Antioxidant Paradox in Male Infertility: ‘A Blind Eye’ on Inflammation

The pathophysiology of male infertility involves various interlinked endogenous pathways. About 50% of the cases of infertility in men are idiopathic, and oxidative stress (OS) reportedly serves as a central mechanism in impairing male fertility parameters. The endogenous antioxidant system operates to conserve the seminal redox homeostasis required for normal male reproduction. OS strikes when a generation of seminal reactive oxygen species (ROS) overwhelms endogenous antioxidant capacity. Thus, antioxidant treatment finds remarkable relevance in the case of idiopathic male infertility or subfertility. However, due to lack of proper detection of OS in male infertility, use of antioxidant(s) in some cases may be arbitrary or lead to overuse and induction of ‘reductive stress’. Moreover, inflammation is closely linked to OS and may establish a vicious loop that is capable of disruption to male reproductive tissues. The result is exaggeration of cellular damage and disruption of male reproductive tissues. Therefore, limitations of antioxidant therapy in treating male infertility are the failure in the selection of specific treatments targeting inflammation and OS simultaneously, two of the core mechanisms of male infertility. The present review aims to elucidate the antioxidant paradox in male infertility treatment, from the viewpoints of both induction of reductive stress as well as overlooking the inflammatory consequences.

Endoplasmic reticulum oxidoreductin (ERO) provides resilience against reductive stress and hypoxic conditions by mediating luminal redox dynamics

Oxidative protein folding in the endoplasmic reticulum (ER) depends on the coordinated action of protein disulfide isomerases and ER oxidoreductins (EROs). Strict dependence of ERO activity on molecular oxygen as the final electron acceptor implies that oxidative protein folding and other ER processes are severely compromised under hypoxia. While many key players involved in oxidative protein folding are known, our understanding of how redox homeostasis in the ER is maintained and how EROs, the Cys residues of nascent proteins, and the luminal glutathione redox buffer interact is limited. Here, we isolated viable ero1 ero2 double mutants largely deficient in ERO activity, which rendered the mutants highly sensitive to reductive stress and hypoxia. To elucidate the specific redox dynamics in the ER lumen in vivo, we expressed the glutathione redox potential (EGSH) sensor Grx1-roGFP2iL-HDEL with a midpoint potential of -240 mV in the ER of Arabidopsis plants. We found EGSH values of -241 mV in wild-type plants, which is less oxidizing than previously estimated. In the ero1 ero2 mutants, luminal EGSH was reduced further to -253 mV. Recovery to reductive ER stress, as induced by acute exposure to dithiothreitol, was delayed in ero1 ero2 mutants. The characteristic signature of EGSH dynamics in the ER lumen triggered by hypoxia was affected in the ero1 ero2 mutant reflecting a disrupted balance of reductive and oxidizing inputs, including nascent polypeptides and glutathione entry. The ER redox dynamics can now be dissected in vivo, revealing a central role of EROs as major redox integrators to promote luminal redox homeostasis.

Redox Balance in Male Infertility: Excellence through Moderation—“Μέτρον ἄριστον”

Male infertility, a relatively common and multifactorial medical condition, affects approximately 15% of couples globally. Based on WHO estimates, a staggering 190 million people struggle with this health condition, and male factor is the sole or contributing factor in roughly 20–50% of these cases. Nowadays, urologists are confronted with a wide spectrum of conditions ranging from the typical infertile male to more complex cases of either unexplained or idiopathic male infertility, requiring a specific patient-tailored diagnostic approach and management. Strikingly enough, no identifiable cause in routine workup can be found in 30% to 50% of infertile males. The medical term male oxidative stress infertility (MOSI) was recently coined to describe infertile men with abnormal sperm parameters and oxidative stress (OS), including those previously classified as having idiopathic infertility. OS is a critical component of male infertility, entailing an imbalance between reactive oxygen species (ROS) and antioxidants. ROS abundance has been implicated in sperm abnormalities, while the exact impact on fertilization and pregnancy has long been a subject of considerable debate. In an attempt to counteract the deleterious effects of OS, urologists resorted to antioxidant supplementation. Mounting evidence indicates that indiscriminate consumption of antioxidants has led in some cases to sperm cell damage through a reductive-stress-induced state. The “antioxidant paradox”, one of the biggest andrological challenges, remains a lurking danger that needs to be carefully avoided and thoroughly investigated. For that reason, oxidation-reduction potential (ORP) emerged as a viable ancillary tool to basic semen analysis, measuring the overall balance between oxidants and antioxidants (reductants). A novel biomarker, the Male infertility Oxidative System (MiOXSYS®), is a paradigm shift towards that goal, offering a quantification of OS via a quick, reliable, and reproducible measurement of the ORP. Moderation or “Μέτρον” according to the ancient Greeks is the key to successfully safeguarding redox balance, with MiOXSYS® earnestly claiming its position as a guarantor of homeostasis in the intracellular redox milieu. In the present paper, we aim to offer a narrative summary of evidence relevant to redox regulation in male reproduction, analyze the impact of OS and reductive stress on sperm function, and shed light on the “antioxidant paradox” phenomenon. Finally, we examine the most up-to-date scientific literature regarding ORP and its measurement by the recently developed MiOXSYS® assay.

Proteome signatures of reductive stress cardiomyopathy

Nuclear factor erythroid 2-related factor 2 (NRF2), a redox sensor, is vital for cellular redox homeostasis. We reported that transgenic mice expressing constitutively active Nrf2 (CaNrf2-TG) exhibit reductive stress (RS). In this study, we identified novel protein biomarkers for RS-induced cardiomyopathy using Tandem Mass Tag (TMT) proteomic analysis in heart tissues of TG (CaNrf2-TG) and non-transgenic (NTg) mice at 6-7 months of age (N= 4/group). A total of 1105 proteins were extracted from 22544 spectra. Of note, about 560 proteins were differentially expressed in TG vs. NTg hearts, indicating a global impact of RS on myocardial proteome. From a closer analysis of the proteome datasets, we identified over 32 proteins that were significantly altered in response to RS. Among these, 20 were upregulated and 12 were downregulated in the hearts of TG vs. NTg mice, suggesting that these proteins could be putative signatures of RS. Scaffold analysis revealed a clear distinction between TG vs NTg hearts. Of note, we observed several proteins with redox (#185; cysteine residues), NEM-adducts (#81), methionine-loss (#21) and acetylation (#1) modifications in TG vs. NTg hearts due to chronic RS. The majority of the differentially expressed proteins (DEPs) that are significantly altered in RS mice were found to be involved in stress related pathways such as antioxidants, NADPH, protein quality control (PQC), etc. Interestingly, proteins that were involved in mitochondrial respiration, lipophagy and cardiac rhythm were dramatically decreased in TG hearts. Of note, we identified the glutathione family of proteins as the significantly changed subset of the proteome in TG heart. Surprisingly, our comparative analysis of NGS based transcriptome and TMT-proteome indicated ∼50% of the altered proteins in TG myocardium was found to be negatively correlated with their transcript levels. Modifications at cysteine/NEM-adducts (redox), methionine or lysine residues in multiple proteins in response to chronic RS might be associated with impaired PQC mechanisms, thus causing pathological cardiac remodeling. Graphical Abstract

Cysteine induces mitochondrial reductive stress in glioblastoma through hydrogen peroxide production

SummaryGlucose and amino acid metabolism are critical for glioblastoma (GBM) growth, but little is known about the specific metabolic alterations in GBM that are targetable with FDA-approved compounds. To investigate tumor metabolism signatures unique to GBM, we interrogated The Cancer Genome Atlas for alterations in glucose and amino acid signatures in GBM relative to other human cancers and found that GBM exhibits the highest levels of cysteine and methionine pathway gene expression of 32 human cancers. Treatment of patient-derived GBM cells with the FDA-approved cysteine compound N-acetylcysteine (NAC) reduce GBM cell growth and mitochondrial oxygen consumption, which was worsened by glucose starvation. Mechanistic experiments revealed that cysteine compounds induce rapid mitochondrial H2O2 production and reductive stress in GBM cells, an effect blocked by oxidized glutathione, thioredoxin, and redox enzyme overexpression. These findings indicate that GBM is uniquely susceptible to NAC-driven reductive stress and could synergize with glucose-lowering treatments for GBM.

Abstract P492: Putative Protein Biomarkers For Reductive Stress Cardiomyopathy

Background: Nuclear factor erythroid 2-related factor 2 (NRF2) signaling is vital for redox homeostasis. We reported that transgenic mice expressing constitutively active Nrf2 (CaNrf2) exhibit reductive stress (RS). Here in, we identified novel protein signatures reacting to RS-induced cardiomyopathy. Methods: Tandem Mass Tag (TMT) proteomic analysis was performed in the heart tissues of Ca-Nrf2-transgenic (TG-low & TG-high) and non-transgenic (NTg) mice at 6 months of age (N= 4/group). Differentially expressed proteins (DEPs) were then identified using Scaffold. Validated the key DEPs using immunoblotting. PANTHER and STRING analysis were used to identify potential targets and their interactions. Results: A total of 1105 proteins were extracted from 24369 spectra. Of note, 226 and 261 proteins were differentially expressed in TG-L and TG-H vs. NTg hearts indicating a unique proteome signature for RS. Heat map analysis revealed a clear distinction between the TG-L and TG-H due to the dose-dependent effects of transgene/RS. Majority of the DEPs that are significantly altered in RS mice found to involve in stress related pathways such as antioxidants, NADPH, protein quality control (PQC), etc. Interestingly, some of these proteins were redox modified at their cysteine residues under chronic RS setting. Conclusions: TMT based proteomic analyses revealed unique proteome signatures for RS. The cysteine modifications in multiple proteins likely to cause pathological alterations via impaired PQC mechanisms. Molecular studies related to RS-mediated redox modifications in structural and functional cardiac proteome are underway.

P13.07 Cysteine induces glioblastoma cytotoxicity through mitochondrial reductive stress

BACKGROUND Glioblastoma (GBM) remains a poorly treatable disease with high mortality. Tumor metabolism in GBM is a critical mechanism responsible for accelerated growth because of upregulation of glucose, amino acid, and fatty acid utilization. However, therapies targeting GBM metabolism, whether through the use of small-molecule compounds or dietary interventions to limit nutrient sources, have failed in clinical trials. Metabolic bypass is an important mechanism that is often overlooked in GBM trials, since many trials have focused instead on combining anti-metabolic therapy with cytotoxic treatments. The goal of this research is to use a multi-pronged treatment approach with targeted drug and dietary therapy to leverage metabolic susceptibilities in GBM. MATERIALS AND METHODS We first interrogated the TCGA database and a cancer metabolite database for alterations in glucose and amino acid signatures in GBM relative to other human cancers and relative to low-grade glioma. We identified the amino acid cysteine as contributing to a novel metabolic susceptibility pathway in GBM. To study the role of cysteine in GBM pathogenesis, we treated patient-derived GBM cells with a variety of FDA-approved cysteine-promoting compounds in vitro, including N-acetylcysteine (NAC). We measured cell proliferation, energy production, mitochondrial metabolism, and reactive oxygen species to study mechanisms of oxidoreductive stress. Results: From our TCGA and cancer metabolite database analyses, we found that GBM exhibits the highest levels of cysteine and methionine pathway gene expression of 32 human cancers and that GBM exhibits high levels of cysteine-related metabolites compared to low-grade gliomas. Cysteine compounds, including NAC, reduce growth of GBM cells, which is exacerbated by glucose deprivation. This growth inhibition is associated with reduced mitochondrial metabolism, manifest by reduction in ATP generation, NADPH/NADP+ ratio, mitochondrial membrane potential, and oxygen consumption rate. Through measurement of mitochondrial hydrogen peroxide, we found that NAC-treated cells exhibit a paradoxical increase in mitochondrial hydrogen peroxide levels, likely due to inhibition of thioreductase and glutathione reductase systems. Through genetic and pharmacological studies, we found that induction of thioredoxin-2 rescues NAC-mediated cytotoxicity and that inhibition of thioreductase and glutathione reductase exacerbates mitochondrial toxicity and reductive stress. CONCLUSIONS We show that cysteine compounds reduce cell growth and induce mitochondrial toxicity in GBM through reductive stress. This metabolic phenotype is exacerbated by glucose deprivation. This pathway is targetable with FDA-approved cysteine-promoting compounds and could synergize with glucose-lowering treatments, including the ketogenic diet, for GBM.