Maura Poli

Heparin: a potent inhibitor of hepcidin expression in vitro and in vivo

Hepcidin is a major regulator of iron homeostasis, and its expression in liver is regulated by iron, inflammation, and erythropoietic activity with mechanisms that involve bone morphogenetic proteins (BMPs) binding their receptors and coreceptors. Here we show that exogenous heparin strongly inhibited hepcidin expression in hepatic HepG2 cells at pharmacologic concentrations, with a mechanism that probably involves bone morphogenetic protein 6 sequestering and the blocking of SMAD signaling. Treatment of mice with pharmacologic doses of heparin inhibited liver hepcidin mRNA expression and SMAD phosphorylation, reduced spleen iron concentration, and increased serum iron. Moreover, we observed a strong reduction of serum hepcidin in 5 patients treated with heparin to prevent deep vein thrombosis, which was accompanied by an increase of serum iron and a reduction of C-reactive protein levels. The data show an unrecognized role for heparin in regulating iron homeostasis and indicate novel approaches to the treatment of iron-restricted iron deficiency anemia.

Metabolic Adaptation to Tissue Iron Overload Confers Tolerance to Malaria

Disease tolerance is a defense strategy that limits the fitness costs of infection irrespectively of pathogen burden. While restricting iron (Fe) availability to pathogens is perceived as a host defense strategy, the resulting tissue Fe overload can be cytotoxic and promote tissue damage to exacerbate disease severity. Examining this interplay during malaria, the disease caused by Plasmodium infection, we find that expression of the Fe sequestering protein ferritin H chain (FtH) in mice, and ferritin in humans, is associated with reduced tissue damage irrespectively of pathogen burden. FtH protection relies on its ferroxidase activity, which prevents labile Fe from sustaining proapoptotic c-Jun N-terminal kinase (JNK) activation. FtH expression is inhibited by JNK activation, promoting tissue Fe overload, tissue damage, and malaria severity. Mimicking FtHs antioxidant effect or inhibiting JNK activation pharmacologically confers therapeutic tolerance to malaria in mice. Thus, FtH provides metabolic adaptation to tissue Fe overload, conferring tolerance to malaria.

Hepcidin antagonists for potential treatments of disorders with hepcidin excess

The discovery of hepcidin clarified the basic mechanism of the control of systemic iron homeostasis. Hepcidin is mainly produced by the liver as a propeptide and processed by furin into the mature active peptide. Hepcidin binds ferroportin, the only cellular iron exporter, causing the internalization and degradation of both. Thus hepcidin blocks iron export from the key cells for dietary iron absorption (enterocytes), recycling of hemoglobin iron (the macrophages) and the release of storage iron from hepatocytes, resulting in the reduction of systemic iron availability. The BMP/HJV/SMAD pathway is the major regulator of hepcidin expression that responds to iron status. Also inflammation stimulates hepcidin via the IL6/STAT3 pathway with a support of an active BMP/HJV/SMAD pathway. In some pathological conditions hepcidin level is inadequately elevated and reduces iron availability in the body, resulting in anemia. These conditions occur in the genetic iron refractory iron deficiency anemia and the common anemia of chronic disease (ACD) or anemia of inflammation. Currently, there is no definite treatment for ACD. Erythropoiesis-stimulating agents and intravenous iron have been proposed in some cases but they are scarcely effective and may have adverse effects. Alternative approaches aimed to a pharmacological control of hepcidin expression have been attempted, targeting different regulatory steps. They include hepcidin sequestering agents (antibodies, anticalins, and aptamers), inhibitors of BMP/SMAD or of IL6/STAT3 pathway or of hepcidin transduction (siRNA/shRNA) or ferroportin stabilizers. In this review we summarized the biochemical interactions of the proteins involved in the BMP/HJV/SMAD pathway and its natural inhibitors, the murine and rat models with high hepcidin levels currently available and finally the progresses in the development of hepcidin antagonists, with particular attention to the role of heparins and heparin sulfate proteoglycans in hepcidin expression and modulation of the BMP6/SMAD pathway.

Ferritin ferroxidase activity: A potent inhibitor of osteogenesis

Hemochromatosis is a known cause of osteoporosis, and iron overload has deleterious effects on bone. Although iron overload and its association with osteoporosis has long been recognized, the pathogenesis and exact role of iron have been undefined. Bone is an active tissue with constant remodeling capacity. Osteoblast (OB) development and maturation are under the influence of core binding factor α‐1 (CBF‐α1), which induces expression of OB‐specific genes, including alkaline phosphatase, an important enzyme in early osteogenesis, and osteocalcin, a noncollagenous protein deposited within the osteoid. This study investigates the mechanism by which iron inhibits human OB activity, which in vivo may lead to decreased mineralization, osteopenia, and osteoporosis. We demonstrate that iron‐provoked inhibition of OB activity is mediated by ferritin and its ferroxidase activity. We confirm this notion by using purified ferritin H‐chain and ceruloplasmin, both known to possess ferroxidase activity that inhibited calcification, whereas a site‐directed mutant of ferritin H‐chain lacking ferroxidase activity failed to provide any inhibition. Furthermore, we are reporting that such suppression is not restricted to inhibition of calcification, but OB‐specific genes such as alkaline phosphatase, osteocalcin, and CBF‐α1 are all downregulated by ferritin in a dose‐responsive manner. This study corroborates that iron decreases mineralization and demonstrates that this suppression is provided by iron‐induced upregulation of ferritin. In addition, we conclude that inhibition of OB activity, mineralization, and specific gene expression is attributed to the ferroxidase activity of ferritin.

Transferrin receptor 2 and HFE regulate furin expression via mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/Erk) signaling. Implications for transferrin-dependent hepcidin regulation

Background Impaired regulation of hepcidin in response to iron is the cause of genetic hemochromatosis associated with defects of HFE and transferrin receptor 2. However, the role of these proteins in the regulation of hepcidin expression is unclear. Design and Methods Hepcidin expression, SMAD and extracellular signal-regulated kinase (Erk) phosphorylation and furin expression were analyzed in hepatic HepG2 cells in which HFE and transferrin receptor 2 were down-regulated or expressed, or furin activity specifically inhibited. Furin expression was also analyzed in the liver of transferrin receptor 2 null mice. Results We showed that the silencing of HFE and transferrin receptor 2 reduced both Erk phosphorylation and furin expression, that the exogenous expression of the two enhanced the induction of phosphoErk1/2 and furin by holotransferrin, but that this did not occur when the pathogenic HFE mutant C282Y was expressed. Furin, phosphoErk1/2 and phosphoSMAD1/5/8 were down-regulated also in transferrin receptor 2-null mice. Treatment of HepG2 cells with an inhibitor of furin activity caused a strong suppression of hepcidin mRNA, probably due to the inhibition of bone morphogenic protein maturation. Conclusions The data indicate that transferrin receptor 2 and HFE are involved in holotransferrin-dependent signaling for the regulation of furin which involved Erk phosphorylation. Furin in turn may control hepcidin expression.

Ferritin Prevents Calcification and Osteoblastic Differentiation of Vascular Smooth Muscle Cells

Vascular calcification plays a role in the pathogenesis of atherosclerosis, diabetes, and chronic kidney disease. Human aortic smooth muscle cells (HSMCs) undergo mineralization in response to elevated levels of inorganic phosphate (Pi) in an active and well-regulated process. This process involves increased activity of alkaline phosphatase and increased expression of core binding factor alpha-1, a bone-specific transcription factor, with the subsequent induction of osteocalcin. Mounting evidence suggests an essential role for the heme oxygenase 1 (HO-1)/ferritin system to maintain homeostasis of vascular function. We examined whether induction of HO-1 and ferritin alters mineralization of HSMCs provoked by high Pi. Upregulation of the HO-1/ferritin system inhibited HSMC calcification and osteoblastic differentiation. Of the products of the system, only ferritin and, to a lesser extent, biliverdin were responsible for the inhibition. Ferritin heavy chain and ceruloplasmin, which both possess ferroxidase activity, inhibited calcification; a site-directed mutant of ferritin heavy chain, which lacked ferroxidase activity, failed to inhibit calcification. In addition, osteoblastic transformation of HSMCs provoked by elevated Pi (assessed by upregulation of core binding factor alpha-1, osteocalcin, and alkaline phosphatase activity) was diminished by ferritin/ferroxidase activity. We conclude that induction of the HO-1/ferritin system prevents Pi-mediated calcification and osteoblastic differentiation of human smooth muscle cells mainly via the ferroxidase activity of ferritin.

Glycol-split nonanticoagulant heparins are inhibitors of hepcidin expression in vitro and in vivo.

Hepcidin controls systemic iron availability, and its excess contributes to the anemia of chronic diseases, the most prevalent anemia in hospitalized patients. We previously reported that heparins are efficient hepcidin inhibitors both in vitro and in vivo, but their anticoagulant activity limits therapeutic use. We studied nonanticoagulant heparins produced by N-acetylation and oxidation/reduction (glycol-split) that lost antithrombin-binding affinity. Four nonanticoagulant heparins inhibited hepcidin expression in hepatic HepG2 cells and primary hepatocytes. The 2 most potent ones used in mice suppressed liver hepcidin expression and serum hepcidin in 6 hours, with a significant decrease of spleen iron. This occurred also in lipopolysaccharide (LPS)-treated animals that mimic inflammation, as well as after chronic 1-week treatments, without evident adverse effects on coagulation. Heparin injections increased iron mobilization and facilitated the recovery from the anemia induced by heat-killed Brucella abortus, a model of inflammatory anemia. The heparins were used also in Bmp6(-/-) mice. A single dose of heparin reduced the already low level of hepcidin of these mice and prevented its induction by LPS. These nonanticoagulant compounds impair bone morphogenetic protein /sons of mothers against decapentaplegic signaling with no evident adverse effect in vivo, even when administered chronically. They may offer a strategy for the treatment of diseases with high hepcidin levels.

Pantothenate kinase-2 (Pank2) silencing causes cell growth reduction, cell-specific ferroportin upregulation and iron deregulation

Pantothenate kinase 2 (Pank2) is a mitochondrial enzyme that catalyses the first regulatory step of Coenzyme A synthesis and that is responsible for a genetic movement disorder named Pank-associated neurodegeneration (PKAN). This is characterized by abnormal iron accumulation in the brain, particularly in the globus pallidus. We downregulated Pank2 in some cell lines by using specific siRNAs to study its effect on iron homeostasis. In HeLa cells this caused a reduction of cell proliferation and of aconitase activity, signs of cytosolic iron deficiency without mitochondrial iron deposition, and a 12-fold induction of ferroportin mRNA. Pank2 silencing caused a strong induction of ferroportin mRNA also in hepatoma HepG2, a modest one in neuroblastoma SH-SY5Y and none in glioma U373 cells. A reduction of cell growth was observed in all these cell types. The strong Pank2-mediated alteration of ferroportin expression in some cell types might alter iron transfer to the brain and be connected with brain iron accumulation.

Mutant ferritin L-chains that cause neurodegeneration act in a dominant-negative manner to reduce ferritin iron incorporation.

Nucleotide insertions that modify the C terminus of ferritin light chain (FTL) cause neurodegenerative movement disorders named neuroferritinopathies, which are inherited with dominant transmission. The disorders are characterized by abnormal brain iron accumulation. Here we describe the biochemical and crystallographic characterization of pathogenic FTL mutant p.Phe167SerfsX26 showing that it is a functional ferritin with an altered conformation of the C terminus. Moreover we analyze functional and stability properties of ferritin heteropolymers made of 20–23 H-chains and 1–4 L-chains with representative pathogenic mutations or the last 10–28 residues truncated. All the heteropolymers containing the pathogenic or truncated mutants had a strongly reduced capacity to incorporate iron, both when expressed in Escherichia coli, and in vitro when iron was supplied as Fe(III) in the presence of ascorbate. The mutations also reduced the physical stability of the heteropolymers. The data indicate that even a few mutated L-chains are sufficient to alter the permeability of 1–2 of the 6 hydrophobic channels and modify ferritin capacity to incorporate iron. The dominant-negative action of the mutations explains the dominant transmission of the disorder. The data support the hypothesis that hereditary ferritinopathies are due to alterations of ferritin functionality and provide new input on the mechanism of the function of isoferritins.

Immunoassay for human serum hemojuvelin.

Background Hemojuvelin, a critical regulator of iron homeostasis, is involved in the regulation of hepcidin expression and iron homeostasis. It is expressed both as a membrane-bound form and as a soluble one. Serum hemojuvelin can be produced by secretion following furin cleavage or by proteolytic cleavage of the membrane-bound form by matriptase 2 (TMPRSS6). These forms contribute to down-regulation of hepcidin expression upon iron deficiency or hypoxia. This study describes the development and validation of the first enzyme-linked immunosorbent assay for hemojuvelin in human serum. Design and Methods This assay is based on the use of a recombinant human repulsive guidance molecule-c peptide and a polyclonal antibody against hemojuvelin able to recognize the recombinant peptide and the native soluble hemojuvelin by immunoprecipitation. Results The enzyme-linked immunosorbent assay was validated and appeared to be a robust method with intra- and inter-coefficients of variance ranging from 2.6% to 15%. The assay was able to quantify hemojuvelin levels in a control population within a range from 0.88 to 1.14 mg/L. Patients with iron-refractory iron-deficiency anemia with a mutation in the TMPRSS6 gene were found to have lower levels of circulating hemojuvelin than those in healthy patients. The enzyme-linked immunosorbent assay also showed that soluble hemojuvelin levels were significantly higher in patients with anemia of chronic disease than in control individuals. Conclusions This enzyme-linked immunosorbent assay has a good specificity and sensitivity for the quantification of soluble hemojuvelin in human serum and could be a valuable aid to understanding the physiological role of this protein.