Liver Sinusoidal Endothelial Cells Suppress Bone Morphogenetic Protein 2 Production in Response to TGFβ Pathway Activation

BACKGROUND AND AIMS

TGFβ/bone morphogenetic protein (BMP) signaling in the liver plays a critical role in liver disease. Growth factors, such as BMP2, BMP6, and TGFβ1, are released from LSECs and signal in a paracrine manner to hepatocytes and hepatic stellate cells to control systemic iron homeostasis and fibrotic processes, respectively. The misregulation of the TGFβ/BMP pathway affects expression of the iron-regulated hormone hepcidin, causing frequent iron overload and deficiency diseases. However, whether LSEC-secreted factors can act in an autocrine manner to maintain liver homeostasis has not been addressed so far.

APPROACH AND RESULTS

We analyzed publicly available RNA-sequencing data of mouse LSECs for ligand-receptor interactions and identified members of the TGFβ family (BMP2, BMP6, and TGFβ1) as ligands with the highest expression levels in LSECs that may signal in an autocrine manner. We next tested the soluble factors identified through in silico analysis in optimized murine LSEC primary cultures and mice. Exposure of murine LSEC primary cultures to these ligands shows that autocrine responses to BMP2 and BMP6 are blocked despite high expression levels of the required receptor complexes partially involving the inhibitor FK-506-binding protein 12. By contrast, LSECs respond efficiently to TGFβ1 treatment, which causes reduced expression of BMP2 through activation of activin receptor-like kinase 5.

CONCLUSIONS

These findings reveal that TGFβ1 signaling is functionally interlinked with BMP signaling in LSECs, suggesting druggable targets for the treatment of iron overload diseases associated with deficiency of the BMP2-regulated hormone hepcidin, such as hereditary hemochromatosis, β-thalassemia, and chronic liver diseases.

Core Cross-Linked Polymeric Micelles for Specific Iron Delivery: Inducing Sterile Inflammation in Macrophages

Iron is an essential co-factor for cellular processes. In the immune system, it can activate macrophages and represents a potential therapeutic for various diseases. To specifically deliver iron to macrophages, iron oxide nanoparticles are embedded in polymeric micelles of reactive polysarcosine-block-poly(S-ethylsulfonyl-l-cysteine). Upon surface functionalization via dihydrolipoic acid, iron oxide cores act as crosslinker themselves and undergo chemoselective disulfide bond formation with the surrounding poly(S-ethylsulfonyl-l-cysteine) block, yielding glutathione-responsive core cross-linked polymeric micelles (CCPMs). When applied to primary murine and human macrophages, these nanoparticles display preferential uptake, sustained intracellular iron release, and induce a strong inflammatory response. This response is also demonstrated in vivo when nanoparticles are intratracheally administered to wild-type C57Bl/6N mice. Most importantly, the controlled release concept to deliver iron oxide in redox-responsive CCPMs induces significantly stronger macrophage activation than any other iron source at identical iron levels (e.g., Feraheme), directing to a new class of immune therapeutics.

Vaccine efficacy and iron deficiency: an intertwined pair?

Vaccines are the most effective measure to prevent deaths and illness from infectious diseases. Nevertheless, the efficacy of several paediatric vaccines is lower in low-income and middle-income countries (LMICs), where mortality from vaccine-preventable infections remains high. Vaccine efficacy can also be decreased in adults in the context of some common comorbidities. Identifying and correcting the specific causes of impaired vaccine efficacy is of substantial value to global health. Iron deficiency is the most common micronutrient deficiency worldwide, affecting more than 2 billion people, and its prevalence in LMICs could increase as food security is threatened by the COVID-19 pandemic. In this Viewpoint, we highlight evidence showing that iron deficiency limits adaptive immunity and responses to vaccines, representing an under-appreciated additional disadvantage to iron deficient populations. We propose a framework for urgent detailed studies of iron-vaccine interactions to investigate and clarify the issue. This framework includes retrospective analysis of newly available datasets derived from trials of COVID-19 and other vaccines, and prospective testing of whether nutritional iron interventions, commonly used worldwide to combat anaemia, improve vaccine performance.

The Macrophage Iron Signature in Health and Disease

Throughout life, macrophages are located in every tissue of the body, where their main roles are to phagocytose cellular debris and recycle aging red blood cells. In the tissue niche, they promote homeostasis through trophic, regulatory, and repair functions by responding to internal and external stimuli. This in turn polarizes macrophages into a broad spectrum of functional activation states, also reflected in their iron-regulated gene profile. The fast adaptation to the environment in which they are located helps to maintain tissue homeostasis under physiological conditions.

Maternal Iron Status in Pregnancy and Child Health Outcomes after Birth: A Systematic Review and Meta-Analysis

In pregnancy, iron deficiency and iron overload increase the risk for adverse pregnancy outcomes, but the effects of maternal iron status on long-term child health are poorly understood. The aim of the study was to systematically review and analyze the literature on maternal iron status in pregnancy and long-term outcomes in the offspring after birth. We report a systematic review on maternal iron status during pregnancy in relation to child health outcomes after birth, from database inception until 21 January 2021, with methodological quality rating (Newcastle-Ottawa tool) and random-effect meta-analysis. (PROSPERO, CRD42020162202). The search identified 8139 studies, of which 44 were included, describing 12,7849 mother–child pairs. Heterogeneity amongst the studies was strong. Methodological quality was predominantly moderate to high. Iron status was measured usually late in pregnancy. The majority of studies compared categories based on maternal ferritin, however, definitions of iron deficiency differed across studies. The follow-up period was predominantly limited to infancy. Fifteen studies reported outcomes on child iron status or hemoglobin, 20 on neurodevelopmental outcomes, and the remainder on a variety of other outcomes. In half of the studies, low maternal iron status or iron deficiency was associated with adverse outcomes in children. Meta-analyses showed an association of maternal ferritin with child soluble transferrin receptor concentrations, though child ferritin, transferrin saturation, or hemoglobin values showed no consistent association. Studies on maternal iron status above normal, or iron excess, suggest deleterious effects on infant growth, cognition, and childhood Type 1 diabetes. Maternal iron status in pregnancy was not consistently associated with child iron status after birth. The very heterogeneous set of studies suggests detrimental effects of iron deficiency, and possibly also of overload, on other outcomes including child neurodevelopment. Studies are needed to determine clinically meaningful definitions of iron deficiency and overload in pregnancy.

Vasculo-toxic and pro-inflammatory action of unbound haemoglobin, haem and iron in transfusion-dependent patients with haemolytic anaemias

Increasing evidence suggests that free haem and iron exert vasculo‐toxic and pro‐inflammatory effects by activating endothelial and immune cells. In the present retrospective study, we compared serum samples from transfusion‐dependent patients with β‐thalassaemia major and intermedia, hereditary spherocytosis and sickle cell disease (SCD). Haemolysis, transfusions and ineffective erythropoiesis contribute to haem and iron overload in haemolytic patients. In all cohorts we observed increased systemic haem and iron levels associated with scavenger depletion and toxic ‘free’ species formation. Endothelial dysfunction, oxidative stress and inflammation markers were significantly increased compared to healthy donors. In multivariable logistic regression analysis, oxidative stress markers remained significantly associated with both haem‐ and iron‐related parameters, while soluble vascular cell adhesion molecule 1 (sVCAM‐1), soluble endothelial selectin (sE‐selectin) and tumour necrosis factor α (TNFα) showed the strongest association with haem‐related parameters and soluble intercellular adhesion molecule 1 (sICAM‐1), sVCAM‐1, interleukin 6 (IL‐6) and vascular endothelial growth factor (VEGF) with iron‐related parameters. While hereditary spherocytosis was associated with the highest IL‐6 and TNFα levels, β‐thalassaemia major showed limited inflammation compared to SCD. The sVCAM1 increase was significantly lower in patients with SCD receiving exchange compared to simple transfusions. The present results support the involvement of free haem/iron species in the pathogenesis of vascular dysfunction and sterile inflammation in haemolytic diseases, irrespective of the underlying haemolytic mechanism, and highlight the potential therapeutic benefit of iron/haem scavenging therapies in these conditions.

Mild Attenuation of the Pulmonary Inflammatory Response in a Mouse Model of Hereditary Hemochromatosis Type 4

The respiratory tract is constantly exposed to pathogens that require iron for proliferation and virulence. Pulmonary iron levels are increased in several lung diseases and associated with increased susceptibility to infections. However, regulation of lung iron homeostasis and its cross talk to pulmonary immune responses are largely unexplored. Here we investigated how increased lung iron levels affect the early pulmonary inflammatory response. We induced acute local pulmonary inflammation via aerosolized LPS in a mouse model of hereditary hemochromatosis type 4 (Slc40a1 ^C326S/C326S), which is hallmarked by systemic and pulmonary iron accumulation, specifically in alveolar macrophages. We show that Slc40a1 ^C326S/C326S mice display a mild attenuation in the LPS-induced pulmonary inflammatory response, with a reduced upregulation of some pro-inflammatory cytokines and chemokines. Despite mildly reduced cytokine levels, there is no short-term impairment in the recruitment of neutrophils into the bronchoalveolar space. These data suggest that increased pulmonary iron levels do not strongly alter the acute inflammatory response of the lung.

Iron deficiency

Iron deficiency is one of the leading contributors to the global burden of disease, and particularly affects children, premenopausal women, and people in low-income and middle-income countries. Anaemia is one of many consequences of iron deficiency, and clinical and functional impairments can occur in the absence of anaemia. Iron deprivation from erythroblasts and other tissues occurs when total body stores of iron are low or when inflammation causes withholding of iron from the plasma, particularly through the action of hepcidin, the main regulator of systemic iron homoeostasis. Oral iron therapy is the first line of treatment in most cases. Hepcidin upregulation by oral iron supplementation limits the absorption efficiency of high-dose oral iron supplementation, and of oral iron during inflammation. Modern parenteral iron formulations have substantially altered iron treatment and enable rapid, safe total-dose iron replacement. An underlying cause should be sought in all patients presenting with iron deficiency: screening for coeliac disease should be considered routinely, and endoscopic investigation to exclude bleeding gastrointestinal lesions is warranted in men and postmenopausal women presenting with iron deficiency anaemia. Iron supplementation programmes in low-income countries comprise part of the solution to meeting WHO Global Nutrition Targets.

HFE and ALK3 act in the same signaling pathway

Hepcidin deficiency leads to iron overload by increased dietary iron uptake and iron release from storage cells. The most frequent mutation in Hfe leads to reduced hepcidin expression and thereby causes iron overload. Recent findings suggested that HFE activates hepcidin expression predominantly via the BMP type I receptor ALK3. Here, we investigated whether HFE exclusively utilizes ALK3 or other signaling mechanisms also. We generated mice with double deficiency of Hfe and hepatocyte-specific Alk3 and compared the iron overload phenotypes of these double knockout mice to single hepatocyte-specific Alk3 deficient or Hfe knockout mice. Double Hfe^-/-/hepatic Alk3^fl/fl;Alb-Cre knockouts develop a similar iron overload phenotype compared to single hepatocyte-specific Alk3 deficient mice hallmarked by serum iron levels, tissue iron content and hepcidin levels of similar grades. HFE protein levels were increased in Alk3^fl/fl;Alb-Cre mice compared to Alk3^fl/fl mice, which was caused by iron overload - and not by Alk3 deficiency. The data provide evidence by genetic means that 1. HFE exclusively uses the BMP type I receptor ALK3 to induce hepcidin expression and 2. HFE protein expression is induced by iron overload, which further emphasizes the iron sensing function of HFE.

Iron Regulation in Elderly Asian Elephants (Elephas maximus) Chronically Infected With Mycobacterium tuberculosis

Restriction of nutrients to pathogens (nutritional immunity) is a critical innate immune response mechanism that operates when pathogens such as Mycobacterium tuberculosis have the potential to evade humoral immunity. Tuberculosis is of growing concern for zoological collections worldwide and is well-illustrated by infections of Asian and African elephants, where tuberculosis is difficult to diagnose. Here, we investigated hematological parameters and iron deposition in liver, lung, and spleen of three Asian elephants (Elephas maximus) infected with Mycobacterium tuberculosis. For reference purposes, we analyzed tissue samples from control M. tuberculosis-negative elephants with and without evidence of inflammation and/or chronic disease. Molecular analyses of bacterial lesions of post mortally collected tissues confirmed M. tuberculosis infection in three elephants. DNA sequencing of the bacterial cultures demonstrated a single source of infection, most likely of human origin. In these elephants, we observed moderate microcytic anemia as well as liver (mild), lung (moderate) and spleen (severe) iron accumulation, the latter mainly occurring in macrophages. Macrophage iron sequestration in response to infection and inflammation is caused by inhibition of iron export via hepcidin-dependent and independent mechanisms. The hepatic mRNA levels of the iron-regulating hormone hepcidin were increased in only one control elephant suffering from chronic inflammation without mycobacterial infection. By contrast, all three tuberculosis-infected elephants showed low hepcidin mRNA levels in the liver and low serum hepcidin concentrations. In addition, hepatic ferroportin mRNA expression was high. This suggests that the hepcidin/ferroportin regulatory system aims to counteract iron restriction in splenic macrophages in M. tuberculosis infected elephants to provide iron for erythropoiesis and to limit iron availability for a pathogen that predominantly proliferates in macrophages. Tuberculosis infections appear to have lingered for more than 30 years in the three infected elephants, and decreased iron availability for mycobacterial proliferation may have forced the bacteria into a persistent, non-proliferative state. As a result, therapeutic iron substitution may not have been beneficial in these elephants, as this therapy may have enhanced progression of the infection.

Cell-specific expression of <i>Hfe</i> determines the outcome of <i>Salmonella enterica</i> serovar Typhimurium infection in mice

Mutations in HFE cause hereditary hemochromatosis type I hallmarked by increased iron absorption, iron accumulation in hepatocytes and iron deficiency in myeloid cells. HFE encodes an MHC-I like molecule, but its function in immune responses to infection remains incompletely understood. Here, we investigated putative roles of Hfe in myeloid cells and hepatocytes, separately, upon infection with Salmonella Typhimurium, an intracellular bacterium with iron-dependent virulence. We found that constitutive and macrophage-specific deletion of Hfe protected infected mice. The propagation of Salmonella in macrophages was reduced due to limited intramacrophage iron availability for bacterial growth and increased expression of the anti-microbial enzyme nitric oxide synthase-2. By contrast, mice with hepatocyte-specific deletion of Hfe succumbed earlier to Salmonella infection because of unrestricted extracellular bacterial replication associated with high iron availability in the serum and impaired expression of essential host defense molecules such as interleukin- 6, interferon-g and nitric oxide synthase-2. Wild-type mice subjected to dietary iron overload phenocopied hepatocyte-specific Hfe deficiency suggesting that increased iron availability in the serum is deleterious in Salmonella infection and underlies impaired host immune responses. Moreover, the macrophage-specific effect is dominant over hepatocytespecific Hfe-depletion, as Hfe knockout mice have increased survival despite the higher parenchymal iron load associated with systemic loss of Hfe. We conclude that cell-specific expression of Hfe in hepatocytes and macrophages differentially affects the course of infections with specific pathogens by determining bacterial iron access and the efficacy of antimicrobial immune effector pathways. This may explain the high frequency and evolutionary conservation of human HFE mutations.

Geographical ancestry affects normal hemoglobin values in high-altitude residents

Increasing the hemoglobin (Hb) concentration is a major mechanism adjusting arterial oxygen content to decreased oxygen partial pressure of inspired air at high altitude. Approximately 5% of the world's population living at altitudes higher than 1,500 m shows this adaptive mechanism. Notably, there is a wide variation in the extent of increase in Hb concentration among different populations. This short review summarizes available information on Hb concentrations of high-altitude residents living at comparable altitudes (3,500-4,500 m) in different regions of the world. An increased Hb concentration is found in all high-altitude populations. The highest mean Hb concentration was found in adult male Andean residents and in Han Chinese living at high altitude, whereas it was lowest in Ethiopians, Tibetans, and Sherpas. A lower plasma volume in Andean high-altitude natives may offer a partial explanation. Indeed, male Andean high-altitude natives have a lower plasma volume than Tibetans and Ethiopians. Moreover, Hb values were lower in adult, nonpregnant females than in males; differences between populations of different ancestry were less pronounced. Various genetic polymorphisms were detected in high-altitude residents thought to favor life in a hypoxic environment, some of which correlate with the relatively low Hb concentration in the Tibetans and Ethiopians, whereas differences in angiotensin-converting enzyme allele distribution may be related to elevated Hb in the Andeans. Taken together, these results indicate different sensitivity of oxygen dependent control of erythropoiesis or plasma volume among populations of different geographical ancestry, offering explanations for differences in the Hb concentration at high altitude.

Iron metabolism in high-altitude residents

Residents at high altitude cope with decreasing inspiratory oxygen partial pressure by stimulating erythropoiesis. The increase in hemoglobin levels requires high amounts of additional iron supplied from the diet. Here, we review available data on how iron metabolism adapts when living in a hypoxic environment. Our analysis reveals that long-term adaptation to high altitude enables healthy individuals to maintain their iron stores within the physiological range despite elevated requirements for erythropoiesis. However, in vulnerable populations with increased iron demand (e.g., pregnant women or exercising individuals), iron stores are less likely to be replenished quickly when living at high altitude. Future studies need to address whether different ethnicities have acquired genetic mechanisms to adapt to the elevated iron demand for erythropoiesis at high altitude.

Chromatin accessibility landscape of pediatric T-lymphoblastic leukemia and human T-cell precursors

We aimed at identifying the developmental stage at which leukemic cells of pediatric T-ALLs are arrested and at defining leukemogenic mechanisms based on ATAC-Seq. Chromatin accessibility maps of seven developmental stages of human healthy T cells revealed progressive chromatin condensation during T-cell maturation. Developmental stages were distinguished by 2,823 signature chromatin regions with 95% accuracy. Open chromatin surrounding SAE1 was identified to best distinguish thymic developmental stages suggesting a potential role of SUMOylation in T-cell development. Deconvolution using signature regions revealed that T-ALLs, including those with mature immunophenotypes, resemble the most immature populations, which was confirmed by TF-binding motif profiles. We integrated ATAC-Seq and RNA-Seq and found DAB1, a gene not related to leukemia previously, to be overexpressed, abnormally spliced and hyper-accessible in T-ALLs. DAB1-negative patients formed a distinct subgroup with particularly immature chromatin profiles and hyper-accessible binding sites for SPI1 (PU.1), a TF crucial for normal T-cell maturation. In conclusion, our analyses of chromatin accessibility and TF-binding motifs showed that pediatric T-ALL cells are most similar to immature thymic precursors, indicating an early developmental arrest.

Disruption of the hepcidin/ferroportin regulatory circuitry causes low axial bone mass in mice

Ferroportin (FPN) is the only known iron exporter. Mutations conferring resistance of FPN to hepcidin-mediated degradation cause the iron overload disorder hereditary hemochromatosis type 4. While iron overload is associated with low bone mass, the mechanisms involved are not completely understood. Here, we aimed to investigate whether the disruption in the hepcidin/FPN axis in Fpn^C326S mice and subsequent systemic iron accumulation impacts on bone tissue to a similar extent as in Hfe^-/- mice, which are hallmarked by a milder iron overload phenotype. Hfe^-/- and Fpn^C326S mice show increased plasma iron levels and liver iron content, whereas iron overload was more pronounced in Fpn^C326S compared to Hfe^-/- mice. Bone volume fraction and trabecular thickness at the femur were not different between 10 and 14-week-old male wild-type (WT), Hfe^-/- and Fpn^C326S mice. By contrast, both Hfe^-/- and Fpn^C326S mice exhibited a lower bone volume fraction [Hfe^-/-, 24%; Fpn^C326S, 33%; p < 0.05] and trabecular thickness [Hfe^-/-, 10%; Fpn^C326S, 15%; p < 0.05] in the fourth lumbar vertebra compared to WT mice. Analysis of the bone formation rate at the tibia showed no difference in both genotypes, but it was reduced in the vertebral bone of Fpn^C326S [36%, p < 0.05] compared to WT mice. Serum levels of the bone formation marker, P1NP, were significantly reduced in both, Hfe^-/- and Fpn^C326S compared with WT mice [Hfe^-/-, 35%; Fpn^C326S, 40%; p < 0.05]. Also, the intrinsic differentiation capacity of Fpn^C326S osteoblasts was impaired. Osteoclast parameters were not grossly affected. Interestingly, the liver iron content and plasma iron levels negatively correlated with the bone formation rate and serum levels of P1NP. Thus, disruption of the hepcidin/ferroportin regulatory axis in Fpn^C326S mice results in axial bone loss due to suppressed bone formation.

Atherosclerosis is aggravated by iron overload and ameliorated by dietary and pharmacological iron restriction

AIMS

Whether and how iron affects the progression of atherosclerosis remains highly debated. Here, we investigate susceptibility to atherosclerosis in a mouse model (ApoE-/- FPNwt/C326S), which develops the disease in the context of elevated non-transferrin bound serum iron (NTBI).

METHODS AND RESULTS

Compared with normo-ferremic ApoE-/- mice, atherosclerosis is profoundly aggravated in iron-loaded ApoE-/- FPNwt/C326S mice, suggesting a pro-atherogenic role for iron. Iron heavily deposits in the arterial media layer, which correlates with plaque formation, vascular oxidative stress and dysfunction. Atherosclerosis is exacerbated by iron-triggered lipid profile alterations, vascular permeabilization, sustained endothelial activation, elevated pro-atherogenic inflammatory mediators, and reduced nitric oxide availability. NTBI causes iron overload, induces reactive oxygen species production and apoptosis in cultured vascular cells, and stimulates massive MCP-1-mediated monocyte recruitment, well-established mechanisms contributing to atherosclerosis. NTBI-mediated toxicity is prevented by transferrin- or chelator-mediated iron scavenging. Consistently, a low-iron diet and iron chelation therapy strongly improved the course of the disease in ApoE-/- FPNwt/C326S mice. Our results are corroborated by analyses of serum samples of haemochromatosis patients, which show an inverse correlation between the degree of iron depletion and hallmarks of endothelial dysfunction and inflammation.

CONCLUSION

Our data demonstrate that NTBI-triggered iron overload aggravates atherosclerosis and unravel a causal link between NTBI and the progression of atherosclerotic lesions. Our findings support clinical applications of iron restriction in iron-loaded individuals to counteract iron-aggravated vascular dysfunction and atherosclerosis.

PDX models recapitulate the genetic and epigenetic landscape of pediatric T-cell leukemia

We compared 24 primary pediatric T‐cell acute lymphoblastic leukemias (T‐ALL) collected at the time of initial diagnosis and relapse from 12 patients and 24 matched patient‐derived xenografts (PDXs). DNA methylation profile was preserved in PDX mice in 97.5% of the promoters (ρ = 0.99). Similarly, the genome‐wide chromatin accessibility (ATAC‐Seq) was preserved remarkably well (ρ = 0.96). Interestingly, both the ATAC regions, which showed a significant decrease in accessibility in PDXs and the regions hypermethylated in PDXs, were associated with immune response, which might reflect the immune deficiency of the mice and potentially the incomplete interaction between murine cytokines and human receptors. The longitudinal approach of this study allowed an observation that samples collected from patients who developed a type 1 relapse (clonal mutations maintained at relapse) preserved their genomic composition; whereas in patients who developed a type 2 relapse (subset of clonal mutations lost at relapse), the preservation of the leukemia's composition was more variable. In sum, this study underlines the remarkable genomic stability, and for the first time documents the preservation of the epigenomic landscape in T‐ALL‐derived PDX models.

The increase in hemoglobin concentration with altitude varies among human populations

Decreased oxygen availability at high altitude requires physiological adjustments allowing for adequate tissue oxygenation. One such mechanism is a slow increase in the hemoglobin concentration ([Hb]) resulting in elevated [Hb] in high-altitude residents. Diagnosis of anemia at different altitudes requires reference values for [Hb]. Our aim was to establish such values based on published data of residents living at different altitudes by applying meta-analysis and multiple regressions. Results show that [Hb] is increased in all high-altitude residents. However, the magnitude of increase varies among the regions analyzed and among ethnic groups within a region. The highest increase was found in residents of the Andes (1 g/dL/1000 m), but this increment was smaller in all other regions of the world (0.6 g/dL/1000 m). While sufficient data exist for adult males and females showing that sex differences in [Hb] persist with altitude, data for infants, children, and pregnant women are incomplete preventing such analyses. Because WHO reference values were originally based on [Hb] of South American people, we conclude that individual reference values have to be defined for ethnic groups to reliably diagnose anemia and erythrocytosis in high-altitude residents. Future studies need to test their applicability for children of different ages and pregnant women.

Glutathione peroxidase 4 and vitamin E control reticulocyte maturation, stress erythropoiesis and iron homeostasis

Glutathione peroxidase 4 (GPX4) is unique as it is the only enzyme that can prevent detrimental lipid peroxidation in vivo by reducing lipid peroxides to the respective alcohols thereby stabilizing oxidation products of unsaturated fatty acids. During reticulocyte maturation, lipid peroxidation mediated by 15-lipoxygenase in humans and rabbits and by 12/15-lipoxygenase (ALOX15) in mice was considered the initiating event for the elimination of mitochondria but is now known to occur through mitophagy. Yet, genetic ablation of the Alox15 gene in mice failed to provide evidence for this hypothesis. We designed a different genetic approach to tackle this open conundrum. Since either other lipoxygenases or non-enzymatic autooxidative mechanisms may compensate for the loss of Alox15, we asked whether ablation of Gpx4 in the hematopoietic system would result in the perturbation of reticulocyte maturation. Quantitative assessment of erythropoiesis indices in the blood, bone marrow (BM) and spleen of chimeric mice with Gpx4 ablated in hematopoietic cells revealed anemia with an increase in the fraction of erythroid precursor cells and reticulocytes. Additional dietary vitamin E depletion strongly aggravated the anemic phenotype. Despite strong extramedullary erythropoiesis reticulocytes failed to mature and accumulated large autophagosomes with engulfed mitochondria. Gpx4-deficiency in hematopoietic cells led to systemic hepatic iron overload and simultaneous severe iron demand in the erythroid system. Despite extremely high erythropoietin and erythroferrone levels in the plasma, hepcidin expression remained unchanged. Conclusively, perturbed reticulocyte maturation in response to Gpx4 loss in hematopoietic cells thus causes ineffective erythropoiesis, a phenotype partially masked by dietary vitamin E supplementation.

Author Correction: Transferrin receptor 2 controls bone mass and pathological bone formation via BMP and Wnt signaling

In the version of this article initially published, affiliation 14 was incorrect, and Deutsche Forschungsgemeinschaft grants SFB1036 and SFB1118 were missing from the Acknowledgements. The errors have been corrected in the HTML and PDF versions of the article.

miR-148a regulates expression of the transferrin receptor 1 in hepatocellular carcinoma

Transferrin receptor 1 (TFR1) is a transmembrane glycoprotein that allows for transferrin-bound iron uptake in mammalian cells. It is overexpressed in various cancers to satisfy the high iron demand of fast proliferating cells. Here we show that in hepatocellular carcinoma (HCC) TFR1 expression is regulated by miR-148a. Within the TFR1 3′UTR we identified and experimentally validated two evolutionarily conserved miRNA response elements (MREs) for miR-148/152 family members, including miR-148a. Interestingly, analyses of RNA sequencing data from patients with liver hepatocellular carcinoma (LIHC) revealed a significant inverse correlation of TFR1 mRNA levels and miR-148a. In addition, TFR1 mRNA levels were significantly increased in the tumor compared to matched normal healthy tissue, while miR-148a levels are decreased. Functional analysis demonstrated post-transcriptional regulation of TFR1 by miR-148a in HCC cells as well as decreased HCC cell proliferation upon either miR-148a overexpression or TFR1 knockdown. We hypothesize that decreased expression of miR-148a in HCC may elevate transferrin-bound iron uptake, increasing cellular iron levels and cell proliferation.

Transferrin receptor 2 controls bone mass and pathological bone formation via BMP and Wnt signaling

Transferrin receptor 2 (Tfr2) is mainly expressed in the liver and controls iron homeostasis. Here, we identify Tfr2 as a regulator of bone homeostasis that inhibits bone formation. Mice lacking Tfr2 display increased bone mass and mineralization independent of iron homeostasis and hepatic Tfr2. Bone marrow transplantation experiments and studies of cell-specific Tfr2 knockout mice demonstrate that Tfr2 impairs BMP-p38MAPK signaling and decreases expression of the Wnt inhibitor sclerostin specifically in osteoblasts. Reactivation of MAPK or overexpression of sclerostin rescues skeletal abnormalities in Tfr2 knockout mice. We further show that the extracellular domain of Tfr2 binds BMPs and inhibits BMP-2-induced heterotopic ossification by acting as a decoy receptor. These data indicate that Tfr2 limits bone formation by modulating BMP signaling, possibly through direct interaction with BMP either as a receptor or as a co-receptor in a complex with other BMP receptors. Finally, the Tfr2 extracellular domain may be effective in the treatment of conditions associated with pathological bone formation.