Giovanni Musci

Ferroxidase activity is required for the stability of cell surface ferroportin in cells expressing GPI-ceruloplasmin

Ferroportin (Fpn), a ferrous iron Fe(II) transporter responsible for the entry of iron into plasma, is regulated post‐translationally through internalization and degradation following binding of the hormone hepcidin. Cellular iron export is impaired in mice and humans with aceruloplasminemia, an iron overload disease due to mutations in the ferroxidase ceruloplasmin (Cp). In the absence of Cp Fpn is rapidly internalized and degraded. Depletion of extracellular Fe(II) by the yeast ferroxidase Fet3p or iron chelators can maintain cell surface Fpn in the absence of Cp. Iron remains bound to Fpn in the absence of multicopper oxidases. Fpn with bound iron is recognized by a ubiquitin ligase, which ubiquitinates Fpn on lysine 253. Mutation of lysine 253 to alanine prevents ubiquitination and maintains Fpn‐iron on cell surface in the absence of ferroxidase activity. The requirement for a ferroxidase to maintain iron transport activity represents a new mechanism of regulating cellular iron export, a new function for Cp and an explanation for brain iron overload in patients with aceruloplasminemia.

Ferroportin-mediated mobilization of ferritin iron precedes ferritin degradation by the proteasome

Ferritin is a cytosolic molecule comprised of subunits that self‐assemble into a nanocage capable of containing up to 4500 iron atoms. Iron stored within ferritin can be mobilized for use within cells or exported from cells. Expression of ferroportin (Fpn) results in export of cytosolic iron and ferritin degradation. Fpn‐mediated iron loss from ferritin occurs in the cytosol and precedes ferritin degradation by the proteasome. Depletion of ferritin iron induces the monoubiquitination of ferritin subunits. Ubiquitination is not required for iron release but is required for disassembly of ferritin nanocages, which is followed by degradation of ferritin by the proteasome. Specific mammalian machinery is not required to extract iron from ferritin. Iron can be removed from ferritin when ferritin is expressed in Saccharomyces cerevisiae, which does not have endogenous ferritin. Expressed ferritin is monoubiquitinated and degraded by the proteasome. Exposure of ubiquitination defective mammalian cells to the iron chelator desferrioxamine leads to degradation of ferritin in the lysosome, which can be prevented by inhibitors of autophagy. Thus, ferritin degradation can occur through two different mechanisms.

Short-time non-enzymatic nitric oxide synthesis from L-arginine and hydrogen peroxide induced by shock waves treatment

The evidence that nitric oxide (NO) production is possible by a non‐enzymatic pathway has already been shown under restrictive experimental conditions. Here we show that NO can non‐enzymatically be formed with short‐time kinetics (min), by ‘bombing’ with shock waves a solution containing 1 mM hydrogen peroxide and 10 mM L‐arginine. This procedure is widening its medical application with surprisingly positive effects in tissue regeneration and our finding could be one of the first steps for the understanding of the biochemical responsible for these therapeutical effects.

Nitric oxide mediates anti‐inflammatory action of extracorporeal shock waves

Here, we show that extracorporeal shock waves (ESW), at a low energy density value, quickly increase neuronal nitric oxide synthase (nNOS) activity and basal nitric oxide (NO) production in the rat glioma cell line C6. In addition, the treatment of C6 cells with ESW reverts the decrease of nNOS activity and NO production induced by a mixture of lipopolysaccharides (LPS), interferon‐γ (IFN‐γ) plus tumour necrosis factor‐α (TNF‐α). Finally, ESW treatment efficiently downregulates NF‐κB activation and NF‐κB‐dependent gene expression, including inducible NOS and TNF‐α. The present report suggests a possible molecular mechanism of the anti‐inflammatory action of ESW treatment.

The physiopathological significance of ceruloplasmin: A possible therapeutic approach

This article reviews and comments on the physiological roles of ceruloplasmin (Cp). We show that, in addition to its ascertained involvement in iron homeostasis, the protein, by virtue of its unique structure among multicopper oxidases, is likely involved in other processes of both an enzymatic and a nonenzymatic nature. In particular, based on the analysis of the kinetic parameters, on the one hand, and of the side-products of the oxidation, on the other, we propose that the long-recognized ability of Cp to interact with and oxidize non-iron substrates may be of physiological relevance. The striking example of 6-hydroxydopamine oxidation is presented, where we show that the catalytic action is carried out readily under physiological conditions, without release of potentially toxic oxygen intermediates.

Beta-amyloid inhibits NOS activity by subtracting NADPH availability.

The amyloid peptides Aβ1–42 and Aβ25–35 strongly inhibited the activity of constitutive neuronal and endothelial nitric oxide synthases (i.e., NOS‐I and NOS‐III, respectively) in cell‐free assays. The molecular mechanism of NOS inhibition by Aβ fragments was studied in detail with Aβ25–35. The inhibitory ability was mostly NADPH‐dependent and specific for the soluble form of Aβ25–35. Optical, fluorescence, and NMR spectroscopy showed that the soluble, but not aggregated, Aβ25–35 interacted with NADPH, thus suggesting that a direct recruitment of NADPH may result in diminished availability of the redox cofactor for NOS functioning. To assess the physiological relevance of our findings, rat neuronal‐like PC12 and glioma C6 cell lines were used as cellular models. After Aβ25–35 internalization into cells was verified, the activity of constitutive NOS was measured using the DAF‐2DA detection system and found to be severely impaired upon Aβ25–35 uptake. Consistent with previous results on the molecular cross‐talk between NOS isoforms, repression of constitutive NOS by Aβ25–35 resulted in enhanced expression of inducible NOS (NOS‐II) mRNA in C6 cells. Our results represent the first evidence that amyloid fragments impair constitutive NOS activity in cell‐free and cellular systems, providing a possible molecular mechanism for the onset and/or maintenance of Alzheimers disease.

Interleukin-1β up-regulates iron efflux in rat C6 glioma cells through modulation of ceruloplasmin and ferroportin-1 synthesis

A number of pathologies, including neurodegeneration and inflammation, have been associated with iron dysmetabolism in the brain. Hence, systems involved in iron homeostasis at the cellular level have aroused considerable interest in recent years. The iron exporter ferroportin-1 (FP) and the multicopper oxidase ceruloplasmin (CP) are essential for iron efflux from cells. By using RT-PCR, we demonstrate that FP and CP gene expression is up-regulated by treatment with the pro-inflammatory cytokine IL-1beta in rat C6 cells, taken as a glial cellular model. Following stimulation with IL-1beta, a higher expression level of CP and FP was also confirmed by Western blotting. Moreover, IL-1beta has been found to increase iron efflux from C6 cells, suggesting that both proteins may play a crucial role in iron homeostasis in pathological brain conditions, such as inflammatory and/or neurodegenerative diseases.

Dominant Mutants of Ceruloplasmin Impair the Copper Loading Machinery in Aceruloplasminemia

The multicopper oxidase ceruloplasmin plays a key role in iron homeostasis, and its ferroxidase activity is required to stabilize cell surface ferroportin, the only known mammalian iron exporter. Missense mutations causing the rare autosomal neurodegenerative disease aceruloplasminemia were investigated by testing their ability to prevent ferroportin degradation in rat glioma C6 cells silenced for endogenous ceruloplasmin. Most of the mutants did not complement (i.e. did not stabilize ferroportin) because of the irreversible loss of copper binding ability. Mutant R701W, which was found in a heterozygous very young patient with severe neurological problems, was unable to complement per se but did so in the presence of copper-glutathione or when the yeast copper ATPase Ccc2p was co-expressed, indicating that the protein was structurally able to bind copper but that metal loading involving the mammalian copper ATPase ATP7B was impaired. Notably, R701W exerted a dominant negative effect on wild type, and it induced the subcellular relocalization of ATP7B. Our results constitute the first evidence of “functional silencing” of ATP7B as a novel molecular defect in aceruloplasminemia. The possibility to reverse the deleterious effects of some aceruloplasminemia mutations may disclose new possible therapeutic strategies.

The essential role of Glu-185 and Tyr-354 residues in the ferroxidase activity of Saccharomyces cerevisiae Fet3.

The structural determinants required for ferroxidase activity by the yeast multicopper oxidase Fet3 have been partially clarified by site‐directed mutagenesis based on homology modeling. Glu‐185 and Tyr‐354 were substituted with Ala and Phe, respectively. Fet3 E185A retained ca. 5% residual ferroxidase catalytic efficiency, and almost 40% oxidase efficiency. On the other hand, Fet3 Y354F exhibited 50% residual efficiency as a ferroxidase and more than 70% as an oxidase. These results provide new insights in the mechanism of iron binding and oxidation by Fet3, establishing the essential role of Glu‐185 and Tyr‐354, and allowing to dissect ferroxidase from non‐iron oxidase activity.

Ceruloplasmin-ferroportin system of iron traffic in vertebrates

Safe trafficking of iron across the cell membrane is a delicate process that requires specific protein carriers. While many proteins involved in iron uptake by cells are known, only one cellular iron export protein has been identified in mammals: ferroportin (SLC40A1). Ceruloplasmin is a multicopper enzyme endowed with ferroxidase activity that is found as a soluble isoform in plasma or as a membrane-associated isoform in specific cell types. According to the currently accepted view, ferrous iron transported out of the cell by ferroportin would be safely oxidized by ceruloplasmin to facilitate loading on transferrin. Therefore, the ceruloplasmin-ferroportin system represents the main pathway for cellular iron egress and it is responsible for physiological regulation of cellular iron levels. The most recent findings regarding the structural and functional features of ceruloplasmin and ferroportin and their relationship will be described in this review.