcidin-modifying strategies and stabilizers of hypoxia inducible factors emerge but their therapeutic efficacy for treatment of AI in ill patients needs to be evaluated in clinical trials.

Management of Adverse Events and Supportive Therapy in Relapsed/Refractory Multiple Myeloma

Simple Summary

Multiple myeloma (MM) patients with relapsing and/or refractory (RR) disease are exposed for a prolonged time to multiple drugs, which increase the risk of toxicity. In addition to tumor response, preserving the quality of life represents an important goal for this patient population. Therefore, supportive therapy plays a pivotal role in their treatment by limiting disease- and drug-related complications. The aim of this review is to outline current standards and future strategies to prevent and treat renal insufficiency, anemia, bone disease, and infection, including COVID-19, in RRMM patients. In addition, the incidence and treatment of side effects of novel anti-MM agents will be discussed.

Abstract

Relapsed/refractory (RR) multiple myeloma (MM) patients are a fragile population because of prolonged drug exposure and advanced age. Preserving a good quality of life is of high priority for these patients and the treatment of disease- and treatment-related complications plays a key role in their management. By preventing and limiting MM-induced complications, supportive care improves patients’ outcome. Erythropoietin-stimulating agents and bisphosphonates are well-established supportive strategies, yet novel agents are under investigation, such as anabolic bone agents and activin receptor-like kinase (ALK) inhibitors. The recent dramatic changes in the treatment landscape of MM pose an additional challenge for the routine care of RRMM patients. Multidrug combinations in first and later lines increase the risk for long-lasting toxicities, including adverse cardiovascular and neurological events. Moreover, recently approved first-in-class drugs have unique side-effect profiles, such as ocular toxicity of belantamab mafodotin or gastrointestinal toxicity of selinexor. This review discusses current standards in supportive treatment of RRMM patients, including recommendations in light of the recent SARS-CoV-19 pandemic, and critically looks at the incidence and management of side effects of standard as well as next generation anti-MM agents.

Isolation of the canine inhibin βB subunit gene and characterization of signalling mediated by canine inhibin βB

Activin B, a homodimer of the inhibin βB subunit, acts as a regulator of gonadal function and as an adipokine. To clarify the role of activin B in dogs, we characterized the canine inhibin βB gene and signalling pathways regulated by the canine inhibin βB. Using 5'- and 3'-rapid amplification of cDNA end (RACE) and RT-PCR on RNA isolated from the ovary of dogs, we identified short and long forms of the inhibin βB gene. Immunoreactive inhibin βB molecules were detected at ~25 and ~14 kDa under nonreducing and reducing conditions, respectively, in culture supernatants from HEK293 cells transfected with a plasmid containing the long form of the inhibin βB gene, indicating activin B production and secretion. Similar to human and murine activin B, the canine activin B-stimulated transcriptions of reporter genes, CAGA-luc and Hepcidin-luc, regulated by the canonical activin/transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) pathway, respectively. Activin B-induced CAGA-luc transcription was not detected in ALK7-deficient MDCK canine-derived cells; however, the forced expression of ALK7 resulted in the activin B-dependent expression in MDCK cells. Unexpectedly, the activin B-induced activation of the BMP pathway was partially blocked by the inhibition of endogenous activin/TGF-β receptor activity. The present study identified an experimentally isolated long form of the canine inhibin βB gene producing activin B that transactivates BMP- and activin/TGF-β-regulated gene expression.

Isoflurane post-conditioning attenuates cerebral ischemia/reperfusion injury by reducing apoptotic through activating the BMP7/SMAD signaling pathway in rats

The expressions of different temporal patterns of bone morphogenetic proteins (BMPs) have changed after ischemic strokes, and ischemic preconditioning-induced neuroprotection was attenuated when BMP7 was inhibited. In the previous study, the neuroprotection of isoflurane postconditioning (ISPOC) against cerebral ischemia-reperfusion (I/R) injury has been addressed, with particular relevance to the role of BMP7. Consequently, in the present study, we continued to explore the mechanisms involved in the BMP7 signal mediated the neuroprotection of ISPOC. A rat model of the middle cerebral artery occlusion was used in this study. Rats were administered 1.5 % isoflurane, 60 min after 90 min of ischemia, followed by a 24 h reperfusion period. The 1.5 % ISPOC significantly ameliorated the cerebral infarct volumes, neurologic deficit scores, damaged neurons, and apoptotic neurons. Moreover, ISPOC unregulated the expressions of BMP7, p-Smad1/5/9, and p-p38. Whereas, the neuroprotective effect was weakened by LDN-193189 and SB203580, respectively, a BMP7/Smad1/5/9 and p38MAPK signaling pathway inhibitor. Furthermore, LDN-193189 downregulated the expression of p-p38. The present results of this study indicated that the neuroprotection of 1.5 % isoflurane postconditioning to cerebral ischemia-reperfusion injury is related to the activating of BMP7/Smad1/5/9 and p38MAPK signal pathway.

Response to iron overload in cultured hepatocytes

Iron is essential for a variety of physiological processes. Hepatic iron overload acts as a trigger for the progression of hepatic steatosis to nonalcoholic steatohepatitis and hepatocellular carcinoma. In the present study, we aimed to study the effects of iron overload on cellular responses in hepatocytes. Rat primary hepatocytes (RPH), mouse primary hepatocytes (MPH), HepG2 human hepatoma cells and Hepa1-6 mouse hepatoma cells were treated with FeCl₃. Treatment with FeCl₃ effectively increased iron accumulation in primary hepatocytes. Expression levels of molecules involved in cellular signaling such as AMPK pathway, TGF-β family pathway, and MAP kinase pathway were decreased by FeCl₃ treatment in RPH. Cell viability in response to FeCl₃ treatment was decreased in RPH but not in HepG2 and Hepa1-6 cells. Treatment with FeCl₃ also decreased expression level of LC-3B, a marker of autophagy in RPH but not in liver-derived cell lines. Ultrastructural observations revealed that cell death resembling ferroptosis and necrosis was induced upon FeCl₃ treatment in RPH. The expression level of genes involved in iron transport varied among different liver-derived cells- iron is thought to be efficiently incorporated as free Fe²⁺ in primary hepatocytes, whereas transferrin-iron is the main route for iron uptake in HepG2 cells. The present study reveals specific cellular responses in different liver-derived cells as a consequence of iron overload.

Expression levels of brown/beige adipocyte-related genes in fat depots of vitamin A-restricted fattening cattle1

Brown/beige adipocytes dissipate energy as heat. We previously showed that brown/beige adipocytes are present in white adipose tissue (WAT) of fattening cattle. The present study examined the effect of vitamin A restriction on mRNA expression of brown/beige adipocyte-related genes. In Japan, fattening cattle are conventionally fed a vitamin A-restricted diet to improve beef marbling. Twelve Japanese Black steers aged 10 mo were fed control feed (n = 6) or vitamin A-restricted feed (n = 6) for 20 mo. Subcutaneous WAT (scWAT) and mesenteric WAT (mesWAT) were collected, and mRNA expression levels of molecules related to the function of brown/beige adipocytes (Ucp1, Cidea, Dio2, Cox7a, and Cox8b) as well as transcriptional regulators related to brown/beige adipogenesis (Zfp516, Nfia, Prdm16, and Pgc-1α) were evaluated. The vitamin A restriction significantly increased or tended to increase expression levels of Cidea and Pgc-1α in scWAT, and Cidea, Dio2, and Nfia in mesWAT. Previous studies revealed that the bone morphogenetic protein (Bmp) pathway was responsible for commitment of mesenchymal stem cells to brown/beige adipocyte-lineage cells. The vitamin A restriction increased expression of Bmp7 and some Bmp receptors in WAT. The interrelationship between gene expression levels indicated that expression levels of Nfia, Prdm16, and Pgc-1α were closely related to those of genes related to the function of brown/beige adipocytes in scWAT. Also, expression levels of Nfia, Prdm16, and Pgc-1α were highly correlated with those of Alk3 in scWAT. In summary, the present results suggest that the vitamin A restriction increases the number or activity of brown/beige adipocytes through regulatory expression of transcriptional regulators to induce brown/beige adipogenesis, especially in scWAT of fattening cattle, which may be governed by the Bmp pathway.

Therapeutic Advances in Regulating the Hepcidin/Ferroportin Axis

The interaction between hepcidin and ferroportin is the key mechanism involved in regulation of systemic iron homeostasis. This axis can be affected by multiple stimuli including plasma iron levels, inflammation and erythropoietic demand. Genetic defects or prolonged inflammatory stimuli results in dysregulation of this axis, which can lead to several disorders including hereditary hemochromatosis and anaemia of chronic disease. An imbalance in iron homeostasis is increasingly being associated with worse disease outcomes in many clinical conditions including multiple cancers and neurological disorders. Currently, there are limited treatment options for regulating iron levels in patients and thus significant efforts are being made to uncover approaches to regulate hepcidin and ferroportin expression. These approaches either target these molecules directly or regulatory steps which mediate hepcidin or ferroportin expression. This review examines the current status of hepcidin and ferroportin agonists and antagonists, as well as inducers and inhibitors of these proteins and their regulatory pathways.

The multifaceted role of iron in renal health and disease

Iron is an essential element that is indispensable for life. The delicate physiological body iron balance is maintained by both systemic and cellular regulatory mechanisms. The iron-regulatory hormone hepcidin assures maintenance of adequate systemic iron levels and is regulated by circulating and stored iron levels, inflammation and erythropoiesis. The kidney has an important role in preventing iron loss from the body by means of reabsorption. Cellular iron levels are dependent on iron import, storage, utilization and export, which are mainly regulated by the iron response element-iron regulatory protein (IRE-IRP) system. In the kidney, iron transport mechanisms independent of the IRE-IRP system have been identified, suggesting additional mechanisms for iron handling in this organ. Yet, knowledge gaps on renal iron handling remain in terms of redundancy in transport mechanisms, the roles of the different tubular segments and related regulatory processes. Disturbances in cellular and systemic iron balance are recognized as causes and consequences of kidney injury. Consequently, iron metabolism has become a focus for novel therapeutic interventions for acute kidney injury and chronic kidney disease, which has fuelled interest in the molecular mechanisms of renal iron handling and renal injury, as well as the complex dynamics between systemic and local cellular iron regulation.

The Importance of Iron Status for Young Children in Low- and Middle-Income Countries: A Narrative Review

Early childhood is characterised by high physiological iron demand to support processes including blood volume expansion, brain development and tissue growth. Iron is also required for other essential functions including the generation of effective immune responses. Adequate iron status is therefore a prerequisite for optimal child development, yet nutritional iron deficiency and inflammation-related iron restriction are widespread amongst young children in low- and middle-income countries (LMICs), meaning iron demands are frequently not met. Consequently, therapeutic iron interventions are commonly recommended. However, iron also influences infection pathogenesis: iron deficiency reduces the risk of malaria, while therapeutic iron may increase susceptibility to malaria, respiratory and gastrointestinal infections, besides reshaping the intestinal microbiome. This means caution should be employed in administering iron interventions to young children in LMIC settings with high infection burdens. In this narrative review, we first examine demand and supply of iron during early childhood, in relation to the molecular understanding of systemic iron control. We then evaluate the importance of iron for distinct aspects of physiology and development, particularly focusing on young LMIC children. We finally discuss the implications and potential for interventions aimed at improving iron status whilst minimising infection-related risks in such settings. Optimal iron intervention strategies will likely need to be individually or setting-specifically adapted according to iron deficiency, inflammation status and infection risk, while maximising iron bioavailability and considering the trade-offs between benefits and risks for different aspects of physiology. The effectiveness of alternative approaches not centred around nutritional iron interventions for children should also be thoroughly evaluated: these include direct targeting of common causes of infection/inflammation, and maternal iron administration during pregnancy.

Iron and liver fibrosis: Mechanistic and clinical aspects

Liver fibrosis is characterised by excessive deposition of extracellular matrix that interrupts normal liver functionality. It is a pathological stage in several untreated chronic liver diseases such as the iron overload syndrome hereditary haemochromatosis, viral hepatitis, alcoholic liver disease, non-alcoholic fatty liver disease, non-alcoholic steatohepatitis and diabetes. Interestingly, regardless of the aetiology, iron-loading is frequently observed in chronic liver diseases. Excess iron can feed the Fenton reaction to generate unquenchable amounts of free radicals that cause grave cellular and tissue damage and thereby contribute to fibrosis. Moreover, excess iron can induce fibrosis-promoting signals in the parenchymal and non-parenchymal cells, which accelerate disease progression and exacerbate liver pathology. Fibrosis regression is achievable following treatment, but if untreated or unsuccessful, it can progress to the irreversible cirrhotic stage leading to organ failure and hepatocellular carcinoma, where resection or transplantation remain the only curative options. Therefore, understanding the role of iron in liver fibrosis is extremely essential as it can help in formulating iron-related diagnostic, prognostic and treatment strategies. These can be implemented in isolation or in combination with the current approaches to prepone detection, and halt or decelerate fibrosis progression before it reaches the irreparable stage. Thus, this review narrates the role of iron in liver fibrosis. It examines the underlying mechanisms by which excess iron can facilitate fibrotic responses. It describes the role of iron in various clinical pathologies and lastly, highlights the significance and potential of iron-related proteins in the diagnosis and therapeutics of liver fibrosis.

Hepcidin and IL-1β

Hepcidin expression is determined through transcriptional regulation by systemic iron status. However, acute or chronic inflammation also increases the expression of hepcidin, which is associated with the dysregulation of iron metabolism in pathological conditions. Interleukin (IL)-6 has been suggested to be a principal molecule to confer inflammation-related hepcidin transcription, which is mediated via signal transducer and activator of transcription (STAT)-binding site on the hepcidin promoter. Recently, it has been uncovered that another pro-inflammatory cytokine IL-1β stimulates hepcidin expression through the distinct mechanism underlying IL-6-mediated hepcidin transcription. In addition to IL-6 induction, IL-1β stimulates expression of CCAAT-enhancer-binding protein (C/EBP)δ, a transcription factor, leading to transcriptional activation of hepcidin via C/EBP-binding site on the hepcidin promoter. Thus, hepcidin transcription is stimulated through multiple elements in response to proinflammatory cytokines. Relationships between increased production of IL-1β and dysregulated iron metabolism have been suggested in various diseases, which may be linked to overproduction of hepcidin.

Regulators of hepcidin expression

Iron, an essential nutrient, is required for many biological processes but is also toxic in excess. The lack of a mechanism to excrete excess iron makes it crucial for the body to regulate the amount of iron absorbed from the diet. This regulation is mediated by the hepatic hormone hepcidin. Hepcidin also controls iron release from macrophages that recycle iron and from hepatocytes that store iron. Hepcidin binds to the only known iron export protein, ferroportin, inducing its internalization and degradation and thus limiting the amount of iron released into the plasma. Important regulators of hepcidin, and therefore of systemic iron homeostasis, include plasma iron concentrations, body iron stores, infection and inflammation, hypoxia and erythropoiesis, and, to a lesser extent, testosterone. Dysregulation of hepcidin production contributes to the pathogenesis of many iron disorders: hepcidin deficiency causes iron overload in hereditary hemochromatosis and non-transfused β-thalassemia, whereas overproduction of hepcidin is associated with iron-restricted anemias seen in patients with chronic inflammatory diseases and inherited iron-refractory iron-deficiency anemia. The present review summarizes our current understanding of the molecular mechanisms and signaling pathways contributing to hepcidin regulation by these factors and highlights the issues that still need clarification.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Hepcidin Therapeutics

Hepcidin is a key hormonal regulator of systemic iron homeostasis and its expression is induced by iron or inflammatory stimuli. Genetic defects in iron signaling to hepcidin lead to “hepcidinopathies” ranging from hereditary hemochromatosis to iron-refractory iron deficiency anemia, which are disorders caused by hepcidin deficiency or excess, respectively. Moreover, dysregulation of hepcidin is a pathogenic cofactor in iron-loading anemias with ineffective erythropoiesis and in anemia of inflammation. Experiments with preclinical animal models provided evidence that restoration of appropriate hepcidin levels can be used for the treatment of these conditions. This fueled the rapidly growing field of hepcidin therapeutics. Several hepcidin agonists and antagonists, as well as inducers and inhibitors of hepcidin expression have been identified to date. Some of them were further developed and are currently being evaluated in clinical trials. This review summarizes the state of the art.

Hepcidin-mediated hypoferremic response to acute inflammation requires a threshold of Bmp6/Hjv/Smad signaling

Systemic iron balance is controlled by hepcidin, a liver hormone that limits iron efflux to the bloodstream by promoting degradation of the iron exporter ferroportin in target cells. Iron-dependent hepcidin induction requires hemojuvelin (HJV), a bone morphogenetic protein (BMP) coreceptor that is disrupted in juvenile hemochromatosis, causing dramatic hepcidin deficiency and tissue iron overload. Hjv^-/- mice recapitulate phenotypic hallmarks of hemochromatosis but exhibit blunted hepcidin induction following lipopolysaccharide (LPS) administration. We show that Hjv^-/- mice fail to mount an appropriate hypoferremic response to acute inflammation caused by LPS, the lipopeptide FSL1, or Escherichia coli infection because residual hepcidin does not suffice to drastically decrease macrophage ferroportin levels. Hfe^-/- mice, a model of milder hemochromatosis, exhibit almost wild-type inflammatory hepcidin expression and associated effects, whereas double Hjv^-/-Hfe^-/- mice phenocopy single Hjv^-/- counterparts. In primary murine hepatocytes, Hjv deficiency does not affect interleukin-6 (IL-6)/Stat, and only slightly inhibits BMP2/Smad signaling to hepcidin; however, it severely impairs BMP6/Smad signaling and thereby abolishes synergism with the IL-6/Stat pathway. Inflammatory induction of hepcidin is suppressed in iron-deficient wild-type mice and recovers after the animals are provided overnight access to an iron-rich diet. We conclude that Hjv is required for inflammatory induction of hepcidin and controls the acute hypoferremic response by maintaining a threshold of Bmp6/Smad signaling. Our data highlight Hjv as a potential pharmacological target against anemia of inflammation.

Regulatory responses of hepatocytes, macrophages and vascular endothelial cells to magnesium deficiency

The liver is the organ that responds to nutritional disturbances including magnesium deficiency. The present study evaluated cellular responses to magnesium deficiency using model cells of the liver, namely, HepG2 cells as hepatocytes, RAW264.7 cells as Kupffer cells and human umbilical vein endothelial cells (HUVECs) as vascular endothelial cells; we examined effects of culture with magnesium deficient medium on cell responses in individual types of cells as well as interactive responses among cells. Metabolomic analyses indicated that magnesium deficiency differentially affected the cellular content of metabolites among HepG2 cells, RAW264.7 cells and HUVECs. The cellular content of the metabolites in HepG2 cells and HUVECs was also affected by the conditioned medium from RAW264.7 cells cultured with the magnesium-deficient media. The changes in HUVECs partly resembled those of the livers of magnesium-deficient rats previously described. RNA-seq analyses indicated that magnesium deficiency modulated the expression levels of molecules related to the ubiquitin-proteasome pathway and oxidative stress/antioxidant response in HepG2 cells and RAW264.7 cells, respectively. Furthermore, when HUVECs were co-cultured with RAW264.7 cells, lipopolysaccharide-induced expression of interleukin (IL)-1β and IL-6 was enhanced by magnesium deficiency, depending on the presence of RAW264.7 cells. The present study reveals that magnesium deficiency affects cellular metabolism in HepG2 liver cells, RAW264.7 macrophages and HUVECs, and that the modulation of cellular responses to extracellular magnesium deficiency in HUVECs depends on the presence of RAW264.7 cells. The complex responses in individual cells and through cell interactions partly explain the regulatory reaction to magnesium deficiency in the liver.

Role of Activins in Hepcidin Regulation during Malaria

Epidemiological observations have linked increased host iron with malaria susceptibility, and perturbed iron handling has been hypothesized to contribute to the potentially life-threatening anemia that may accompany blood-stage malaria infection. To improve our understanding of these relationships, we examined the pathways involved in regulation of the master controller of iron metabolism, the hormone hepcidin, in malaria infection. We show that hepcidin upregulation in Plasmodium berghei murine malaria infection was accompanied by changes in expression of bone morphogenetic protein (BMP)/sons of mothers against decapentaplegic (SMAD) pathway target genes, a key pathway involved in hepcidin regulation. We therefore investigated known agonists of the BMP/SMAD pathway and found that Bmp gene expression was not increased in infection. In contrast, activin B, which can signal through the BMP/SMAD pathway and has been associated with increased hepcidin during inflammation, was upregulated in the livers of Plasmodium berghei-infected mice; hepatic activin B was also upregulated at peak parasitemia during infection with Plasmodium chabaudi Concentrations of the closely related protein activin A increased in parallel with hepcidin in serum from malaria-naive volunteers infected in controlled human malaria infection (CHMI) clinical trials. However, antibody-mediated neutralization of activin activity during murine malaria infection did not affect hepcidin expression, suggesting that these proteins do not stimulate hepcidin upregulation directly. In conclusion, we present evidence that the BMP/SMAD signaling pathway is perturbed in malaria infection but that activins, although raised in malaria infection, may not have a critical role in hepcidin upregulation in this setting.

Iron and inflammation - the gut reaction

Anemia is a frequent complication of many inflammatory disorders, including inflammatory bowel disease. Although the pathogenesis of this problem is multifactorial, a key component is the abnormal elevation of the hormone hepcidin, the central regulator of systemic iron homeostasis. Investigations over the last decade have resulted in important insights into the role of hepcidin in iron metabolism and the mechanisms that lead to hepcidin dysregulation in the context of inflammation. These insights provide the foundation for novel strategies to prevent and treat the anemia associated with inflammatory diseases.

The immunophilin FKBP12 inhibits hepcidin expression by binding the BMP type I receptor ALK2 in hepatocytes

The expression of the key regulator of iron homeostasis hepcidin is activated by the BMP-SMAD pathway in response to iron and inflammation and among drugs, by rapamycin, which inhibits mTOR in complex with the immunophilin FKBP12. FKBP12 interacts with BMP type I receptors to avoid uncontrolled signaling. By pharmacologic and genetic studies, we identify FKBP12 as a novel hepcidin regulator. Sequestration of FKBP12 by rapamycin or tacrolimus activates hepcidin both in vitro and in murine hepatocytes. Acute tacrolimus treatment transiently increases hepcidin in wild-type mice. FKBP12 preferentially targets the BMP receptor ALK2. ALK2 mutants defective in binding FKBP12 increase hepcidin expression in a ligand-independent manner, through BMP-SMAD signaling. ALK2 free of FKBP12 becomes responsive to the noncanonical inflammatory ligand Activin A. Our results identify a novel hepcidin regulator and a potential therapeutic target to increase defective BMP signaling in disorders of low hepcidin.

Hepcidin-(In)dependent Mechanisms of Iron Metabolism Regulation during Infection by Listeria and Salmonella

During bacterial infection, the pathogenic agent and the host battle for iron, due to its importance for fundamental cellular processes. However, iron redistribution and sequestration during infection can culminate in anemia. Although hepcidin has been recognized as the key regulator of iron metabolism, in some infections its levels remain unaffected, suggesting the involvement of other players in iron metabolism deregulation. In this work, we use a mouse model to elucidate the main cellular and molecular mechanisms that lead to iron redistribution during infection with two different pathogens: Listeria monocytogenes and Salmonella enterica serovar Typhimurium. Both infections clearly impacted iron metabolism, causing iron redistribution, decreasing serum iron levels, decreasing the saturation of transferrin, and increasing iron accumulation in the liver. Both infections were accompanied by the release of proinflammatory cytokines. However, when analyzing iron-related gene expression in the liver, we observed that hepcidin was induced by S Typhimurium but not by L. monocytogenes In the latter model, the downregulation of hepatic ferroportin mRNA and protein levels suggested that ferroportin plays a major role in iron redistribution. On the other hand, S Typhimurium infection induced the expression of hepcidin mRNA, and we show here, for the first time in vivo, that this induction is Toll-like receptor 4 (TLR4) dependent. In this work, we compare several aspects of iron metabolism alterations induced by two different pathogens and suggest that hepcidin-(in)dependent mechanisms contribute to iron redistribution upon infection.

Interleukin-1β (IL-1β) transcriptionally activates hepcidin by inducing CCAAT enhancer-binding protein δ (C/EBPδ) expression in hepatocytes

Hepcidin is a liver-derived hormone that negatively regulates serum iron levels and is mainly regulated at the transcriptional level. Previous studies have clarified that in addition to hepatic iron levels, inflammation also efficiently increases hepatic hepcidin expression. The principle regions responsible for efficient hepcidin transcription are bone morphogenetic protein-responsive elements (BMP-REs) 1 and 2 as well as the signal transducer and activator of transcription 3-binding site (STAT-BS). Here, we show that the proinflammatory cytokine interleukin-1β (IL-1β) efficiently increases hepcidin expression in human HepG2 liver-derived cells and primary mouse hepatocytes. The primary region responsible for IL-1β-mediated hepcidin transcription was the putative CCAAT enhancer-binding protein (C/EBP)-binding site (C/EBP-BS) at the hepcidin promoter spanning nucleotides -329 to -320. IL-1β induces the expression of C/EBPδ but neither C/EBPα nor C/EBPβ in hepatocytes, and C/EBPδ bound to the C/EBP-BS in an IL-1β-dependent manner. Lipopolysaccharide (LPS) induced the expression of IL-1β in Kupffer cells and hepatocytes in the mouse liver; furthermore, the culture supernatants from the macrophage-like cell line RAW264.7 treated with LPS potentiated the stimulation of hepcidin expression in hepatocytes. The present study reveals that: 1) inflammation induces IL-1β production in Kupffer cells and hepatocytes; 2) IL-1β increases C/EBPδ expression in hepatocytes; and 3) induction of C/EBPδ activates hepcidin transcription via the C/EBP-BS that has been uncharacterized yet. In cooperation with the other pathways activated by inflammation, IL-1β pathway stimulation leads to excess production of hepcidin, which could be causative to anemia of inflammation.