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

The small molecule peroxiredoxin mimetics restore growth factor signalings and reverse vascular remodeling

Epidithio-diketopiperazine (ETP) compound is the family of natural fungal metabolites that are known to exert diverse biological effects, such as immunosuppression and anti-cancer activity, in higher animals. However, an enzyme-like catalytic activity or function of the ETP derivatives has not been reported. Here, we report the generation of novel thiol peroxidase mimetics that possess peroxide-reducing activity through strategic derivatization of the core ETP ring structure. The ETP derivatives with small side chains are the bona fide 2-Cys peroxiredoxin (PRX) mimetics that catalyze the H2O2-reducing reaction specifically coupled to the thioredoxin/thioredoxin reductase system. In contrast, the ETP derivatives with linear chains or a heterocyclic group show H2O2-reducing activity in coupling with both thioredoxin and glutathione systems. Moreover, the ETP derivatives with bulky heterocyclic groups almost lose catalytic activity. The 2-Cys PRX mimetics regulate intracellular H2O2 levels, thereby restoring the receptor Tyr kinase signaling and cellular functions disrupted by the absence of 2-Cys PRX in vascular cells. In a rodent model, the 2-Cys PRX mimetics reverse vascular occlusion in the injured carotid arteries by inhibiting smooth muscle hyperplasia and promoting reendothelialization. Thus, this study reveals a novel chemical platform for complementing defective 2-Cys PRX enzymes in biological systems.

Reactive oxygen species in hypertension

Hypertension is a leading risk factor for stroke, heart disease and chronic kidney disease. Multiple interacting factors and organ systems increase blood pressure and cause target-organ damage. Among the many molecular elements involved in the development of hypertension are reactive oxygen species (ROS), which influence cellular processes in systems that contribute to blood pressure elevation (such as the cardiovascular, renal, immune and central nervous systems, or the renin-angiotensin-aldosterone system). Dysregulated ROS production (oxidative stress) is a hallmark of hypertension in humans and experimental models. Of the many ROS-generating enzymes, NADPH oxidases are the most important in the development of hypertension. At the cellular level, ROS influence signalling pathways that define cell fate and function. Oxidative stress promotes aberrant redox signalling and cell injury, causing endothelial dysfunction, vascular damage, cardiovascular remodelling, inflammation and renal injury, which are all important in both the causes and consequences of hypertension. ROS scavengers reduce blood pressure in almost all experimental models of hypertension; however, clinical trials of antioxidants have yielded mixed results. In this Review, we highlight the latest advances in the understanding of the role and the clinical implications of ROS in hypertension. We focus on cellular sources of ROS, molecular mechanisms of oxidative stress and alterations in redox signalling in organ systems, and their contributions to hypertension.

Human mitochondrial peroxiredoxin Prdx3 is dually localized in the intermembrane space and matrix subcompartments

Peroxiredoxin 3 (Prdx3) is the major sink for H2O2 and other hydroperoxides within mitochondria, yet the mechanisms guiding the import of its cytosolic precursor into mitochondrial sub-compartments remain elusive. Prdx3 is synthesized in the cytosol as a precursor with an N-terminal cleavable presequence, which is frequently proposed to target the protein exclusively to the mitochondrial matrix. Here, we present a comprehensive analysis of the human Prdx3 biogenesis, using highly purified mitochondria from HEK293T cells. Subfractionation and probing for specific mitochondrial markers confirmed Prdx3 localization in the matrix, while unexpectedly revealed its presence in the mitochondrial intermembrane space (IMS). Both matrix and IMS isoforms were found to be soluble proteins, as demonstrated by alkaline carbonate extraction. By combining in silico analysis, in organello import assays and heterologous expression in yeast, we found that Prdx3 undergoes sequential proteolytic processing steps by mitochondrial processing peptidase (MPP) and mitochondrial intermediate peptidase (MIP) during its import into the matrix. Additionally, heterologous expression of Prdx3 in yeast revealed that its sorting to the IMS is dependent on the inner membrane peptidase (IMP) complex. Collectively, these findings uncover a complex submitochondrial distribution of Prdx3, supporting its multifaceted role in mitochondrial H2O2 metabolism.

Screening of pathologically significant diagnostic biomarkers in tears of thyroid eye disease based on bioinformatic analysis and machine learning

Background

Lacrimal gland enlargement is a common pathological change in patients with thyroid eye disease (TED). Tear fluid has emerged as a new source of diagnostic biomarkers, but tear-based diagnostic biomarkers for TED with high efficacy are still lacking.

Objective

We aim to investigate genes associated with TED-associated lacrimal gland lesions. Additionally, we seek to identify potential biomarkers for diagnosing TED in tear fluid.

Methods

We obtained two expression profiling datasets related to TED lacrimal gland samples from the Gene Expression Omnibus (GEO). Subsequently, we combined the two separate datasets and conducted differential gene expression analysis and weighted gene co-expression network analysis (WGCNA) on the obtained integrated dataset. The genes were employed for Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. The genes were intersected with the secretory proteins profile to get the potential proteins in the tear fluid. Machine learning techniques were then employed to identify optimal biomarkers and develop a diagnostic nomogram for predicting TED. Finally, gene set enrichment analysis (GSEA) and immune infiltration analysis were conducted on screened hub genes to further elucidate their potential mechanisms in TED.

Results

In our analysis of the integrated TED dataset, we identified 2,918 key module genes and 157 differentially expressed genes and finally obtained 84 lacrimal-associated key genes. Enrichment analysis disclosed that these 84 genes primarily pertain to endoplasmic reticulum organization. After intersecting with the secretory proteins, 13 lacrimal gland-associated secretory protein genes (LaSGs) were identified. The results from machine learning indicated the substantial diagnostic value of dyslexia associated gene (KIAA0319) and peroxiredoxin4 (PRDX4) in TED-associated lacrimal gland lesions. The two hub genes were chosen as candidate biomarkers in tear fluid and employed to establish a diagnostic nomogram. Furthermore, single-gene GSEA results and immune cell infiltration analysis unveiled immune dysregulation in the lacrimal gland of TED, with KIAA0319 and PRDX4 showing significant associations with infiltrating immune cells.

Conclusions

We uncovered the distinct pathophysiology of TED-associated lacrimal gland enlargement compared to TED-associated orbital adipose tissue enlargement. We have demonstrated the endoplasmic reticulum-related pathways involved in TED-associated lacrimal gland lesions and established a diagnostic nomogram for TED utilizing KIAA0319 and PRDX4 through integrated bioinformatics analysis. This contribution offers novel insights for non-invasive, prospective diagnostic approaches in the context of TED.

Peroxiredoxin 4 deficiency induces accelerated ovarian aging through destroyed proteostasis in granulosa cells

Ovarian aging, a complex and challenging concern within the realm of reproductive medicine, is associated with reduced fertility, menopausal symptoms and long-term health risks. Our previous investigation revealed a correlation between Peroxiredoxin 4 (PRDX4) and human ovarian aging. The purpose of this research was to substantiate the protective role of PRDX4 against ovarian aging and elucidate the underlying molecular mechanism in mice. In this study, a Prdx4-/- mouse model was established and it was observed that the deficiency of PRDX4 led to only an accelerated decline of ovarian function in comparison to wild-type (WT) mice. The impaired ovarian function observed in this study can be attributed to an imbalance in protein homeostasis, an exacerbation of endoplasmic reticulum stress (ER stress), and ultimately an increase in apoptosis of granulosa cells. Furthermore, our research reveals a noteworthy decline in the expression of Follicle-stimulating hormone receptor (FSHR) in aging Prdx4-/- mice, especially the functional trimer, due to impaired disulfide bond formation. Contrarily, the overexpression of PRDX4 facilitated the maintenance of protein homeostasis, mitigated ER stress, and consequently elevated E2 levels in a simulated KGN cell aging model. Additionally, the overexpression of PRDX4 restored the expression of the correct spatial conformation of FSHR, the functional trimer. In summary, our research reveals the significant contribution of PRDX4 in delaying ovarian aging, presenting a novel and promising therapeutic target for ovarian aging from the perspective of endoplasmic reticulum protein homeostasis.

Lycium barbarum berry extract improves female fertility against aging-related oxidative stress in the ovary

Reproductive aging in female mammals is characterized by ovarian senescence, leading to a significant fertility decline. Lycium barbarum berry, or goji berry, is a food and medicine that appears in various formulas for treating infertility in traditional Chinese medicine. We investigated the function of an aqueous extract of Lycium barbarum berry (LB extract) to improve health status, fertility, and offspring development during female aging. Aged female mice were supplemented with LB extract, and its effects on fertility, locomotor activity, and offspring development were assessed. The results demonstrated that LB extract significantly increased pregnancy and live birth rates in naturally aged female mice. It also effectively improved aged animals' locomotor activity. Moreover, LB extract promoted the growth and development of offspring delivered from the aged animals and reduced the offspring's anxiety. During aging, fertility-related hormones gradually decline. However, the decline of anti-Müllerian hormone (AMH) and estradiol (E2) in the serum of aged mice was restored by LB extract supplementation. Immunohistochemical analysis revealed that the levels of oxidation and the inflammatory IL-6 in intra-ovarian cells were reduced by LB extract, while the antioxidant-associated proteins peroxiredoxin 4 (PRDX4) and nuclear factor erythroid 2-related factor 2 (NRF2) were increased. Bioinformatics analysis revealed a decline in egg PRDX4 expression with age across various species. This suggests that the antioxidant function protected by LB extract through PRDX4 may consistently promote fertility enhancement by improving ovarian function across different species. Importantly, LB extract did not induce significant adverse effects on aged female mice and their offspring. These findings highlight the potential of LB as a protective agent against ovarian oxidative stress, which preserves ovarian function and improves fertility rates in naturally senescent females.

Temporary alleviation of MAPK by arbutin alleviates oxidative damage in the retina and ARPE-19 cells

Dry age-related macular degeneration (AMD) is one of the main diseases that causes blindness in humans, and the number of cases is increasing yearly. However, effective treatments are unavailable, and arbutin (ARB) has been reported to have antioxidant, anti-inflammatory, and anti-aging effects in other age-related diseases. However, whether ARB can be used to treat dry AMD remains unknown. To explore the therapeutic potential and molecular mechanism of arbutin in the treatment of dry AMD. MTT assays, reactive oxygen species (ROS) production assays, flow cytometry assays, qPCR and western blotting were used to assess the impact of ARB on human RPECs induced by H2O2. A transcriptome sequencing assay was used to further explore how ARB acts on human RPECs treated with H2O2. Hematoxylin and eosin (H&E) staining and total antioxidant capacity (T-AOC) assays were used to observe the impact of ARB on mouse retina induced by sodium iodate. ARB counteracted the H2O2-induced reduction in human RPECs viability, ARB reversed H2O2-induced cellular ROS production by increasing the expression of antioxidant-related genes and proteins, ARB also reversed H2O2-induced cell apoptosis by altering the expression of apoptosis-related genes and proteins. Transcriptome sequencing and western blotting showed that ARB reduced ERK1/2 and P-38 phosphorylation to prevent H2O2-induced oxidation damage. The in vivo experiments demonstrated that ARB protected against retinal morphology injury in mice, increased serum T-AOC levels and increased antioxidant oxidase gene expression levels in the mouse retina induced by sodium iodate. We concluded that ARB reversed the H2O2-induced decrease in human RPECs viability through the inhibition of ROS production and apoptosis. The ERK1/2 and P38 MAPK signaling pathways may mediate this process. ARB maintained retinal morphology, increased serum T-AOC level and improved the expression of antioxidant oxidase genes in mice.

Biological mechanisms and clinical significance of endoplasmic reticulum oxidoreductase 1 alpha (ERO1α) in human cancer

A firm link between endoplasmic reticulum (ER) stress and tumors has been wildly reported. Endoplasmic reticulum oxidoreductase 1 alpha (ERO1α), an ER-resident thiol oxidoreductase, is confirmed to be highly upregulated in various cancer types and associated with a significantly worse prognosis. Of importance, under ER stress, the functional interplay of ERO1α/PDI axis plays a pivotal role to orchestrate proper protein folding and other key processes. Multiple lines of evidence propose ERO1α as an attractive potential target for cancer treatment. However, the unavailability of specific inhibitor for ERO1α, its molecular inter-relatedness with closely related paralog ERO1β and the tightly regulated processes with other members of flavoenzyme family of enzymes, raises several concerns about its clinical translation. Herein, we have provided a detailed description of ERO1α in human cancers and its vulnerability towards the aforementioned concerns. Besides, we have discussed a few key considerations that may improve our understanding about ERO1α in tumors.

Analysis of the Differentially Expressed Proteins in Donkey Milk in Different Lactation Stages

Proteins in donkey milk (DM) have special biological activities. However, the bioactive proteins and their expression regulation in donkey milk are still unclear. Thus, the differentially expressed proteins (DEPs) in DM in different lactation stages were first investigated by data-independent acquisition (DIA) proteomics. A total of 805 proteins were characterized in DM. The composition and content of milk proteins varied with the lactation stage. A total of 445 candidate DEPs related to biological processes and molecular functions were identified between mature milk and colostrum. The 219 down-regulated DEPs were mainly related to complement and coagulation cascades, staphylococcus aureus infection, systemic lupus erythematosus, prion diseases, AGE-RAGE signaling pathways in diabetic complications, and pertussis. The 226 up-regulated DEPs were mainly involved in metabolic pathways related to nutrient (fat, carbohydrate, nucleic acid, and vitamin) metabolism. Some other DEPs in milk from the lactation period of 30 to 180 days also had activities such as promoting cell proliferation, promoting antioxidant, immunoregulation, anti-inflammatory, and antibacterial effects, and enhancing skin moisture. DM can be used as a nutritional substitute for infants, as well as for cosmetic and medical purposes. Our results provide important insights for understanding the bioactive protein differences in DM in different lactation stages.

In silico analysis reveals PRDX4 as a prognostic and oncogenic marker in renal papillary cell carcinoma

BACKGROUND

Alterations in the tumor microenvironment leads to the accumulation of reactive oxygen species (ROS). When in low levels, ROS act as a signaling molecule and contribute to tumor cell proliferation whereas its elevation results in oxidative stress and eventually cell death. It is known that antioxidant systems regulate the ROS levels and thus cell fate. Among these systems, peroxiredoxins (PRDXs) were found to be upregulated in various cancers. However their exact contribution to carcinogenesis is not yet clear.

AIM

Herein, the expression pattern and prognostic value of PRDXs were explored in cancer setting by using in silico analysis tools and publicly available datasets.

RESULTS

Pan-cancer analysis revealed that PRDXs are differentially expressed in normal and tumor tissues. Further analysis showed that higher PRDX4 levels was associated with poor prognosis and clinicopathological and histological features associated with a more aggressive renal papillary cell carcinoma (KIRP) profile. Hypoxia, ER stress and protein folding were shown to be pathways positively correlated with PRDX4 levels. Furthermore, PRDX4 was found to be strong regulator of protein homeostasis. Kaplan-Meier analysis revealed that PRDX4 is a potent prognostic marker in Type 2 KIRP and this might be due to increased ER stress and oxidative stress levels in this subtype.

CONCLUSIONS

The data suggest that PRDX4 can be used as a prognostic marker for KIRP patients. Its association with more aggressive tumor characteristics also underlines that it might be used for targeted therapy.

Proteome changes in a human retinal pigment epithelial cell line during oxidative stress and following antioxidant treatment

Age related macular degeneration (AMD) is the most common cause of blindness in the elderly. Oxidative stress contributes to retinal pigment epithelium (RPE) dysfunction and cell death thereby leading to AMD. Using improved RPE cell model systems, such as human telomerase transcriptase-overexpressing (hTERT) RPE cells (hTERT-RPE), pathophysiological changes in RPE during oxidative stress can be better understood. Using this model system, we identified changes in the expression of proteins involved in the cellular antioxidant responses after induction of oxidative stress. Some antioxidants such as vitamin E (tocopherols and tocotrienols) are powerful antioxidants that can reduce oxidative damage in cells. Alpha-tocopherol (α-Toc or αT) and gamma-tocopherol (γ-Toc or γT) are well-studied tocopherols, but signaling mechanisms underlying their respective cytoprotective properties may be distinct. Here, we determined what effect oxidative stress, induced by extracellularly applied tBHP in the presence and absence of αT and/or γT, has on the expression of antioxidant proteins and related signaling networks. Using proteomics approaches, we identified differential protein expression in cellular antioxidant response pathways during oxidative stress and after tocopherol treatment. We identified three groups of proteins based on biochemical function: glutathione metabolism/transfer, peroxidases and redox-sensitive proteins involved in cytoprotective signaling. We found that oxidative stress and tocopherol treatment resulted in unique changes in these three groups of antioxidant proteins indicate that αT and γT independently and by themselves can induce the expression of antioxidant proteins in RPE cells. These results provide novel rationales for potential therapeutic strategies to protect RPE cells from oxidative stress.

Endoplasmic reticulum oxidoreductin 1-alpha deficiency and activation of protein translation synergistically impair breast tumour resilience

BACKGROUND AND PURPOSE

Endoplasmic reticulum (ER) stress triggers an adaptive response in tumours which fosters cell survival and resilience to stress. Activation of the ER stress response, through its PERK branch, promotes phosphorylation of the α-subunit of the translation initiation factor eIF2, thereby repressing general protein translation and augmenting the translation of ATF4 with the downstream CHOP transcription factor and the protein disulfide oxidase, ERO1-alpha EXPERIMENTAL APPROACH: Here, we show that ISRIB, a small molecule that inhibits the action of phosphorylated eIF2alpha, activating protein translation, synergistically interacts with the genetic deficiency of protein disulfide oxidase ERO1-alpha, enfeebling breast tumour growth and spread.

KEY RESULTS

ISRIB represses the CHOP signal, but does not inhibit ERO1. Mechanistically, ISRIB increases the ER protein load with a marked perturbing effect on ERO1-deficient triple-negative breast cancer cells, which display impaired proteostasis and have adapted to a low client protein load in hypoxia, and ERO1 deficiency impairs VEGF-dependent angiogenesis. ERO1-deficient triple-negative breast cancer xenografts have an augmented ER stress response and its PERK branch. ISRIB acts synergistically with ERO1 deficiency, inhibiting the growth of triple-negative breast cancer xenografts by impairing proliferation and angiogenesis.

CONCLUSION AND IMPLICATIONS

These results demonstrate that ISRIB together with ERO1 deficiency synergistically shatter the PERK-dependent adaptive ER stress response, by restarting protein synthesis in the setting of impaired proteostasis, finally promoting tumour cytotoxicity. Our findings suggest two surprising features in breast tumours: ERO1 is not regulated via CHOP under hypoxic conditions, and ISRIB offers a therapeutic option to efficiently inhibit tumour progression in conditions of impaired proteostasis.

ER stress and calcium-dependent arrhythmias

The sarcoplasmic reticulum (SR) plays the key role in cardiac function as the major source of Ca²⁺ that activates cardiomyocyte contractile machinery. Disturbances in finely-tuned SR Ca²⁺ release by SR Ca²⁺ channel ryanodine receptor (RyR2) and SR Ca²⁺ reuptake by SR Ca²⁺-ATPase (SERCa2a) not only impair contraction, but also contribute to cardiac arrhythmia trigger and reentry. Besides being the main Ca²⁺ storage organelle, SR in cardiomyocytes performs all the functions of endoplasmic reticulum (ER) in other cell types including protein synthesis, folding and degradation. In recent years ER stress has become recognized as an important contributing factor in many cardiac pathologies, including deadly ventricular arrhythmias. This brief review will therefore focus on ER stress mechanisms in the heart and how these changes can lead to pro-arrhythmic defects in SR Ca²⁺ handling machinery.

Dimethylarginine dimethylaminohydrolase 1 protects PM2.5 exposure-induced lung injury in mice by repressing inflammation and oxidative stress

BACKGROUND

Airborne fine particulate matter with aerodynamic diameter ≤ 2.5 μm (PM_2.5) pollution is associated with the prevalence of respiratory diseases, including asthma, bronchitis and chronic obstructive pulmonary disease. In patients with those diseases, circulating asymmetric dimethylarginine (ADMA) levels are increased, which contributes to airway nitric oxide deficiency, oxidative stress and inflammation. Overexpression of dimethylarginine dimethylaminohydrolase 1 (DDAH1), an enzyme degrading ADMA, exerts protective effects in animal models. However, the impact of DDAH1/ADMA on PM_2.5-induced lung injury has not been investigated.

METHODS

Ddah1^-/- and DDAH1-transgenic mice, as well as their respective wild-type (WT) littermates, were exposed to either filtered air or airborne PM_2.5 (mean daily concentration ~ 50 µg/m³) for 6 months through a whole-body exposure system. Mice were also acutely exposed to 10 mg/kg PM_2.5 and/or exogenous ADMA (2 mg/kg) via intratracheal instillation every other day for 2 weeks. Inflammatory response, oxidative stress and related gene expressions in the lungs were examined. In addition, RAW264.7 cells were exposed to PM_2.5 and/or ADMA and the changes in intracellular oxidative stress and inflammatory response were determined.

RESULTS

Ddah1^-/- mice developed more severe lung injury than WT mice after long-term PM_2.5 exposure, which was associated with greater induction of pulmonary oxidative stress and inflammation. In the lungs of PM_2.5-exposed mice, Ddah1 deficiency increased protein expression of p-p65, iNOS and Bax, and decreased protein expression of Bcl-2, SOD1 and peroxiredoxin 4. Conversely, DDAH1 overexpression significantly alleviated lung injury, attenuated pulmonary oxidative stress and inflammation, and exerted opposite effects on those proteins in PM_2.5-exposed mice. In addition, exogenous ADMA administration could mimic the effect of Ddah1 deficiency on PM_2.5-induced lung injury, oxidative stress and inflammation. In PM_2.5-exposed macrophages, ADMA aggravated the inflammatory response and oxidative stress in an iNOS-dependent manner.

CONCLUSION

Our data revealed that DDAH1 has a marked protective effect on long-term PM_2.5 exposure-induced lung injury.

Peroxiredoxins as Potential Targets for Cardiovascular Disease

Increased oxidative stress (OS) is considered a common etiology in the pathogenesis of cardiovascular disease (CVD). Therefore, the precise regulation of reactive oxygen species (ROS) in cardiovascular cells is essential to maintain normal physiological functions. Numerous regulators of cellular homeostasis are reportedly influenced by ROS. Hydrogen peroxide (H₂O₂), as an endogenous ROS in aerobic cells, is a toxic substance that can induce OS. However, many studies conducted over the past two decades have provided substantial evidence that H₂O₂ acts as a diffusible intracellular signaling messenger. Antioxidant enzymes, including superoxide dismutases, catalase, glutathione peroxidases, and peroxiredoxins (Prdxs), maintain the balance of ROS levels against augmentation of ROS production during the pathogenesis of CVD. Especially, Prdxs are regulatory sensors of transduced intracellular signals. The intracellular abundance of Prdxs that specifically react with H₂O₂ act as regulatory proteins. In this review, we focus on the role of Prdxs in the regulation of ROS-induced pathological changes in the development of CVD.

Regulation of redox signaling in HIF-1-dependent tumor angiogenesis

Angiogenesis is the process of blood vessel growth. The angiogenic switch consists of new blood vessel formation that, in carcinogenesis, can lead to the transition from a harmless cluster of dormant cells to a large tumorigenic mass with metastatic potential. Hypoxia, that is, the scarcity of oxygen, is a hallmark of solid tumors to which they adapt by activating hypoxia-inducible factor-1 (HIF-1), a transcription factor triggering de novo angiogenesis. HIF-1 and the angiogenic molecules that are expressed upon its activation are modulated by redox status. Modulations of the redox environment can influence the angiogenesis signaling at different levels, thereby impinging on the angiogenic switch. This review provides a molecular overview of the redox-sensitive steps in angiogenic signaling, the main molecular players involved, and their crosstalk with the unfolded protein response. New classes of inhibitors of these modulators which might act as antiangiogenic drugs in cancer are also discussed.

NOX4 links metabolic regulation in pancreatic cancer to endoplasmic reticulum redox vulnerability and dependence on PRDX4

There is an urgent need to identify vulnerabilities in pancreatic ductal adenocarcinoma (PDAC). PDAC cells acquire metabolic changes that augment NADPH production and cytosolic redox homeostasis. Here, we show that high NADPH levels drive activity of NADPH oxidase 4 (NOX4) expressed in the endoplasmic reticulum (ER) membrane. NOX4 produces H₂O₂ metabolized by peroxiredoxin 4 (PRDX4) in the ER lumen. Using functional genomics and subsequent in vitro and in vivo validations, we find that PDAC cell lines with high NADPH levels are dependent on PRDX4 for their growth and survival. PRDX4 addiction is associated with increased reactive oxygen species, a DNA-PKcs-governed DNA damage response and radiosensitivity, which can be rescued by depletion of NOX4 or NADPH. Hence, this study has identified NOX4 as a protein that paradoxically converts the reducing power of the cytosol to an ER-specific oxidative stress vulnerability in PDAC that may be therapeutically exploited by targeting PRDX4.

Oxidation of peroxiredoxin-4 induces oligomerization and promotes interaction with proteins governing protein folding and endoplasmic reticulum stress

Peroxiredoxins (PRDXs) catalyze the reduction of hydrogen peroxide (H₂O₂). PRDX4 is the only peroxiredoxin located within the endoplasmic reticulum (ER) and is the most highly expressed H₂O₂ scavenger in the ER. PRDX4 has emerged as an important player in numerous diseases, such as fibrosis and metabolic syndromes, and its overoxidation is a potential indicator of ER redox stress. It is unclear how overoxidation of PRDX4 governs its oligomerization state and interacting partners. Herein, we addressed these questions via nonreducing Western blots, mass spectrometry, and site-directed mutagenesis. We report that the oxidation of PRDX4 in lung epithelial cells treated with tertbutyl hydroperoxide caused a shift of PRDX4 from monomer/dimer to high molecular weight (HMW) species, which contain PRDX4 modified with sulfonic acid residues (PRDX4-SO₃), as well as of a complement of ER-associated proteins, including protein disulfide isomerases important in protein folding, thioredoxin domain-containing protein 5, and heat shock protein A5, a key regulator of the ER stress response. Mutation of any of the four cysteines in PRDX4 altered the HMW species in response to tertbutyl hydroperoxide as well as the secretion of PRDX4. We also demonstrate that the expression of ER oxidoreductase 1 alpha, which generates H₂O₂ in the ER, increased PRDX4 HMW formation and secretion. These results suggest a link between SO₃ modification in the formation of HMW PRDX4 complexes in cells, whereas the association of key regulators of ER homeostasis with HMW oxidized PRDX4 point to a putative role of PRDX4 in regulating ER stress responses.

Ascorbic Acid Route to the Endoplasmic Reticulum: Function and Role in Disease

Significance: Humans cannot synthesize ascorbic acid (AscH₂) (vitamin C), so deficiencies in dietary AscH₂ cause the life-threatening disease of scurvy and many other diseases. After oral ingestion, plasma AscH₂ concentrations are strictly controlled by transporters, which are required for entry into the cell and into intracellular organelles. Recent Advances: Besides its general antioxidant function, AscH₂ is a cofactor for endoplasmic reticulum (ER)-localized collagen hydroxylases. Its important role in ER homeostasis is also highlighted by the fact that AscH₂ deficiency in auxotrophic species triggers ER stress. Critical Issues: Characterizations of the molecular basis of diseases suggest that intracellular AscH₂ deficiency is due not only to limited dietary access but also to its limited intracellular transport and net loss under conditions of intracellular hyperoxidation in the ER. This essay will offer an overview of the different transporters of vitamin C regulating its intracellular concentration, its function inside the ER, and the phenotypes of the diseases that can be triggered by increased depletion of this vitamin in the ER. Future Directions: When considering the benefits of increasing dietary AscH₂, it is important to consider pharmacokinetic differences in the bioavailability between orally and intravenously administered AscH₂: the latter bypasses intestinal absorption and is, therefore, the only route that can lead to the high plasma concentrations that may provide some health effects, and it is this route that needs to be chosen in clinical trials for those diseases associated with a deficiency of AscH₂. Antioxid. Redox Signal. 34, 845-855.

Arterial Tortuosity Syndrome: An Ascorbate Compartmentalization Disorder?

Significance: Cardiovascular disorders are the most important cause of morbidity and mortality in the Western world. Monogenic developmental disorders of the heart and vessels are highly valuable to study the physiological and pathological processes in cardiovascular system homeostasis. The arterial tortuosity syndrome (ATS) is a rare, autosomal recessive connective tissue disorder showing lengthening, tortuosity, and stenosis of the large arteries, with a propensity for aneurysm formation. In histopathology, it associates with fragmentation and disorganization of elastic fibers in several tissues, including the arterial wall. ATS is caused by pathogenic variants in SLC2A10 encoding the facilitative glucose transporter (GLUT)10. Critical Issues: Although several hypotheses have been forwarded, the molecular mechanisms linking disrupted GLUT10 activity with arterial malformations are largely unknown. Recent Advances: The vascular and systemic manifestations and natural history of ATS patients have been largely delineated. GLUT10 was identified as an intracellular transporter of dehydroascorbic acid, which contributes to collagen and elastin cross-linking in the endoplasmic reticulum, redox homeostasis in the mitochondria, and global and gene-specific methylation/hydroxymethylation affecting epigenetic regulation in the nucleus. We revise here the current knowledge on ATS and the role of GLUT10 within the compartmentalization of ascorbate in physiological and diseased states. Future Directions: Centralization of clinical, treatment, and outcome data will enable better management for ATS patients. Establishment of representative animal disease models could facilitate the study of pathomechanisms underlying ATS. This might be relevant for other forms of vascular dysplasia, such as isolated aneurysm formation, hypertensive vasculopathy, and neovascularization. Antioxid. Redox Signal. 34, 875-889.

In situ detection of protein interactions for recombinant therapeutic enzymes

Despite their therapeutic potential, many protein drugs remain inaccessible to patients since they are difficult to secrete. Each recombinant protein has unique physicochemical properties and requires different machinery for proper folding, assembly, and posttranslational modifications (PTMs). Here we aimed to identify the machinery supporting recombinant protein secretion by measuring the protein-protein interaction (PPI) networks of four different recombinant proteins (SERPINA1, SERPINC1, SERPING1, and SeAP) with various PTMs and structural motifs using the proximity-dependent biotin identification (BioID) method. We identified PPIs associated with specific features of the secreted proteins using a Bayesian statistical model and found proteins involved in protein folding, disulfide bond formation, and N-glycosylation were positively correlated with the corresponding features of the four model proteins. Among others, oxidative folding enzymes showed the strongest association with disulfide bond formation, supporting their critical roles in proper folding and maintaining the ER stability. Knockdown of disulfide-isomerase PDIA4, a measured interactor with significance for SERPINC1 but not SERPINA1, led to the decreased secretion of SERPINC1, which relies on its extensive disulfide bonds, compared to SERPINA1, which has no disulfide bonds. Proximity-dependent labeling successfully identified the transient interactions supporting synthesis of secreted recombinant proteins and refined our understanding of key molecular mechanisms of the secretory pathway during recombinant protein production.

The ER stress response mediator ERO1 triggers cancer metastasis by favoring the angiogenic switch in hypoxic conditions

Solid tumors are often characterized by a hypoxic microenvironment which contributes, through the hypoxia-inducible factor HIF-1, to the invasion-metastasis cascade. Endoplasmic reticulum (ER) stress also leads tumor cells to thrive and spread by inducing a transcriptional and translational program, the Unfolded Protein Response (UPR), aimed at restoring ER homeostasis. We studied ERO1 alpha (henceforth ERO1), a protein disulfide oxidase with the tumor-relevant characteristic of being positively regulated by both ER stress and hypoxia. Analysis of the redox secretome indicated that pro-angiogenic HIF-1 targets, were blunted in ERO1-devoid breast cancer cells under hypoxic conditions. ERO1 deficiency reduced tumor cell migration and lung metastases by impinging on tumor angiogenesis, negatively regulating the upstream ATF4/CHOP branch of the UPR and selectively impeding oxidative folding of angiogenic factors, among which VEGF-A. Thus, ERO1 deficiency acted synergistically with the otherwise feeble curative effects of anti-angiogenic therapy in aggressive breast cancer murine models and it might be exploited to treat cancers with pathological HIF-1-dependent angiogenesis. Furthermore, ERO1 levels are higher in the more aggressive basal breast tumors and correlate inversely with the disease- and metastasis-free interval of breast cancer patients. Thus, taking advantage of our in vitro data on ERO1-regulated gene products we identified a gene set associated with ERO1 expression in basal tumors and related to UPR, hypoxia, and angiogenesis, whose levels might be investigated in patients as a hallmark of tumor aggressiveness and orient those with lower levels toward an effective anti-angiogenic therapy.

Peroxiredoxin 4 protects against ovarian ageing by ameliorating D-galactose-induced oxidative damage in mice

Peroxiredoxin 4 (Prdx4), a member of the Prdx family, is a vital ER-resident antioxidant in cells. As revealed in our previous study, Prdx4 expression was detected in ovarian granulosa cells and was closely related to ovarian function. This research aimed to explore the effect and underlying molecular mechanism of the protective role of Prdx4 against D-gal-induced ovarian ageing in mice. The D-gal-induced ovarian ageing model has been extensively used to study the mechanisms of premature ovarian failure (POF). In this study, adult Prdx4^-/- and wild-type mice were intraperitoneally injected with D-gal (150 mg/kg/day) daily for 6 weeks. Ovarian function, granulosa cell apoptosis, oxidative damage and ER stress in the ovaries were evaluated in the two groups. Ovarian weight was significantly lower, the HPO axis was more strongly disrupted, and the numbers of atretic follicles and apoptotic granulosa cells were obviously higher in Prdx4^-/- mice. In addition, Prdx4^-/- mice showed increased expression of oxidative damage-related factors and the ovarian senescence-related protein P16. Moreover, the levels of the proapoptotic factors CHOP and activated caspase-12 protein, which are involved in the ER stress pathway, and the level of the apoptosis-related BAX protein were elevated in the ovaries of Prdx4^-/- mice. Thus, D-gal-induced ovarian ageing is accelerated in Prdx4^-/- mice due to granulosa cell apoptosis via oxidative damage and ER stress-related pathways, suggesting that Prdx4 is a protective agent against POF.

The pivotal role of ERp44 in patrolling protein secretion

Interactions between protein ligands and receptors are the main language of intercellular communication; hence, how cells select proteins to be secreted or presented on the plasma membrane is a central concern in cell biology. A series of checkpoints are located along the secretory pathway, which ensure the fidelity of such protein signals (quality control). Proteins that pass the checkpoints operated in the endoplasmic reticulum (ER) by the binding immunoglobulin protein (BiP; also known as HSPA5 and GRP78) and the calnexin-calreticulin systems, must still overcome additional scrutiny in the ER-Golgi intermediate compartment (ERGIC) and the Golgi. One of the main players of this process in all metazoans is the ER-resident protein 44 (ERp44); by cycling between the ER and the Golgi, ERp44 controls the localization of key enzymes designed to act in the ER but that are devoid of suitable localization motifs. ERp44 also patrols the secretion of correctly assembled disulfide-linked oligomeric proteins. Here, we discuss the mechanisms driving ERp44 substrate recognition, with important consequences on the definition of 'thiol-mediated quality control'. We also describe how pH and zinc gradients regulate the functional cycle of ERp44, coupling quality control and membrane trafficking along the early secretory compartment.

Unbiased Profiling of the Human Proinsulin Biosynthetic Interaction Network Reveals a Role for Peroxiredoxin 4 in Proinsulin Folding

The β-cell protein synthetic machinery is dedicated to the production of mature insulin, which requires the proper folding and trafficking of its precursor, proinsulin. The complete network of proteins that mediate proinsulin folding and advancement through the secretory pathway, however, remains poorly defined. Here we used affinity purification and mass spectrometry to identify, for the first time, the proinsulin biosynthetic interaction network in human islets. Stringent analysis established a central node of proinsulin interactions with endoplasmic reticulum (ER) folding factors, including chaperones and oxidoreductases, that is remarkably conserved in both sexes and across three ethnicities. The ER-localized peroxiredoxin PRDX4 was identified as a prominent proinsulin-interacting protein. In β-cells, gene silencing of PRDX4 rendered proinsulin susceptible to misfolding, particularly in response to oxidative stress, while exogenous PRDX4 improved proinsulin folding. Moreover, proinsulin misfolding induced by oxidative stress or high glucose was accompanied by sulfonylation of PRDX4, a modification known to inactivate peroxiredoxins. Notably, islets from patients with type 2 diabetes (T2D) exhibited significantly higher levels of sulfonylated PRDX4 than islets from healthy individuals. In conclusion, we have generated the first reference map of the human proinsulin interactome to identify critical factors controlling insulin biosynthesis, β-cell function, and T2D.

Targeting the Redox Landscape in Cancer Therapy

Reactive oxygen species (ROS) are produced predominantly by the mitochondrial electron transport chain and by NADPH oxidases in peroxisomes and in the endoplasmic reticulum. The antioxidative defense counters overproduction of ROS with detoxifying enzymes and molecular scavengers, for instance, superoxide dismutase and glutathione, in order to restore redox homeostasis. Mutations in the redox landscape can induce carcinogenesis, whereas increased ROS production can perpetuate cancer development. Moreover, cancer cells can increase production of antioxidants, leading to resistance against chemo- or radiotherapy. Research has been developing pharmaceuticals to target the redox landscape in cancer. For instance, inhibition of key players in the redox landscape aims to modulate ROS production in order to prevent tumor development or to sensitize cancer cells in radiotherapy. Besides the redox landscape of a single cell, alternative strategies take aim at the multi-cellular level. Extracellular vesicles, such as exosomes, are crucial for the development of the hypoxic tumor microenvironment, and hence are explored as target and as drug delivery systems in cancer therapy. This review summarizes the current pharmaceutical and experimental interventions of the cancer redox landscape.

The effect of exposure time and concentration of airborne PM2.5 on lung injury in mice: A transcriptome analysis

The association between airborne fine particulate matter (PM2.5) concentration and the risk of respiratory diseases has been well documented by epidemiological studies. However, the mechanism underlying the harmful effect of PM2.5 has not been fully understood. In this study, we exposed the C57BL/6J mice to airborne PM2.5 for 3 months (mean daily concentration ~50 or ~110 μg/m3, defined as PM2.5-3L or PM2.5-3H) or 6 months (mean daily concentration ~50 μg/m3, defined as PM2.5-6L) through a whole-body exposure system. Histological and biochemical analysis revealed that PM2.5-3H exposure caused more severe lung injury than did PM2.5-3L, and the difference was greater than that of PM2.5-6L vs PM2.5-3L exposure. With RNA-sequencing technique, we found that the lungs exposed with different concentration of PM2.5 have distinct transcriptional profiles. PM2.5-3H exposure caused more differentially expressed genes (DEGs) in lungs than did PM2.5-3L or PM2.5-6L. The DEGs induced by PM2.5-3L or PM2.5-6L exposure were mainly enriched in immune pathways, including Hematopoietic cell lineage and Cytokine-cytokine receptor interaction, while the DEGs induced by PM2.5-3H exposure were mainly enriched in cardiovascular disease pathways, including Hypertrophic cardiomyopathy and Dilated cardiomyopathy. In addition, we found that upregulation of Cd5l and reduction of Hspa1 and peroxiredoxin-4 was associated with PM2.5-induced pulmonary inflammation and oxidative stress. These results may provide new insight into the cytotoxicity mechanism of PM2.5 and help to development of new strategies to attenuate air pollution associated respiratory disease.

Targeting ER stress/ER stress response in myopathies

There is more skeletal muscle tissue in the body than any other tissue and, as it is the organ of the majority of metabolic activity, muscle defect can affect the health of the entire body. Endoplasmic reticulum (ER) stress due to defects in protein folding/degradation balance, altered calcium and lipid levels and alterations in ER-mitochondria contacts has recently been recognised as the pathogenic cause of many different myopathies. In addition, a maladaptive ER stress response triggered by ER stress and mediated by three ER stress sensors (PERK, IRE1 and ATF6) is involved in a failure to relieve muscle tissue from this stress. Targeting ER stress and the ER stress response pathway offers a broad range of opportunities for treating myopathies but, as the inhibition of the three ER stress sensors may not be safe because it could lead to unexpected effects; it therefore calls for careful analysis of the changes in downstream signal transduction in the different myopathies so these sub-pathways can be pharmacologically targeted. This review summarises the known inhibitors of the ER stress response and the successful results obtained using some of them in mouse models of muscle diseases caused by ER stress/ER stress response.

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ER stress and the ER stress response are pathogenic causes of myopathies.

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Pre-clinical models improve our understanding of the safest branch or sub-branch of the ER stress response to inhibit.

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The inhibitors of signalling downstream of the three ER stress sensors is the safest pharmacological option.

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Chemical chaperones are promising pharmacological means of treating myopathies.

Decreased Nuclear Ascorbate Accumulation Accompanied with Altered Genomic Methylation Pattern in Fibroblasts from Arterial Tortuosity Syndrome Patients

Ascorbate requiring Fe²⁺/2-oxoglutarate-dependent dioxygenases located in the nucleoplasm have been shown to participate in epigenetic regulation of gene expression via histone and DNA demethylation. Transport of dehydroascorbic acid is impaired in the endomembranes of fibroblasts from arterial tortuosity syndrome (ATS) patients, due to the mutation in the gene coding for glucose transporter GLUT10. We hypothesized that altered nuclear ascorbate concentration might be present in ATS fibroblasts, affecting dioxygenase activity and DNA demethylation. Therefore, our aim was to characterize the subcellular distribution of vitamin C, the global and site-specific changes in 5-methylcytosine and 5-hydroxymethylcytosine levels, and the effect of ascorbate supplementation in control and ATS fibroblast cultures. Diminished nuclear accumulation of ascorbate was found in ATS fibroblasts upon ascorbate or dehydroascorbic acid addition. Analyzing DNA samples of cultured fibroblasts from controls and ATS patients, a lower global 5-hydroxymethylcytosine level was found in ATS fibroblasts, which could not be significantly modified by ascorbate addition. Investigation of the (hydroxy)methylation status of specific regions in six candidate genes related to ascorbate metabolism and function showed that ascorbate addition could stimulate hydroxymethylation and active DNA demethylation at the PPAR-γ gene region in control fibroblasts only. The altered DNA hydroxymethylation patterns in patient cells both at the global level and at specific gene regions accompanied with decreased nuclear accumulation of ascorbate suggests the epigenetic role of vitamin C in the pathomechanism of ATS. The present findings represent the first example for the role of vitamin C transport in epigenetic regulation suggesting that ATS is a compartmentalization disease.

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.

Evidence for the interaction of peroxiredoxin-4 with the store-operated calcium channel activator STIM1 in liver cells

Ca²⁺ entry through store-operated Ca²⁺ channels (SOCs) in the plasma membrane (PM) of hepatocytes plays a central role in the hormonal regulation of liver metabolism. SOCs are composed of Orai1, the channel pore protein, and STIM1, the activator protein, and are regulated by hormones and reactive oxygen species (ROS). In addition to Orai1, STIM1 also interacts with several other intracellular proteins. Most previous studies of the cellular functions of Orai1 and STIM1 have employed immortalised cells in culture expressing ectopic proteins tagged with a fluorescent polypeptide such as GFP. Little is known about the intracellular distributions of endogenous Orai1 and STIM1. The aims are to determine the intracellular distribution of endogenous Orai1 and STIM1 in hepatocytes and to identify novel STIM1 binding proteins. Subcellular fractions of rat liver were prepared by homogenisation and differential centrifugation. Orai1 and STIM1 were identified and quantified by western blot. Orai1 was found in the PM (0.03%), heavy (44%) and light (27%) microsomal fractions, and STIM1 in the PM (0.09%), and heavy (85%) and light (13%) microsomal fractions. Immunoprecipitation of STIM1 followed by LC/MS or western blot identified peroxiredoxin-4 (Prx-4) as a potential STIM1 binding protein. Prx-4 was found principally in the heavy microsomal fraction. Knockdown of Prx-4 using siRNA, or inhibition of Prx-4 using conoidin A, did not affect Ca²⁺ entry through SOCs but rendered SOCs susceptible to inhibition by H₂O₂. It is concluded that, in hepatocytes, a considerable proportion of endogenous Orai1 and STIM1 is located in the rough ER. In the rough ER, STIM1 interacts with Prx-4, and this interaction may contribute to the regulation by ROS of STIM1 and SOCs.

Chemistry and biology of reactive species with special reference to the antioxidative defence status in pancreatic β-cells

BACKGROUND

Diabetes mellitus is a serious metabolic disease. Dysfunction and subsequent loss of the β-cells in the islets of Langerhans through apoptosis ultimately cause a life-threatening insulin deficiency. The underlying reason for the particular vulnerability of the β-cells is an extraordinary sensitivity to the toxicity of reactive oxygen and nitrogen species (ROS and RNS) due to its low antioxidative defense status.

SCOPE REVIEW

This review considers the different aspects of the chemistry and biology of the biologically most important reactive species and their chemico-biological interactions in the β-cell toxicity of proinflammatory cytokines in type 1 diabetes and of lipotoxicity in type 2 diabetes development.

MAJOR CONCLUSION

The weak antioxidative defense equipment in the different subcellular organelles makes the β-cells particularly vulnerable and prone to mitochondrial, peroxisomal and ER stress. Looking upon the enzyme deficiencies which are responsible for the low antioxidative defense status of the pancreatic β-cells it is the lack of enzymatic capacity for H₂O₂ inactivation at all major subcellular sites.

GENERAL SIGNIFICANCE

Diabetes is the most prevalent metabolic disorder with a steadily increasing incidence of both type 1 and type 2 diabetes worldwide. The weak protection of the pancreatic β-cells against oxidative stress is a major reason for their particular vulnerability. Thus, careful protection of the β-cells is required for prevention of the disease.

Regulation of Calcium Homeostasis by ER Redox: A Close-Up of the ER/Mitochondria Connection

Calcium signaling plays an important role in cell survival by influencing mitochondria-related processes such as energy production and apoptosis. The endoplasmic reticulum (ER) is the main storage compartment for cell calcium (Ca²⁺; ~60-500μM), and the Ca²⁺ released by the ER has a prompt effect on the homeostasis of the juxtaposed mitochondria. Recent findings have highlighted a close connection between ER redox and Ca²⁺ signaling that is mediated by Ca²⁺-handling proteins. This paper describes the redox-regulated mediators and mechanisms that orchestrate Ca²⁺ signals from the ER to mitochondria.

Endoplasmic Reticulum Oxidative Stress Triggers Tgf-Beta-Dependent Muscle Dysfunction by Accelerating Ascorbic Acid Turnover

Endoplasmic reticulum (ER) and oxidative stress are two related phenomena that have important metabolic consequences. As many skeletal muscle diseases are triggered by oxidative stress, we explored the chain of events linking a hyperoxidized ER (which causes ER and oxidative stress) with skeletal muscle dysfunction. An unbiased exon expression array showed that the combined genetic modulation of the two master ER redox proteins, selenoprotein N (SEPN1) and endoplasmic oxidoreductin 1 (ERO1), led to an SEPN1-related myopathic phenotype due to excessive signalling of transforming growth factor (TGF)-beta. The increased TGF-beta activity in the genetic mutants was caused by accelerated turnover of the ER localized (anti-oxidant) ascorbic acid that affected collagen deposition in the extracellular matrix. In a mouse mutant of SEPN1, which is dependent on exogenous ascorbic acid, a limited intake of ascorbic acid revealed a myopathic phenotype as a consequence of an altered TGF-beta signalling. Indeed, systemic antagonism of TGF-beta re-established skeletal muscle function in SEPN1 mutant mice. In conclusion, this study sheds new light on the molecular mechanism of SEPN1-related myopathies and indicates that the TGF-beta/ERO1/ascorbic acid axis offers potential for their treatment.

Adiporedoxin, an upstream regulator of ER oxidative folding and protein secretion in adipocytes

Objective

Adipocytes are robust protein secretors, most notably of adipokines, hormone-like polypeptides, which act in an endocrine and paracrine fashion to affect numerous physiological processes such as energy balance and insulin sensitivity. To understand how such proteins are assembled for secretion we describe the function of a novel endoplasmic reticulum oxidoreductase, adiporedoxin (Adrx).

Methods

Adrx knockdown and overexpressing 3T3-L1 murine adipocyte cell lines and a knockout mouse model were used to assess the influence of Adrx on secreted proteins as well as the redox state of ER resident chaperones. The metabolic phenotypes of Adrx null mice were characterized and compared to WT mice. The correlation of Adrx levels BMI, adiponectin levels, and other inflammatory markers from adipose tissue of human subjects was also studied.

Results

Adiporedoxin functions via a CXXC active site, and is upstream of protein disulfide isomerase whose direct function is disulfide bond formation, and ultimately protein secretion. Over and under expression of Adrx in vitro enhances and reduces, respectively, the secretion of the disulfide-bonded proteins including adiponectin and collagen isoforms. On a chow diet, Adrx null mice have normal body weights, and glucose tolerance, are moderately hyperinsulinemic, have reduced levels of circulating adiponectin and are virtually free of adipocyte fibrosis resulting in a complex phenotype tending towards insulin resistance. Adrx protein levels in human adipose tissue correlate positively with adiponectin levels and negatively with the inflammatory marker phospho-Jun kinase.

Conclusion

These data support the notion that Adrx plays a critical role in adipocyte biology and in the regulation of mouse and human metabolism via its modulation of adipocyte protein secretion.

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Adrx is an adipocyte specific, endoplasmic reticulum oxidoreductase upstream of disulfide bond formation.

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Adrx over and under expression in vitro results enhanced and decreased protein secretion, respectively.

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Mice lacking Adrx have lower levels of circulating adiponectin and decreased fibrosis.

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Adrx is expressed in human adipocytes and down regulated in proportion to the level of inflammation.

Altered protein expression pattern in skin fibroblasts from parkin-mutant early-onset Parkinson's disease patients

Parkinson's disease (PD) is the most common neurodegenerative movement disorder caused primarily by selective degeneration of the dopaminergic neurons in substantia nigra. In this work the proteomes extracted from primary fibroblasts of two unrelated, hereditary cases of PD patients, with different parkin mutations, were compared with the proteomes extracted from commercial adult normal human dermal fibroblasts (NHDF) and primary fibroblasts from the healthy mother of one of the two patients. The results show that the fibroblasts from the two different cases of parkin-mutant patients display analogous alterations in the expression level of proteins involved in different cellular functions, like cytoskeleton structure-dynamics, calcium homeostasis, oxidative stress response, protein and RNA processing.

Composition of the redox environment of the endoplasmic reticulum and sources of hydrogen peroxide

The endoplasmic reticulum (ER) is a metabolically active organelle, which has a central role in proteostasis by translating, modifying, folding, and occasionally degrading secretory and membrane proteins. The lumen of the ER represents a separate compartment of the eukaryotic cell, with a characteristic proteome and metabolome. Although the redox metabolome and proteome of the compartment have not been holistically explored, it is evident that proper redox conditions are necessary for the functioning of many luminal pathways. These redox conditions are defined by local oxidoreductases and the membrane transport of electron donors and acceptors. The main electron carriers of the compartment are identical with those of the other organelles: glutathione, pyridine and flavin nucleotides, ascorbate, and others. However, their composition, concentration, and redox state in the ER lumen can be different from those observed in other compartments. The terminal oxidases of oxidative protein folding generate and maintain an "oxidative environment" by oxidizing protein thiols and producing hydrogen peroxide. ER-specific mechanisms reutilize hydrogen peroxide as an electron acceptor of oxidative folding. These mechanisms, together with membrane and kinetic barriers, guarantee that redox systems in the reduced or oxidized state can be present simultaneously in the lumen. The present knowledge on the in vivo conditions of ER redox is rather limited; development of new genetically encoded targetable sensors for the measurement of the luminal state of redox systems other than thiol/disulfide will contribute to a better understanding of ER redox homeostasis.

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.

Prdx4 is a compartment-specific H2O2 sensor that regulates neurogenesis by controlling surface expression of GDE2

Neural progenitors and terminally differentiated neurons show distinct redox profiles, suggesting that coupled-redox cascades regulate the initiation and progression of neuronal differentiation. Discrete cellular compartments have different redox environments and how they contribute to differentiation is unclear. Here we show that Prdx4, an endoplasmic reticulum (ER) enzyme that metabolizes H₂O₂, acts as a tunable regulator of neurogenesis via its compartmentalized thiol-oxidative function. Prdx4 ablation causes premature motor neuron differentiation and progenitor depletion, leading to imbalances in subtype-specific motor neurons. GDE2, a six-transmembrane protein that induces differentiation by downregulating Notch signalling through surface cleavage of GPI-anchored proteins, is targeted by Prdx4 oxidative activity. Prdx4 dimers generated by H₂O₂ metabolism oxidize two cysteine residues within the GDE2 enzymatic domain, which blocks GDE2 trafficking to the plasma membrane and prevents GDE2 neurogeneic function. Thus, Prdx4 oxidative activity acts as a sensor to directly couple neuronal differentiation with redox environments in the ER.

Neuron differentiation is marked by changes in intracellular redox status. Here Yan et al. show that ER-resident peroxiredoxin 4 senses increased H₂O₂ and prevents the surface expression of differentiation-promoting GDE2 by modifying cysteine residues within GDE2.

Oxidative protein folding: from thiol-disulfide exchange reactions to the redox poise of the endoplasmic reticulum

This review examines oxidative protein folding within the mammalian endoplasmic reticulum (ER) from an enzymological perspective. In protein disulfide isomerase-first (PDI-first) pathways of oxidative protein folding, PDI is the immediate oxidant of reduced client proteins and then addresses disulfide mispairings in a second isomerization phase. In PDI-second pathways the initial oxidation is PDI-independent. Evidence for the rapid reduction of PDI by reduced glutathione is presented in the context of PDI-first pathways. Strategies and challenges are discussed for determination of the concentrations of reduced and oxidized glutathione and of the ratios of PDI(red):PDI(ox). The preponderance of evidence suggests that the mammalian ER is more reducing than first envisaged. The average redox state of major PDI-family members is largely to almost totally reduced. These observations are consistent with model studies showing that oxidative protein folding proceeds most efficiently at a reducing redox poise consistent with a stoichiometric insertion of disulfides into client proteins. After a discussion of the use of natively encoded fluorescent probes to report the glutathione redox poise of the ER, this review concludes with an elaboration of a complementary strategy to discontinuously survey the redox state of as many redox-active disulfides as can be identified by ratiometric LC-MS-MS methods. Consortia of oxidoreductases that are in redox equilibrium can then be identified and compared to the glutathione redox poise of the ER to gain a more detailed understanding of the factors that influence oxidative protein folding within the secretory compartment.

Endoplasmic reticulum proteins quality control and the unfolded protein response: the regulative mechanism of organisms against stress injuries

The endoplasmic reticulum is the cellular compartment in which secretory proteins are synthesized and folded. Perturbations of endoplasmic reticulum homeostasis lead to the accumulation of unfolded proteins. The activation of the unfolded protein response during endoplasmic reticulum stress transmits information about the status of protein folding to the cytosol and nucleus. The unfolded protein response leads to the upregulation of genes encoding endoplasmic reticulum chaperones, attenuation of translation, and initiation of the endoplasmic reticulum quality control system to restore endoplasmic reticulum homeostasis. When the unfolded protein response is insufficient to rebuild the steady state in endoplasmic reticulum, the programmed cell death or apoptosis would be initiated, by triggering cell injuries, even to cell death through apoptosis signals. In this review, we briefly outline research on the chaperones and foldases conserved in eukaryotes and plants, and describe the general principles and mechanisms of the endoplasmic reticulum quality control and the unfolded protein response. We describe the current models for the molecular mechanism of the unfolded protein response in plants, and emphasize the role of inositol requiring enzyme-1-dependent network in the unfolded protein response. Finally, we give a general overview of the directions for future research on the unfolded protein response in plants and its role in the response to environmental stresses.

Peroxiredoxin 4 improves insulin biosynthesis and glucose-induced insulin secretion in insulin-secreting INS-1E cells

Oxidative folding of (pro)insulin is crucial for its assembly and biological function. This process takes place in the endoplasmic reticulum (ER) and is accomplished by protein disulfide isomerase and ER oxidoreductin 1β, generating stoichiometric amounts of hydrogen peroxide (H2O2) as byproduct. During insulin resistance in the prediabetic state, increased insulin biosynthesis can overwhelm the ER antioxidative and folding capacity, causing an imbalance in the ER redox homeostasis and oxidative stress. Peroxiredoxin 4 (Prdx4), an ER-specific antioxidative peroxidase can utilize luminal H2O2 as driving force for reoxidizing protein disulfide isomerase family members, thus efficiently contributing to disulfide bond formation. Here, we examined the functional significance of Prdx4 on β-cell function with emphasis on insulin content and secretion during stimulation with nutrient secretagogues. Overexpression of Prdx4 in glucose-responsive insulin-secreting INS-1E cells significantly metabolized luminal H2O2 and improved the glucose-induced insulin secretion, which was accompanied by the enhanced proinsulin mRNA transcription and insulin content. This β-cell beneficial effect was also observed upon stimulation with the nutrient insulin secretagogue combination of leucine plus glutamine, indicating that the effect is not restricted to glucose. However, knockdown of Prdx4 had no impact on H2O2 metabolism or β-cell function due to the fact that Prdx4 expression is negligibly low in pancreatic β-cells. Moreover, we provide evidence that the constitutively low expression of Prdx4 is highly susceptible to hyperoxidation in the presence of high glucose. Overall, these data suggest an important role of Prdx4 in maintaining insulin levels and improving the ER folding capacity also under conditions of a high insulin requirement.

Intact protein folding in the glutathione-depleted endoplasmic reticulum implicates alternative protein thiol reductants

Protein folding homeostasis in the endoplasmic reticulum (ER) requires efficient protein thiol oxidation, but also relies on a parallel reductive process to edit disulfides during the maturation or degradation of secreted proteins. To critically examine the widely held assumption that reduced ER glutathione fuels disulfide reduction, we expressed a modified form of a cytosolic glutathione-degrading enzyme, ChaC1, in the ER lumen. ChaC1(CtoS) purged the ER of glutathione eliciting the expected kinetic defect in oxidation of an ER-localized glutathione-coupled Grx1-roGFP2 optical probe, but had no effect on the disulfide editing-dependent maturation of the LDL receptor or the reduction-dependent degradation of misfolded alpha-1 antitrypsin. Furthermore, glutathione depletion had no measurable effect on induction of the unfolded protein response (UPR); a sensitive measure of ER protein folding homeostasis. These findings challenge the importance of reduced ER glutathione and suggest the existence of alternative electron donor(s) that maintain the reductive capacity of the ER.DOI: http://dx.doi.org/10.7554/eLife.03421.001.

The thioredoxin superfamily in oxidative protein folding

SIGNIFICANCE

The thioredoxin (Trx) superfamily proteins, including protein disulfide isomerases (PDI) and Dsb protein family, are major players in oxidative protein folding, which involves native disulfide bond formation. These proteins contain Trx folds with CXXC active sites and fulfill their physiological functions in oxidative cellular compartments such as the endoplasmic reticulum (ER) or the bacterial periplasm.

RECENT ADVANCES

The structure of the Trx superfamily protein PDI has been solved by X-ray crystallography and shown to be a flexible molecule, having a horseshoe shape with a closed reduced and an open oxidized conformation, which is important for exerting its catalytic activity. Atomic force microscopy revealed that PDI works as a placeholder to prevent early non-native disulfide bond formation and further misfolding. S-nitrosylation of the active site of PDI inhibits the PDI activity and links protein misfolding to neurodegenerative diseases like Alzheimer's and Parkinson's diseases.

CRITICAL ISSUES

Electron transfer pathways of the oxidative protein folding show conserved Trx-like thiol-disulfide chemistry. Overall, mammalian cells have a large number of disulfide-containing proteins, the folding of which involves non-native disulfide bond isomerization. The process is sensitive to oxidative stress and ER stress.

FUTURE DIRECTIONS

The correct oxidative protein folding is critical for the substrate protein stability and function, and protein misfolding is linked to, for example, neurodegenerative diseases. Further understanding on the mechanisms and specific roles of Trx superfamily proteins in oxidative protein folding may lead to drug development for the treatment of bacterial infection and various human diseases in aging and neurodegeneration.

The role of ascorbate in protein folding

Ascorbate was linked to protein folding a long time ago. At the first level of this connection, it had been shown that ascorbate functions as an essential cofactor in the hydroxylation enzymes involved in collagen synthesis. Although the hydroxylation reactions catalyzed by the members of the prolyl 4-hydroxylase family are considered to be ascorbate dependent, the hydroxylation of proline alone does not need ascorbate. Prolyl 4-hydroxylases participate in two catalytic reactions: one in which proline residues are hydroxylated, while 2-oxoglutarate is decarboxylated and molecular oxygen is consumed. This reaction is ascorbate independent. However, in another reaction, prolyl 4-hydroxylases catalyze the decarboxylation of 2-oxoglutarate uncoupled from proline hydroxylation but still needing molecular oxygen. At this time, ferrous iron is oxidized and the protein is rendered catalytically inactive until reduced by ascorbate. At the second level of the connection, the oxidation and the oxidized form of ascorbate, dehydroascorbate, is involved in the formation of disulfide bonds of secretory proteins. The significance of the dehydroascorbate reductase activity of protein disulfide isomerase was debated because protein disulfide isomerase as a dehydroascorbate reductase was found to be too slow to be the major route for the reduction of dehydroascorbate (and formation of disulfides) in the endoplasmic reticulum lumen. However, very recently, low tissue ascorbate levels and a noncanonical scurvy were observed in endoplasmic reticulum thiol oxidase- and peroxiredoxin 4-compromised mice. This novel observation implies that ascorbate may be involved in oxidative protein folding and creates a link between the disulfide bond formation (oxidative protein folding) and hydroxylation.

Destroy and exploit: catalyzed removal of hydroperoxides from the endoplasmic reticulum

Peroxidases are enzymes that reduce hydroperoxide substrates. In many cases, hydroperoxide reduction is coupled to the formation of a disulfide bond, which is transferred onto specific acceptor molecules, the so-called reducing substrates. As such, peroxidases control the spatiotemporal distribution of diffusible second messengers such as hydrogen peroxide (H₂O₂) and generate new disulfides. Members of two families of peroxidases, peroxiredoxins (Prxs) and glutathione peroxidases (GPxs), reside in different subcellular compartments or are secreted from cells. This review discusses the properties and physiological roles of PrxIV, GPx7, and GPx8 in the endoplasmic reticulum (ER) of higher eukaryotic cells where H₂O₂ and—possibly—lipid hydroperoxides are regularly produced. Different peroxide sources and reducing substrates for ER peroxidases are critically evaluated. Peroxidase-catalyzed detoxification of hydroperoxides coupled to the productive use of disulfides, for instance, in the ER-associated process of oxidative protein folding, appears to emerge as a common theme. Nonetheless, in vitro and in vivo studies have demonstrated that individual peroxidases serve specific, nonoverlapping roles in ER physiology.