Identification and characterization of alternatively transcribed form of peroxiredoxin IV gene that is specifically expressed in spermatids of postpubertal mouse testis

Essential Roles of Peroxiredoxin IV in Inflammation and Cancer

Peroxiredoxin IV (Prx4) is a 2-Cysteine peroxidase with ubiquitous expression in human tissues. Prx4 scavenges hydrogen peroxide and participates in oxidative protein folding in the endoplasmic reticulum. In addition, Prx4 is secreted outside the cell. Prx4 is upregulated in several cancers and is a potential therapeutic target. We have summarized historical and recent advances in the structure, function and biological roles of Prx4, focusing on inflammatory diseases and cancer. Oxidative stress is known to activate pro-inflammatory pathways. Chronic inflammation is a risk factor for cancer development. Hence, redox enzymes such as Prx4 are important players in the crosstalk between inflammation and cancer. Understanding molecular mechanisms of regulation of Prx4 expression and associated signaling pathways in normal physiological and disease conditions should reveal new therapeutic strategies. Thus, although Prx4 is a promising therapeutic target for inflammatory diseases and cancer, further research needs to be conducted to bridge the gap to clinical application.

Impact of Oxidative Stress on Embryogenesis and Fetal Development

Multiple cellular processes are regulated by oxygen radicals or reactive oxygen species (ROS) where they play crucial roles as primary or secondary messengers, particularly during cell proliferation, differentiation, and apoptosis. Embryogenesis and organogenesis encompass all these processes; therefore, their role during these crucial life events cannot be ignored, more so when there is an imbalance in redox homeostasis. Perturbed redox homeostasis is responsible for damaging the biomolecules such as lipids, proteins, and nucleic acids resulting in leaky membrane, altered protein, enzyme function, and DNA damage which have adverse impact on the embryo and fetal development. In this article, we attempt to summarize the available data in literature for an in-depth understanding of redox regulation during development that may help in optimizing the pregnancy outcome both under natural and assisted conditions.

Pathological Role of Reactive Oxygen Species on Female Reproduction

Oxidative stress (OS), a clinical predicament characterized by a shift in homeostatic imbalance among prooxidant molecules embracing reactive oxygen species (ROS) and reactive nitrogen species (RNS), along with antioxidant defenses, has been established to play an indispensable part in the pathophysiology of subfertility in both human males and females. ROS are highly reactive oxidizing by-products generated during critical oxygen-consuming processes or aerobic metabolism. A healthy body system has its own course of action to maintain the equilibrium between prooxidants and antioxidants with an efficient defense system to fight against ROS. But when ROS production crosses its threshold, the disturbance in homeostatic balance results in OS. Besides their noxious effects, literature studies have depicted that controlled and adequate ROS concentrations exert physiologic functions, especially that gynecologic OS is an important mediator of conception in females. Yet the impact of ROS on oocytes and reproductive functions still needs a strong attestation for further analysis because the disruption in prooxidant and antioxidant balance leads to abrupt ROS generation initiating multiple reproductive diseases such as polycystic ovary syndrome (PCOS), endometriosis, and unexplained infertility in addition to other impediments in pregnancy such as recurrent pregnancy loss, spontaneous abortion, and preeclampsia. The current article elucidates the skeptical state of affairs created by ROS that influences female fertility.

Testis-specific peroxiredoxin 4 variant is not absolutely required for spermatogenesis and fertility in mice

PRDX4, a member of peroxiredoxin family, is largely concentrated in the endoplasmic reticulum (ER) and plays a pivotal role in the redox relay during oxidative protein folding as well as in peroxidase reactions. A testis-specific PRDX4 variant transcript (PRDX4t) lacks the conventional exon 1, which encodes the signal peptide that is required for entry into the ER lumen, but instead carries alternative exon 1, which is transcribed from the upstream promoter in a testis-specific manner and results in the PRDX4t protein being localized in the cytosol. However, the potential roles of PRDX4t in male genital action remain unknown. Using a CRISPR/Cas9 system, we first disrupted the testis-specific promoter/exon 1 and generated mice that were specifically deficient in PRDX4t. The resulting PRDX4t knockout (KO) mice underwent normal spermatogenesis and showed no overt abnormalities in the testis. Mating PRDX4t KO male mice with wild-type (WT) female mice produced normal numbers of offspring, indicating that a PRDX4t deficiency alone had no effect on fertility in the male mice. We then generated mice lacking both PRDX4 and PRDX4t by disrupting exon 2, which is communal to these variants. The resulting double knockout (DKO) mice were again fertile, and mature sperm isolated from the epididymis of DKO mice exhibited a normal fertilizing ability in vitro. In the meantime, the protein levels of glutathione peroxidase 4 (GPX4), which plays an essential role in the disulfide bond formation during spermatogenesis, were significantly increased in the testis and caput epididymis of the DKO mice compared with the WT mice. Based on these results, we conclude that the disruption of the function of PRDX4t in the spermatogenic process appears to be compensated by other factors including GPX4.

Oxidative Stress and Reproductive Function in the Aging Male

With the delay of parenthood becoming more common, the age at which men father children is on the rise. While the effects of advanced maternal age have been well documented, only recently have studies started to focus on the impact of advanced paternal age (APA) in the context of male reproduction. As men age, the antioxidant defense system gradually becomes less efficient and elevated levels of reactive oxygen species (ROS) accumulate in spermatozoa; this can impair their functional and structural integrity. In this review, we present an overview of how oxidative stress is implicated in male reproductive aging by providing a summary of the sources and roles of ROS, the theories of aging, and the current animal and human studies that demonstrate the impacts of APA on the male germ line, the health of progeny and fertility, and how treatment with antioxidants may reverse these effects.

Knockout Mouse Models for Peroxiredoxins

Peroxiredoxins (PRDXs) are members of a highly conserved peroxidase family and maintain intracellular reactive oxygen species (ROS) homeostasis. The family members are expressed in most organisms and involved in various biological processes, such as cellular protection against ROS, inflammation, carcinogenesis, atherosclerosis, heart diseases, and metabolism. In mammals, six PRDX members have been identified and are subdivided into three subfamilies: typical 2-Cys (PRDX1, PRDX2, PRDX3, and PRDX4), atypical 2-Cys (PRDX5), and 1-Cys (PRDX6) subfamilies. Knockout mouse models of PRDXs have been developed to investigate their in vivo roles. This review presents an overview of the knockout mouse models of PRDXs with emphases on the biological and physiological changes of these model mice.

PRDX4 and Its Roles in Various Cancers

Reactive oxygen species play a vital role in cell survival by regulating physiological metabolism and signal transduction of cells. The imbalance of oxidant and antioxidant states induces oxidative stress within a cell. Redox regulation and oxidative stress are closely related to survival and proliferation of stem cells, cancer cells, and cancer stem cells. Peroxiredoxin 4, a typical endoplasmic reticulum-resident 2-Cys antioxidant of peroxiredoxins, can fine-tune hydrogen peroxide catabolism which affects cell survival by affecting redox balance, oxidative protein folding, and regulation of hydrogen peroxide signaling. Recent studies revealed the overexpression of peroxiredoxin 4 in several kinds of cancers, such as breast cancer, prostate cancer, ovarian cancer, colorectal cancer, and lung cancer. And it has been demonstrated that peroxiredoxin 4 causally contributes to tumorigenesis, therapeutic resistance, metastasis, and recurrence of tumors. In this article, the characteristics of peroxiredoxin 4 in physiological functions and the cancer-related research progress of mammalian peroxiredoxin 4 is reviewed. We believe that peroxiredoxin 4 has the potential of serving as a novel target for multiple cancers.

Mutual interaction between oxidative stress and endoplasmic reticulum stress in the pathogenesis of diseases specifically focusing on non-alcoholic fatty liver disease

Reactive oxygen species (ROS) are produced during normal physiologic processes with the consumption of oxygen. While ROS play signaling roles, when they are produced in excess beyond normal antioxidative capacity this can cause pathogenic damage to cells. The majority of such oxidation occurs in polyunsaturated fatty acids and sulfhydryl group in proteins, resulting in lipid peroxidation and protein misfolding, respectively. The accumulation of misfolded proteins in the endoplasmic reticulum (ER) is enhanced under conditions of oxidative stress and results in ER stress, which, together, leads to the malfunction of cellular homeostasis. Multiple types of defensive machinery are activated in unfolded protein response under ER stress to resolve this unfavorable situation. ER stress triggers the malfunction of protein secretion and is associated with a variety of pathogenic conditions including defective insulin secretion from pancreatic β-cells and accelerated lipid droplet formation in hepatocytes. Herein we use nonalcoholic fatty liver disease (NAFLD) as an illustration of such pathological liver conditions that result from ER stress in association with oxidative stress. Protecting the ER by eliminating excessive ROS via the administration of antioxidants or by enhancing lipid-metabolizing capacity via the activation of peroxisome proliferator-activated receptors represent promising therapeutics for NAFLD.

Double Knockout of Peroxiredoxin 4 (Prdx4) and Superoxide Dismutase 1 (Sod1) in Mice Results in Severe Liver Failure

Mice that are deficient in superoxide dismutase 1 (Sod1), an antioxidative enzyme, are susceptible to developing liver steatosis. Peroxiredoxin 4 (Prdx4) catalyzes disulfide bond formation in proteins via the action of hydrogen peroxide and hence decreases oxidative stress and supports oxidative protein folding for the secretion of lipoproteins. Because elevated reactive oxygen species induce endoplasmic reticulum stress, this negative chain reaction is likely involved in the development of nonalcoholic fatty liver diseases and more advanced steatohepatitis (NASH). In the current study, we generated Prdx4 and Sod1 double knockout (DKO; Prdx4^-/ySod1^-/-) mice and examined whether the combined deletion of Prdx4 and Sod1 aggravated liver pathology compared to single knockout and wild-type mice. The secretion of triglyceride-rich lipoprotein was strikingly impaired in the DKO mice, leading to aggravated liver steatosis. Simultaneously, the activation of caspase-3 in the liver was observed. The hyperoxidation of Prdxs, a hallmark of oxidative stress, occurred in different isoforms that are uniquely associated with Sod1^-/- and Prdx4^-/y mice, and the effect was additive in DKO mouse livers. Because DKO mice spontaneously develop severe liver failure at a relatively young stage, they have the potential for use as a model for hepatic disorders and for testing other potential treatments.

Protective role of testis-specific peroxiredoxin 4 against cellular oxidative stress

Peroxiredoxin (PRDX), a newly discovered antioxidant enzyme, has an important role in hydrogen peroxide reduction. Among six PRDX genes (PRDX1–6) in mammals, PRDX4 gene is alternatively spliced to produce the somatic cell form (PRDX4) and the testis specific form (PRDX4t). In our previous study, PRDX4 knockout mice displayed testicular atrophy with an increase in cell death due to oxidative stress. However, the antioxidant function of PRDX4t is unknown. In this study, we demonstrate that PRDX4t plays a protective role against oxidative stress in the mammalian cell line HEK293T. The PRDX4t-EGFP plasmid was transferred into HEK293T cells; protein expression was confirmed in the cytoplasm. To determine the protective role of PRDX4t in cells, we performed image-based analysis of PRDX4t-EGFP expressed cells exposed to UV irradiation and hydrogen peroxide using fluorescent probe CellROX. Our results suggested that PRDX4t-EGFP expressed cells had reduced levels of oxidative stress compared with cells that express only EGFP. This study highlights that PRDX4t plays an important role in cellular antioxidant defense.

Multiple Functions and Regulation of Mammalian Peroxiredoxins

Peroxiredoxins (Prxs) constitute a major family of peroxidases, with mammalian cells expressing six Prx isoforms (PrxI to PrxVI). Cells produce hydrogen peroxide (H₂O₂) at various intracellular locations where it can serve as a signaling molecule. Given that Prxs are abundant and possess a structure that renders the cysteine (Cys) residue at the active site highly sensitive to oxidation by H₂O₂, the signaling function of this oxidant requires extensive and highly localized regulation. Recent findings on the reversible regulation of PrxI through phosphorylation at the centrosome and on the hyperoxidation of the Cys at the active site of PrxIII in mitochondria are described in this review as examples of such local regulation of H₂O₂ signaling. Moreover, their high affinity for and sensitivity to oxidation by H₂O₂ confer on Prxs the ability to serve as sensors and transducers of H₂O₂ signaling through transfer of their oxidation state to bound effector proteins.

Negative biomarker based male fertility evaluation: Sperm phenotypes associated with molecular-level anomalies

Biomarker-based sperm analysis elevates the treatment of human infertility and ameliorates reproductive performance in livestock. The negative biomarker-based approach focuses on proteins and ligands unique to defective spermatozoa, regardless of their morphological phenotype, lending itself to analysis by flow cytometry (FC). A prime example is the spermatid specific thioredoxin SPTRX3/TXNDC8, retained in the nuclear vacuoles and superfluous cytoplasm of defective human spermatozoa. Infertile couples with high semen SPTRX3 are less likely to conceive by assisted reproductive therapies (ART) and more prone to recurrent miscarriage while low SPTRX3 has been associated with multiple ART births. Ubiquitin, a small, proteolysis-promoting covalent posttranslational protein modifier is found on the surface of defective posttesticular spermatozoa and in the damaged protein aggregates, the aggresomes of spermiogenic origin. Semen ubiquitin content correlates negatively with fertility and conventional semen parameters, and with sperm binding of lectins LCA (Lens culinaris agglutinin; reveals altered sperm surface) and PNA (Arachis hypogaea/peanut agglutinin; reveals acrosomal malformation or damage). The Postacrosomal Sheath WWI Domain Binding Protein (PAWP), implicated in oocyte activation during fertilization, is ectopic or absent from defective human and animal spermatozoa. Consequently, FC-parameters of PAWP correlate with ART outcomes in infertile couples and with fertility in bulls. Assays based on the above biomarkers have been combined into multiplex FC semen screening protocols, and the surface expression of lectins and ubiquitin has been utilized to develop nanoparticle-based bull semen purification method validated by field artificial insemination trials. These advances go hand-in-hand with the innovation of FC-technology and genomics/proteomics-based biomarker discovery.

Physiological and pathological views of peroxiredoxin 4

Peroxiredoxins (PRDXs) form an enzyme family that exhibits peroxidase activity using electrons from thioredoxin and other donor molecules. As the signaling roles of hydrogen peroxide in response to extracellular stimuli have emerged, the involvement of PRDX in the hydrogen peroxide-mediated signaling has become evident. Among six PRDX members in mammalian cells, PRDX4 uniquely possesses a hydrophobic signal peptide at the amino terminus, and, hence, it undergoes either secretion or retention by the endoplasmic reticulum (ER) lumen. The role of PRDX4 as a sulfoxidase in ER is now attracting much attention regarding the oxidative protein folding of nascent proteins. Contrary to this role in the ER, the functional significance of PRDX4 in the extracellular milieu is virtually unknown despite its implications as a biomarker under pathological conditions in some diseases. Other than its systemically expressed form, a variant form of PRDX4 is transcribed from the upstream promoter/exon 1 of the systemic promoter/exon 1 and is uniquely expressed in sexually matured testes. Circumstantial evidence, together with deduced functions from the systemic form, suggests that there are potential roles for testicular PRDX4 in the reproductive processes such as the regulation of hormonal signals and the oxidative packaging of sperm chromatin. Elucidation of these PRDX4 functions under in vivo situations is expected to show the whole picture of how PRDX4 has evolved in multicellular organisms.

ERO1: A protein disulfide oxidase and H2O2 producer

Oxidative protein folding in the endoplasmic reticulum (ER) is an essential function of eukaryotic cells that requires the relaying of electrons between the proteinaceous components of the pathway. During this process, protein disulfide isomerase (PDI) chaperones oxidatively fold their client proteins before endoplasmic reticulum oxireductin 1 (ERO1) oxidase transfers electrons from the reduced PDI to the terminal acceptor, which is usually molecular oxygen and is subsequently reduced to H2O2. ERO1 function is essential for disulfide bond formation in yeast, whereas in mammals its function is compensated for by alternative pathways. ERO1 activity is allosterically and transcriptionally regulated by the ER unfolded protein response (UPR). The ER stress-induced upregulation of ERO1 and other genes contributes to a cell's ability to cope with ER stress as a result of an adaptive homeostatic response, but the stress persists if a "maladaptive UPR" fails to reestablish ER homeostasis. As the oxidative activity of ERO1 is related to the production of H2O2 and consequently burdens cells with potentially toxic reactive oxygen species, deregulated ERO1 activity is likely to impair cell fitness under certain conditions of severe ER stress and may therefore lead to a change from an adaptive to a maladaptive UPR. This review summarizes the evidence of the double-edged sword activity of ERO1 by highlighting its role as a protein disulfide oxidase and H2O2 producer.

ROS, thiols and thiol-regulating systems in male gametogenesis

BACKGROUND

During maturation and storage, spermatozoa generate substantial amounts of reactive oxygen species (ROS) and are thus forced to cope with an increasingly oxidative environment that is both needed and detrimental to their biology. Such a janus-faceted intermediate needs to be tightly controlled and this is done by a wide array of redox enzymes. These enzymes not only have to prevent unspecific modifications of essential cellular biomolecules by quenching undesired ROS, but they are also required and often directly involved in critical protein modifications.

SCOPE OF REVIEW

The present review is conceived to present an update on what is known about critical roles of redox enzymes, whereby special emphasis is put on the family of glutathione peroxidases, which for the time being presents the best characterized tasks during gametogenesis.

MAJOR CONCLUSIONS

We therefore demonstrate that understanding the function of (seleno)thiol-based oxidases/reductases is not a trivial task and relevant knowledge will be mainly gained by using robust systems, as exemplified by several (conditional) knockout studies. We thus stress the importance of using such models for providing unequivocal evidence in the molecular understanding of redox regulatory mechanisms in sperm maturation.

GENERAL SIGNIFICANCE

ROS are not merely detrimental by-products of metabolism and their proper generation and usage by specific enzymes is essential for vital functions as beautifully exemplified during male gametogenesis. As such, lessons learnt from thiol-based oxidases/reductases in male gametogenesis could be used as a general principle for other organs as it is most likely not only restricted to this developmental phase. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Redox reactions in mammalian spermatogenesis and the potential targets of reactive oxygen species under oxidative stress

Reduction-oxidation (Redox) reactions are ubiquitous mechanisms for vital activities in all organisms, and they play pivotal roles in the regulation of spermatogenesis as well. Here we focus on 3 redox-involved processes that have drawn much recent attention: the regulation of signal transduction by reactive oxygen species (ROS) such as hydrogen peroxide, oxidative protein folding in the endoplasmic reticulum (ER), and sulfoxidation of protamines during sperm chromatin condensation. The first 2 of these processes are emerging topics in cell biology and are applicable to most living cells, which includes spermatogenic cells. The roles of ROS in signal transduction have been elucidated in the last 2 decades and have received broad attention, most notably from the viewpoint of the proper control of mitotic signals. Redox processes in the ER are important because this is the organelle where secretory and membrane proteins are synthesized and proceed toward their functional structure, so that malfunction of the ER affects not only the involved cells but also the accepting cells of the secreted proteins in multicellular organisms. Sulfoxidation is the third of these processes, and the sulfoxidation of chromatin is a unique process in sperm maturation. During recent sulfoxidase research, GPX4 has emerged as a promising enzyme that plays essential roles in the production of fertile sperm, but the involvement of other redox proteins is also becoming evident. Because the molecules involved in the redox reactions are prone to oxidation, they can be sensitive to oxidative damage, which makes them potential targets for antioxidant therapy.

Oxidative stress and redox regulation of gametogenesis, fertilization, and embryonic development

Oxidative stress caused by elevated reactive oxygen species (ROS) is one of the predominant causes of both male and female infertility. Oxidative stress conditions cause either cell death or senescence by oxidation of cellular molecules including nucleic acid, proteins, and lipids. It is particularly important to minimize oxidative stress when in vitro fertilization is performed for the purpose of assisted reproduction. The problems associated with assisted reproductive technology are becoming evident, and it is now the time to clarify its mechanisms and cope with them. On the other hand, the beneficial roles of ROS, such as intracellular signaling, have become evident. The antithetical functions of ROS make it more difficult to overcome the problems caused by oxidative stress. Despite the difficulty in understanding mammalian reproduction, the mechanisms and problems can be gradually unveiled by advanced technology such as genetic modification of animals.

PRDX4, an endoplasmic reticulum-localized peroxiredoxin at the crossroads between enzymatic oxidative protein folding and nonenzymatic protein oxidation

SIGNIFICANCE

Peroxiredoxin 4 (PRDX4) is an endoplasmic reticulum (ER)-resident peroxiredoxin that has the characteristic of coupling hydrogen peroxide (H(2)O(2)) catabolism with oxidative protein folding. This enzymatic arrangement involves the ingenious use of H(2)O(2) as a substrate to streamline protein metabolism.

RECENT ADVANCES

Mice with compound mutations in Prdx4 and Ero1 have revealed the physiological implication that PRDX4 is a fine-tuned enzymatic mediator of oxidative folding. Remarkably, by simultaneously triggering slow disulfide bond formation and the buildup of H(2)O(2), the lack of PRDX4 and endoplasmic oxidoreductin 1 (ERO1) exposes the thiols of new client proteins to competing H(2)O(2)-mediated oxidation, which leads to an increase in sulfenylated proteins. Such oxygenated thiol derivatives exploit ascorbate as their reductant, thus accelerating its clearance. This relay of events culminates in an altered extracellular matrix (ECM) and a senescent phenotype.

CRITICAL ISSUES AND FUTURE DIRECTIONS

By combining H(2)O(2) metabolism with oxidative folding, PRDX4 protects nascent proteins from an alternative oxidative fate, and cells from the consequences of having misfolded proteins. This highlights the importance of kinetic-regulated disulfide formation at physiological level, and the presence of an exquisite backup system to protect ER redox homeostasis. By altering ECM architecture, ascorbate depletion in the cells triggers an integrated signaling cascade. This sequence of events is part of a multifaceted response linking the ER and the nucleus, which helps cells to overcome ER redox impairment. Furthermore, the relationship between the protein sulfenylation and ascorbate depletion suggests that it would be interesting to explore the metabolism of ascorbate in pathological conditions accompanied by oxidative stress and a defective ECM.

Should autism be considered a canary bird telling that Homo sapiens may be on its way to extinction?

There has been a dramatic enhancement of the reported incidence of autism in different parts of the world over the last 30 years. This can apparently not be explained only as a result of improved diagnosis and reporting, but may also reflect a real change. The causes of this change are unknown, but if we shall follow T.C. Chamberlin's principle of multiple working hypotheses, we need to take into consideration the possibility that it partly may reflect an enhancement of the average frequency of responsible alleles in large populations. If this hypothesis is correct, it means that the average germline mutation rate must now be much higher in the populations concerned, compared with the natural mutation rate in hominid ancestors before the agricultural and industrial revolutions. This is compatible with the high prevalence of impaired human semen quality in several countries and also with what is known about high levels of total exposure to several different unnatural chemical mutagens, plus some natural ones at unnaturally high levels. Moreover, dietary deficiency conditions that may lead to enhancement of mutation rates are also very widespread, affecting billions of people. However, the natural mutation rate in hominids has been found to be so high that there is apparently no tolerance for further enhancement of the germline mutation rate before the Eigen error threshold will be exceeded and our species will go extinct because of mutational meltdown. This threat, if real, should be considered far more serious than any disease causing the death only of individual patients. It should therefore be considered the first and highest priority of the best biomedical scientists in the world, of research-funding agencies and of all medical doctors to try to stop the express train carrying all humankind as passengers on board before it arrives at the end station of our civilization.

Peroxides and peroxidases in the endoplasmic reticulum: integrating redox homeostasis and oxidative folding

SIGNIFICANCE

The endoplasmic reticulum (ER), the port of entry into the secretory pathway, is a complex organelle that performs many fundamental functions, including protein synthesis and quality control, Ca(2+) storage and signaling. Redox homeostasis is of paramount importance for allowing the efficient folding of secretory proteins, most of which contain essential disulfide bonds.

RECENT ADVANCES

revealed that an intricate protein network sustains the processes of disulfide bond formation and reshuffling in the ER. Remarkably, H(2)O(2), which is a known by-product of Ero1 flavoproteins in cells, is utilized by peroxiredoxin-4 and glutathione peroxidases-7 and -8, which reside in the mammalian secretory compartment and further fuel oxidative protein folding while limiting oxidative damage.

CRITICAL ISSUES

that remain to be addressed are the sources, diffusibility and signaling role(s) of H(2)O(2) in and between organelles and cells, how the emerging redundancy in the systems is coupled to precise regulation, and how the distinct pathways operating in the early secretory compartment are integrated with one another.

FUTURE DIRECTIONS

A further dissection of the pathways that integrate folding, redox homeostasis, and signaling in the early secretory pathway may allow to manipulate protein homeostasis and survival-death decisions in degenerative diseases or cancer.

Peroxiredoxin 4: a multifunctional biomarker worthy of further exploration

Currently, there is much interest in identifying clinically relevant biomarkers, as they have the potential to be high utility non-invasive tools for early diagnosis and reliable patient monitoring in numerous conditions. Since its discovery almost 15 years ago, research on the ubiquitous antioxidant enzyme peroxiredoxin 4 (Prx4) has culminated in the recognition that Prx4 levels are different in blood drawn from the healthy general population and patients with acute or chronic diseases. In this commentary, the most striking research data from different in vitro approaches, animal models and human observational studies are discussed collectively, highlighting the clinical importance of Prx4 as a multifunctional staging and prognosis biomarker. In this context, the oxidative state of patients may be reflected by intra- and extracellular Prx4 levels, redox state, oligomerization and nitro-oxidative modifications of the enzyme. A consolidated model of the potential role and origin of circulating Prx4 is presented to stimulate further investigations in light of the current biomarker situation.