Mevalonate pathway, selenoproteins, redox balance, immune system, Covid-19: Reasoning about connections

It has been proposed that a degraded immune system is (one of) the condition(s) that predispose certain subjects to fatal consequences from infection by SARS-CoV-2. It is unknown whether therapeutic regimens to which these patients may have been subjected to in the months/years preceding the infection could be immunocompromising. Statins are among the most widely prescribed cholesterol-lowering drugs. As competitive inhibitors of HMG-CoA-reductase, the key enzyme of the "mevalonate pathway" through which essential compounds, not only cholesterol, are synthesized, statins decrease the levels of cholesterol, and thus LDLs, as an innate defense mechanism, with controversial results in decreasing mortality from cardiovascular disease. Moreover, statins have pleiotropic, mostly deleterious effects on many cell types, including immune cells. In the attempt to decipher the enigma of SARS-CoV-2 infectivology, the hypothesis should be tested whether the population of subjects who succumbed to Covid-19 may have developed a compromised immunity at sub-clinical levels and have become more susceptible to fatal consequences from SARS-Cov-2 infection due to statin therapy.

Oxidative Post-translational Modifications Accelerate Proteolytic Degradation of Calprotectin

Oxidative post-translational modifications affect the structure and function of many biomolecules. Herein we examine the biophysical and functional consequences of oxidative post-translational modifications to human calprotectin (CP, S100A8/S100A9 oligomer, MRP8/MRP14 oligomer, calgranulins A/B oligomer). This abundant metal-sequestering protein contributes to innate immunity by starving invading microbial pathogens of transition metal nutrients in the extracellular space. It also participates in the inflammatory response. Despite many decades of study, little is known about the fate of CP at sites of infection and inflammation. We present compelling evidence for methionine oxidation of CP in vivo, supported by using 15N-labeled CP-Ser (S100A8(C42S)/S100A9(C3S)) to monitor for adventitious oxidation following human sample collection. To elucidate the biochemical and functional consequences of oxidative post-translational modifications, we examine recombinant CP-Ser with methionine sulfoxide modifications generated by exposing the protein to hydrogen peroxide. These oxidized species coordinate transition metal ions and exert antibacterial activity. Nevertheless, oxidation of M81 in the S100A9 subunit disrupts Ca(II)-induced tetramerization and, in the absence of a transition metal ion bound at the His6 site, accelerates proteolytic degradation of CP. We demonstrate that native CP, which contains one Cys residue in each full-length subunit, forms disulfide bonds within and between S100A8/S100A9 heterodimers when exposed to hydrogen peroxide. Remarkably, disulfide bond formation accelerates proteolytic degradation of CP. We propose a new extension to the working model for extracellular CP where post-translational oxidation by reactive oxygen species generated during the neutrophil oxidative burst modulates its lifetime in the extracellular space.

The Oxidized Protein Repair Enzymes Methionine Sulfoxide Reductases and Their Roles in Protecting against Oxidative Stress, in Ageing and in Regulating Protein Function

Cysteine and methionine residues are the amino acids most sensitive to oxidation by reactive oxygen species. However, in contrast to other amino acids, certain cysteine and methionine oxidation products can be reduced within proteins by dedicated enzymatic repair systems. Oxidation of cysteine first results in either the formation of a disulfide bridge or a sulfenic acid. Sulfenic acid can be converted to disulfide or sulfenamide or further oxidized to sulfinic acid. Disulfide can be easily reversed by different enzymatic systems such as the thioredoxin/thioredoxin reductase and the glutaredoxin/glutathione/glutathione reductase systems. Methionine side chains can also be oxidized by reactive oxygen species. Methionine oxidation, by the addition of an extra oxygen atom, leads to the generation of methionine sulfoxide. Enzymatically catalyzed reduction of methionine sulfoxide is achieved by either methionine sulfoxide reductase A or methionine sulfoxide reductase B, also referred as to the methionine sulfoxide reductases system. This oxidized protein repair system is further described in this review article in terms of its discovery and biologically relevant characteristics, and its important physiological roles in protecting against oxidative stress, in ageing and in regulating protein function.

Brain, immune system and selenium: a starting point for a new diagnostic marker for Alzheimer's disease?

The clinical diagnosis of Alzheimer's disease (AD) is based primarily on neuropsychological tests, which assess the involutive damage, and imaging techniques that evaluate morphologic changes in the brain. Currently available diagnostic tests do not show complete specificity and do not permit accurate differentiation between AD and other forms of senile dementia. The correlation of these tests with laboratory investigations based on biochemical parameters could increase the certainty of diagnosis. In recent years, several biochemical markers for the diagnosis of AD have been proposed, but in most cases they show a limited specificity and their application is invasive, requiring, in general, sampling of cerebrospinal fluid. Thus, the use of a peripheral biochemical marker could represent a valuable complement for the diagnosis of this disease. Several studies have shown a relationship between neurodegenerative disorders typical of the ageing process, weakening of the immune system and alterations in the levels of selenium and of the antioxidant selenoenzymes in brain tissues and blood cells. Among blood cells, neutrophil granulocytes uniquely express the selenoenzyme methionine sulfoxide reductase B1 (MsrB1). In a preliminary analysis carried out on neutrophils from subjects affected by AD, we observed a significant decline in MsrB1 activity compared to normal subjects. Therefore, we deem it of particular interest to explore the potential use of MsrB1 as a selective peripheral marker for the diagnosis of AD.

Methionine 35 sulphoxide reduces toxicity of Aβ in red blood cell

BACKGROUND

The oxidation of methionine residue in position 35 of Ab to sulphoxide (Ab-sulphoxide) has the ability to deeply modify wild-type Ab 1-42 (Ab) neurotoxic action. Our previous studies suggest that in nucleated cells, lower toxicity of Ab-sulphoxide might result not from structural alteration, but from elevation of methionine sulphoxide reductase A (MsrA) activity and mRNA levels.

DESIGN

On this basis, we hypothesised that red blood cell (RBC), a cell devoid almost completely of MsrA activity, shares with nucleated cells an antioxidant system induced by methionine 35 sulphoxide, responsible for the lower toxicity of Ab-sulphoxide in RBC. (Results) Supporting this hypothesis, we found that the low toxicity of Ab-sulphoxide in RBC correlated with pentose phosphate pathway (PPP) flux increase, and this event was associated with a low level of methionine oxidation in total proteins. None of these effects were observed when cells were exposed to Ab native.

DISCUSSION

These results outline the importance of the redox state of methionine 35 in the modulation of Ab-mediated events and suggest an important protective role for PPP in RBC of patients affected by Alzheimer's disease.

The discovery of methionine sulfoxide reductase enzymes: An historical account and future perspectives

L-Methionine (L-Met) is the only sulphur-containing proteinogenic amino acid together with cysteine. Its importance is highlighted by it being the initiator amino acid for protein synthesis in all known living organisms. L-Met, free or inserted into proteins, is sensitive to oxidation of its sulfide moiety, with formation of L-Met sulfoxide. The sulfoxide could not be inserted into proteins, and the oxidation of L-Met in proteins often leads to the loss of biological activity of the affected molecule. Key discoveries revealed the existence, in rats, of a metabolic pathway for the reduction of free L-Met sulfoxide and, later, in Escherichia coli, of the enzymatic reduction of L-Met sulfoxide inserted in proteins. Upon oxidation, the sulphur atom becomes a new stereogenic center, and two stable diastereoisomers of L-Met sulfoxide exist. A fundamental discovery revealed the existence of two unrelated families of enzymes, MsrA and MsrB, whose members display opposite stereospecificity of reduction for the two sulfoxides. The importance of Msrs is additionally emphasized by the discovery that one of the only 25 selenoproteins expressed in humans is a Msr. The milestones on the road that led to the discovery and characterization of this group of antioxidant enzymes are recounted in this review.

Hypochlorite-induced structural modifications enhance the chaperone activity of human α2-macroglobulin

Hypochlorite, an oxidant generated in vivo by the innate immune system, kills invading pathogens largely by inducing the misfolding of microbial proteins. Concomitantly, the nonspecific activity of hypochlorite also damages host proteins, and the accumulation of damaged (misfolded) proteins is implicated in the pathology of a variety of debilitating human disorders (e.g., Alzheimer's disease, atherosclerosis, and arthritis). It is well-known that cells respond to oxidative stress by up-regulating proteostasis machinery, but the direct activation of mammalian chaperones by hypochlorite has not, to our knowledge, been previously reported. In this study, we show that hypochlorite-induced modifications of human α2-macroglobulin (α2M) markedly increase its chaperone activity by generating species, particularly dimers formed by dissociation of the native tetramer, which have enhanced surface hydrophobicity. Moreover, dimeric α2M is generated in whole-blood plasma in the presence of physiologically relevant amounts of hypochlorite. The chaperone activity of hypochlorite-modified α2M involves the formation of stable soluble complexes with misfolded client proteins, including heat-denatured enzymes, oxidized fibrinogen, oxidized LDL, and native or oxidized amyloid β-peptide (Aβ1-42). Here, we show that hypochlorite-modified α2M delivers its misfolded cargo to lipoprotein receptors on macrophages and reduces Aβ1-42 neurotoxicity. Our results support the conclusion that α2M is a specialized chaperone that prevents the extracellular accumulation of misfolded and potentially pathogenic proteins, particularly during innate immune system activity.

Recharging oxidative protein repair: catalysis by methionine sulfoxide reductases towards their amino acid, protein, and model substrates

The sulfur-containing amino acid methionine (Met) in its free and amino acid residue forms can be readily oxidized to the R and S diastereomers of methionine sulfoxide (MetO). Methionine sulfoxide reductases A (MSRA) and B (MSRB) reduce MetO back to Met in a stereospecific manner, acting on the S and R forms, respectively. A third MSR type, fRMSR, reduces the R form of free MetO. MSRA and MSRB are spread across the three domains of life, whereas fRMSR is restricted to bacteria and unicellular eukaryotes. These enzymes protect against abiotic and biotic stresses and regulate lifespan. MSRs are thiol oxidoreductases containing catalytic redox-active cysteine or selenocysteine residues, which become oxidized by the substrate, requiring regeneration for the next catalytic cycle. These enzymes can be classified according to the number of redox-active cysteines (selenocysteines) and the strategies to regenerate their active forms by thioredoxin and glutaredoxin systems. For each MSR type, we review catalytic parameters for the reduction of free MetO, low molecular weight MetO-containing compounds, and oxidized proteins. Analysis of these data reinforces the concept that MSRAs reduce various types of MetO-containing substrates with similar efficiency, whereas MSRBs are specialized for the reduction of MetO in proteins.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Methionine sulfoxide reductases preferentially reduce unfolded oxidized proteins and protect cells from oxidative protein unfolding

Reduction of methionine sulfoxide (MetO) residues in proteins is catalyzed by methionine sulfoxide reductases A (MSRA) and B (MSRB), which act in a stereospecific manner. Catalytic properties of these enzymes were previously established mostly using low molecular weight MetO-containing compounds, whereas little is known about the catalysis of MetO reduction in proteins, the physiological substrates of MSRA and MSRB. In this work we exploited an NADPH-dependent thioredoxin system and determined the kinetic parameters of yeast MSRA and MSRB using three different MetO-containing proteins. Both enzymes showed Michaelis-Menten kinetics with the K(m) lower for protein than for small MetO-containing substrates. MSRA reduced both oxidized proteins and low molecular weight MetO-containing compounds with similar catalytic efficiencies, whereas MSRB was specialized for the reduction of MetO in proteins. Using oxidized glutathione S-transferase as a model substrate, we showed that both MSR types were more efficient in reducing MetO in unfolded than in folded proteins and that their activities increased with the unfolding state. Biochemical quantification and identification of MetO reduced in the substrates by mass spectrometry revealed that the increased activity was due to better access to oxidized MetO in unfolded proteins; it also showed that MSRA was intrinsically more active with unfolded proteins regardless of MetO availability. Moreover, MSRs most efficiently protected cells from oxidative stress that was accompanied by protein unfolding. Overall, this study indicates that MSRs serve a critical function in the folding process by repairing oxidatively damaged nascent polypeptides and unfolded proteins.

Iron homeostasis and methionine-centred redox cycle in nasal polyposis

Nasal polyposis is a multifactorial disease with a strong inflammatory component. Its pathogenesis is often associated with ROS production catalysed by redox-active iron. This study aimed to characterize the roles of iron homeostasis and redox status in the pathogenesis of polyposis. Nasal polyps (NP) from asthmatics and non-asthmatics and turbinates from controls and NP-patients were analysed for ferritin, ferritin-bound iron (FBI) and levels of methionine-centred redox cycle proteins. The ferritin content in both NPs was significantly higher than in adjacent turbinates. No differences in FBI were observed between both NP groups and both turbinates groups, while in NPs it was significantly higher. In NP-turbinates the highest levels of redox proteins were observed. In conclusion, re-distribution of iron occurs upon the development of NP. While FBI is elevated in NPs, the adjacent turbinate remain iron-poor and low-inflammatory, suggesting the formation of virtual boundary between these tissues.

Specific activation of human neutrophils by scorpion venom: a flow cytometry assessment

Acute lung injury following envenomation by Tityus scorpion species is due in part to activation of the inflammatory response leading to release of cytotoxic leukocyte-derived products, including cytokines and possibly reactive oxygen species (ROS). Tityus zulianus envenomation in Venezuela produces cardiorespiratory complications and death by lung injury whereas stings by Tityus discrepans produce mainly gastrointestinal and pancreatic alterations. To ascertain the role played by granulocytes in the envenomation by T. zulianus (TzV) and T. discrepans (TdV), human peripheral blood neutrophils, eosinophils, and monocytes were exposed to scorpion venoms (0.001-5 μg/mL) and the kinetics (5-15 min) of peroxide production determined by flow cytometry, using 2',7'-dichlorodihydrofluorescein diacetate (succinimidyl ester) as a fluorescent substrate. TzV induced a significantly (p<0.01) more potent increase in peroxide production in neutrophils (for 5 and 10 min of incubation), and to a lesser extent in monocytes (5-15 min), compared to TdV. TzV induced necrosis in neutrophils at doses higher than 5 μg/mL. No effect was observed on eosinophils, suggesting that TzV specifically targets neutrophil intracellular ROS production. The TzV-stimulated pathway is protein kinase C-dependent because it was almost completely (>90%) abolished by staurosporine. The stimulatory effect is associated with the lowest molecular mass venom peptides as gel filtration fractions TzII and TzIII significantly enhanced peroxide production. The combined used of the intracellular ROS agonist, phorbol myristate acetate (PMA), and TzV produced a modest but significant increase in peroxide production suggesting the possibility of overlapping signaling cascades amongst PMA and TzV. Up-regulation of intracellular neutrophil ROS production may be an important in vivo target for TzV which could have a role to play in the cardiorespiratory complications elicited after envenomation by this species.

S-glutathionylation regulates inflammatory activities of S100A9

Reactive oxygen species generated by activated neutrophils can cause oxidative stress and tissue damage. S100A8 (A8) and S100A9 (A9), abundant in neutrophil cytoplasm, are exquisitely sensitive to oxidation, which may alter their functions. Murine A8 is a neutrophil chemoattractant, but it suppresses leukocyte transmigration in the microcirculation when S-nitrosylated. Glutathione (GSH) modulates intracellular redox, and S-glutathionylation can protect susceptible proteins from oxidative damage and regulate function. We characterized S-glutathionylation of A9; GSSG and GSNO generated S-glutathionylated A8 (A8-SSG) and A9 (A9-SSG) in vitro, whereas only A9-SSG was detected in cytosol of neutrophils activated with phorbol myristate acetate (PMA) but not with fMLP or opsonized zymosan. S-Glutathionylation exposed more hydrophobic regions in Zn(2+)-bound A9 but did not alter Zn(2+) binding affinity. A9-SSG had reduced capacity to form heterocomplexes with A8, but the arachidonic acid binding capacities of A8/A9 and A8/A9-SSG were similar. A9 and A8/A9 bind endothelial cells; S-glutathionylation reduced binding. We found little effect of A9 or A9-SSG on neutrophil CD11b/CD18 expression or neutrophil adhesion to endothelial cells. However, A9, A9-SSG and A8/A9 promoted neutrophil adhesion to fibronectin but, in the presence of A8, A9-mediated adhesion was abrogated by glutathionylation. S-Glutathionylation of A9 may protect its oxidation to higher oligomers and reduce neutrophil binding to the extracellular matrix. This may regulate the magnitude of neutrophil migration in the extravasculature, and together with the functional changes we reported for S-nitrosylated A8, particular oxidative modifications of these proteins may limit tissue damage in acute inflammation.

Overexpression of methionine sulfoxide reductases A and B2 protects MOLT-4 cells against zinc-induced oxidative stress

Among the amino acids, methionine is the most susceptible to oxidation, and methionine sulfoxide can be catalytically reduced within proteins by methionine sulfoxide reductase A (MsrA) and B (MsrB). As one of the very few repair systems for oxidized proteins, MsrA and MsrB enzymes play a major role in protein homeostasis during aging and have also been involved in cellular defenses against oxidative stress, by scavenging reactive oxygen species. To elucidate the role of zinc on the Msr system, the effects of zinc treatment on control and stably overexpressing MsrA and MsrB2 MOLT-4 leukemia cells have been analyzed. Here we show that zinc treatment has a pro-antioxidant effect in MOLT-4 cells by inducing the transcription of metallothioneins and positively modulating the activity of the Msr enzymes. In contrast, due to its pro-oxidant effect, zinc also led to increased cell death, reactive oxygen species production, and protein damage. Our results indicate that overexpression of the Msr enzymes, due to their antioxidant properties, counteracts the pro-oxidant effects of zinc treatment, which lead to a cellular protection against protein oxidative damage and cell death, by reducing the production of reactive oxygen species.

Cytokine generation, promoter activation, and oxidant-independent NF-kappaB activation in a transfectable human neutrophilic cellular model

Background

Human neutrophils are key players of innate immunity, and influence inflammatory and immune reactions through the production of numerous cytokines and chemokines. Despite major advances in our understanding of this important functional response of neutrophils, the short lifespan of these cells and their resistance to transfection have always been an obstacle to the detailed dissection of signaling pathways and effector responses that is often possible in other cell types.

Results

Here, we report that granulocytic differentiation of human PLB-985 cells with DMSO yields cells that are neutrophil-like with respect to surface markers, acquisition of responsiveness to physiological neutrophil stimuli (fMLP, LPS), cytokine expression and production profile, and transcription factor activation profile (NF-κB, C/EBP, AP-1, STAT). We also show that granulocytic PLB-985 cells can be reliably tranfected by nucleofection in a rapid and efficient manner. Indeed, we overexpressed several proteins and luciferase constructs into these cells. In particular, overexpression of a dominant negative IκB-α confirmed the central role of NF-κB in the production of cytokines by granulocytes. Moreover, the use of PLB-985 granulocytes in which the NADPH oxidase is inactive due to the targeted disruption of a key component (gp91phox) revealed that NF-κB activation and κB-dependent responses are independent of endogenous reactive oxygen intermediates in these cells. Antioxidant studies performed in primary human neutrophils support this conclusion.

Conclusion

Our results unveil a new facet of the NF-κB system of human granulocytes, and pave the way for deciphering signal transduction pathways and promoter activation in these cells.