Redox regulation of the tumor suppressor PTEN by glutaredoxin 5 and Ycp4

Redox Regulation of PTEN by Reactive Oxygen Species: Its Role in Physiological Processes

Phosphatase and tensin homolog (PTEN) is a tumor suppressor due to its ability to regulate cell survival, growth, and proliferation by downregulating the PI3K/AKT signaling pathway. In addition, PTEN plays an essential role in other physiological events associated with cell growth demands, such as ischemia-reperfusion, nerve injury, and immune responsiveness. Therefore, recently, PTEN inhibition has emerged as a potential therapeutic intervention in these situations. Increasing evidence demonstrates that reactive oxygen species (ROS), especially hydrogen peroxide (H2O2), are produced and required for the signaling in many important cellular processes under such physiological conditions. ROS have been shown to oxidize PTEN at the cysteine residue of its active site, consequently inhibiting its function. Herein, we provide an overview of studies that highlight the role of the oxidative inhibition of PTEN in physiological processes.

Disruption of YCP4 enhances freeze-thaw tolerance in Saccharomyces cerevisiae

OBJECTIVE

This study aimed to identify genes related to freeze-thaw tolerance and elucidate the tolerance mechanism in yeast Saccharomyces cerevisiae as an appropriate eukaryote model.

RESULTS

In this study, one tolerant strain exposed to freeze-thaw stress was isolated by screening a transposon-mediated mutant library and the disrupted gene was identified to be YCP4. In addition, this phenotype related to freeze-thaw tolerance was confirmed by deletion and overexpressing of this corresponding gene. This mutant strain showed a freeze-thaw tolerance by reducing the intracellular level of reactive oxygen species and the activation of the MSN2/4 and STRE-mediated genes such as CTT1 and HSP12.

CONCLUSIONS

Disruption of YCP4 in S. cerevisiae results in increased tolerance to freeze-thaw stress.

Glutaredoxin: Discovery, redox defense and much more

Glutaredoxin, Grx, is a small protein containing an active site cysteine pair and was discovered in 1976 by Arne Holmgren. The Grx system, comprised of Grx, glutathione, glutathione reductase, and NADPH, was first described as an electron donor for Ribonucleotide Reductase but, from the first discovery in E.coli, the Grx family has impressively grown, particularly in the last two decades. Several isoforms have been described in different organisms (from bacteria to humans) and with different functions. The unique characteristic of Grxs is their ability to catalyse glutathione-dependent redox regulation via glutathionylation, the conjugation of glutathione to a substrate, and its reverse reaction, deglutathionylation. Grxs have also recently been enrolled in iron sulphur cluster formation. These functions have been implied in various physiological and pathological conditions, from immune defense to neurodegeneration and cancer development thus making Grx a possible drug target. This review aims to give an overview on Grxs, starting by a phylogenetic analysis of vertebrate Grxs, followed by an analysis of the mechanisms of action, the specific characteristics of the different human isoforms and a discussion on aspects related to human physiology and diseases.

Role of Reductive versus Oxidative Stress in Tumor Progression and Anticancer Drug Resistance

Redox homeostasis is not only essential for the maintenance of normal physiological functions, but also plays an important role in the growth, survival, and therapy resistance of cancer cells. Altered redox balance and consequent disruption of redox signaling are implicated in the proliferation and progression of cancer cells and their resistance to chemo- and radiotherapy. The nuclear factor erythroid 2 p45-related factor (Nrf2) is the principal stress-responsive transcription factor that plays a pivotal role in maintaining cellular redox homeostasis. Aberrant Nrf2 overactivation has been observed in many cancerous and transformed cells. Uncontrolled amplification of Nrf2-mediated antioxidant signaling results in reductive stress. Some metabolic pathways altered due to reductive stress have been identified as major contributors to tumorigenesis. This review highlights the multifaceted role of reductive stress in cancer development and progression.

Differential Roles of a Family of Flavodoxin-Like Proteins That Promote Resistance to Quinone-Mediated Oxidative Stress in Candida albicans

Survival of the fungal pathogen Candida albicans within a mammalian host relies on its ability to resist oxidative stress. The four flavodoxin-like proteins (Pst1, Pst2, Pst3, and Ycp4) that reside on the inner surface of the C. albicans plasma membrane represent a recently discovered antioxidant mechanism that is essential for virulence. Flavodoxin-like proteins combat oxidative stress by promoting a two-electron reduction of quinone molecules, which prevents the formation of toxic semiquinone radicals. Previous studies indicated that Pst3 played a major role in promoting resistance to the small quinone molecules p-benzoquinone and menadione. Analysis of additional quinones confirmed this role for Pst3. To better define their function, antibodies were raised against each of the four flavodoxin-like proteins and used to quantify protein levels. Interestingly, the basal level of flavodoxin-like proteins differed, with Pst3 and Ycp4 being the most abundant. However, after induction with p-benzoquinone, Pst1 and Pst3 were the most highly induced, resulting in Pst3 becoming the most abundant. Constitutive expression of the flavodoxin-like protein genes from a TDH3 promoter resulted in similar protein levels and showed that Pst1 and Pst3 were better at protecting C. albicans against p-benzoquinone than Pst2 or Ycp4. In contrast, Pst1 and Ycp4 provided better protection against oxidative damage induced by tert-butyl hydroperoxide. Thus, both the functional properties and the relative abundance contribute to the distinct roles of the flavodoxin-like proteins in resisting oxidative stress. These results further define how C. albicans combats the host immune response and survives in an environment rich in oxidative stress.

Cigarette smoke extract amplifies NADPH oxidase-dependent ROS production to inactivate PTEN by oxidation in BEAS-2B cells

Chronic obstructive pulmonary disease (COPD) is widely recognized as a global public health problem and the third leading cause of mortality worldwide by 2020. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a dual-specificity protein and lipid phosphatase that plays an important role in COPD. However, the redox regulation of PTEN in the development of COPD was poorly studied. Our results showed that cigarette smoke extract (CSE) could oxidize PTEN in a time-dependent manner in BEAS-2B cells, whereas PTEN oxidation exposed to CSE was delayed compared to that of H₂O_2. Additionally, we found that ROS derived from DUOX1 and 2 of NADPH oxidases were mainly responsible for oxidative inactivation PTEN, also simultaneously led to Trx-1 inactivation by dimerization. Oxidative mechanism of PTEN exposed to CSE was mediated by forming a disulfide bond between Cys⁷¹and Cys¹²⁴, similar to H₂O₂. Inactivation of PTEN resulted in the increased phosphorylation of Akt. In conclusion, CSE exposure could elevate the intracellular ROS mainly from DUOX1 and 2 to oxidize PTEN and Trx-1 resulting in Akt activation, eventually cause the occurrence of COPD, suggesting that PTEN is a potential target for new therapies in COPD.

Plasma Membrane MCC/Eisosome Domains Promote Stress Resistance in Fungi

There is growing appreciation that the plasma membrane orchestrates a diverse array of functions by segregating different activities into specialized domains that vary in size, stability, and composition. Studies with the budding yeast Saccharomyces cerevisiae have identified a novel type of plasma membrane domain known as the MCC (membrane compartment of Can1)/eisosomes that correspond to stable furrows in the plasma membrane. MCC/eisosomes maintain proteins at the cell surface, such as nutrient transporters like the Can1 arginine symporter, by protecting them from endocytosis and degradation. Recent studies from several fungal species are now revealing new functional roles for MCC/eisosomes that enable cells to respond to a wide range of stressors, including changes in membrane tension, nutrition, cell wall integrity, oxidation, and copper toxicity. The different MCC/eisosome functions are often intertwined through the roles of these domains in lipid homeostasis, which is important for proper plasma membrane architecture and cell signaling. Therefore, this review will emphasize the emerging models that explain how MCC/eisosomes act as hubs to coordinate cellular responses to stress. The importance of MCC/eisosomes is underscored by their roles in virulence for fungal pathogens of plants, animals, and humans, which also highlights the potential of these domains to act as novel therapeutic targets.

Control of tumor angiogenesis and metastasis through modulation of cell redox state

Redox reactions pervade all biology. The control of cellular redox state is essential for bioenergetics and for the proper functioning of many biological functions. This review traces a timeline of findings regarding the connections between redox and cancer. There is ample evidence of the involvement of cellular redox state on the different hallmarks of cancer. Evidence of the control of tumor angiogenesis and metastasis through modulation of cell redox state is reviewed and highlighted.

PTEN Inhibition in Human Disease Therapy

The tumor suppressor PTEN is a major homeostatic regulator, by virtue of its lipid phosphatase activity against phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3], which downregulates the PI3K/AKT/mTOR prosurvival signaling, as well as by its protein phosphatase activity towards specific protein targets. PTEN catalytic activity is crucial to control cell growth under physiologic and pathologic situations, and it impacts not only in preventing tumor cell survival and proliferation, but also in restraining several cellular regeneration processes, such as those associated with nerve injury recovery, cardiac ischemia, or wound healing. In these conditions, inhibition of PTEN catalysis is being explored as a potentially beneficial therapeutic intervention. Here, an overview of human diseases and conditions in which PTEN inhibition could be beneficial is presented, together with an update on the current status of specific small molecule inhibitors of PTEN enzymatic activity, their use in experimental models, and their limitations as research or therapeutic drugs.

MCC/Eisosomes Regulate Cell Wall Synthesis and Stress Responses in Fungi

The fungal plasma membrane is critical for cell wall synthesis and other important processes including nutrient uptake, secretion, endocytosis, morphogenesis, and response to stress. To coordinate these diverse functions, the plasma membrane is organized into specialized compartments that vary in size, stability, and composition. One recently identified domain known as the Membrane Compartment of Can1 (MCC)/eisosome is distinctive in that it corresponds to a furrow-like invagination in the plasma membrane. MCC/eisosomes have been shown to be formed by the Bin/Amphiphysin/Rvs (BAR) domain proteins Lsp1 and Pil1 in a range of fungi. MCC/eisosome domains influence multiple cellular functions; but a very pronounced defect in cell wall synthesis has been observed for mutants with defects in MCC/eisosomes in some yeast species. For example, Candida albicans MCC/eisosome mutants display abnormal spatial regulation of cell wall synthesis, including large invaginations and altered chemical composition of the walls. Recent studies indicate that MCC/eisosomes affect cell wall synthesis in part by regulating the levels of the key regulatory lipid phosphatidylinositol 4,5-bisphosphate (PI_4,5P₂) in the plasma membrane. One general way MCC/eisosomes function is by acting as protected islands in the plasma membrane, since these domains are very stable. They also act as scaffolds to recruit >20 proteins. Genetic studies aimed at defining the function of the MCC/eisosome proteins have identified important roles in resistance to stress, such as resistance to oxidative stress mediated by the flavodoxin-like proteins Pst1, Pst2, Pst3 and Ycp4. Thus, MCC/eisosomes play multiple roles in plasma membrane organization that protect fungal cells from the environment.

Redox regulation of the tumor suppressor PTEN by the thioredoxin system and cumene hydroperoxide

Intracellular redox status influences the oxidation and enzyme activity of the tumor suppressor phosphatase and tensin homolog on chromosome 10 (PTEN). Cumene hydroperoxide (CuHP), an organic hydroperoxide, is a known tumor promoter. However, molecular targets and action mechanism of CuHP in tumor promotion have not been well characterized. In this study, we investigated the effect of CuHP on the redox state of PTEN in HeLa cells. In addition, the intracellular reducing system of oxidized PTEN was analyzed using a biochemical approach and the effect of CuHP on this reducing system was also analyzed. While PTEN oxidized by hydrogen peroxide is progressively converted to its reduced form, PTEN was irreversibly oxidized by exposure to CuHP in HeLa cells. A combination of protein fractionation and mass analysis showed that the reducing system of PTEN was comprised of NADPH, thioredoxin reductase (TrxR), and thioredoxin (Trx). Although CuHP-mediated PTEN oxidation was not reversible in cells, CuHP-oxidized PTEN was reactivated by the exogenous Trx system, indicating that the cellular Trx redox system for PTEN is inactivated by CuHP. We present evidence that PTEN oxidation and the concomitant inhibition of thioredoxin by CuHP results in irreversible oxidation of PTEN in HeLa cells. In addition, ablation of peroxiredoxin (Prdx) enhanced CuHP-induced PTEN oxidation in cells. These results provide a new line of evidence that PTEN might be a crucial determinant of cell fate in response to cellular oxidative stress induced by organic hydroperoxides.

Flavodoxin-Like Proteins Protect Candida albicans from Oxidative Stress and Promote Virulence

The fungal pathogen Candida albicans causes lethal systemic infections in humans. To better define how pathogens resist oxidative attack by the immune system, we examined a family of four Flavodoxin-Like Proteins (FLPs) in C. albicans. In agreement with previous studies showing that FLPs in bacteria and plants act as NAD(P)H quinone oxidoreductases, a C. albicans quadruple mutant lacking all four FLPs (pst1Δ, pst2Δ, pst3Δ, ycp4Δ) was more sensitive to benzoquinone. Interestingly, the quadruple mutant was also more sensitive to a variety of oxidants. Quinone reductase activity confers important antioxidant effects because resistance to oxidation was restored in the quadruple mutant by expressing either Escherichia coli wrbA or mammalian NQO1, two distinct types of quinone reductases. FLPs were detected at the plasma membrane in C. albicans, and the quadruple mutant was more sensitive to linolenic acid, a polyunsaturated fatty acid that can auto-oxidize and promote lipid peroxidation. These observations suggested that FLPs reduce ubiquinone (coenzyme Q), enabling it to serve as an antioxidant in the membrane. In support of this, a C. albicans coq3Δ mutant that fails to synthesize ubiquinone was also highly sensitive to oxidative stress. FLPs are critical for survival in the host, as the quadruple mutant was avirulent in a mouse model of systemic candidiasis under conditions where infection with wild type C. albicans was lethal. The quadruple mutant cells initially grew well in kidneys, the major site of C. albicans growth in mice, but then declined after the influx of neutrophils and by day 4 post-infection 33% of the mice cleared the infection. Thus, FLPs and ubiquinone are important new antioxidant mechanisms that are critical for fungal virulence. The potential of FLPs as novel targets for antifungal therapy is further underscored by their absence in mammalian cells.

The neurodegenerative effects of selenium are inhibited by FOXO and PINK1/PTEN regulation of insulin/insulin-like growth factor signaling in Caenorhabditis elegans

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Insulin/insulin-like signaling reduction alters selenium-induced neurodegeneration.

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Selenium induces nuclear translocation of DAF-16/FOXO3a.

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DAF-16 overexpression decreases GABAergic and cholinergic motor neuron degeneration.

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Loss of DAF-18/PTEN increases sensitivity to selenium-induced movement deficits.

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Glutathione requires DAF-18/PINK-1 to improve selenium-induced movement deficits.

Exposures to high levels of environmental selenium have been associated with motor neuron disease in both animals and humans and high levels of selenite have been identified in the cerebrospinal fluid of patients with amyotrophic lateral sclerosis (ALS). We have shown previously that exposures to high levels of sodium selenite in the environment of Caenorhabditis elegans adult animals can induce neurodegeneration and cell loss resulting in motor deficits and death and that this is at least partially caused by a reduction in cholinergic signaling across the neuromuscular junction. Here we provide evidence that reduction in insulin/insulin-like (IIS) signaling alters response to high dose levels of environmental selenium which in turn can regulate the IIS pathway. Most specifically we show that nuclear localization and thus activation of the DAF-16/forkhead box transcription factor occurs in response to selenium exposure although this was not observed in motor neurons of the ventral cord. Yet, tissue specific expression and generalized overexpression of DAF-16 can partially rescue the neurodegenerative and behavioral deficits observed with high dose selenium exposures in not only the cholinergic, but also the GABAergic motor neurons. In addition, two modifiers of IIS signaling, PTEN (phosphatase and tensin homolog, deleted on chromosome 10) and PINK1 (PTEN-induced putative kinase 1) are required for the cellular antioxidant reduced glutathione to mitigate the selenium-induced movement deficits. Studies have suggested that environmental exposures can lead to ALS or other neurological diseases and this model of selenium-induced neurodegeneration developed in a genetically tractable organism provides a tool for examining the combined roles of genetics and environment in the neuro-pathologic disease process.

Protein-thiol oxidation and cell death: regulatory role of glutaredoxins

SIGNIFICANCE

Glutaredoxin (Grx) is the primary enzyme responsible for catalysis of deglutathionylation of protein-mixed disulfides with glutathione (GSH) (protein-SSG). This reversible post-translational modification alters the activity and function of many proteins important in regulation of critical cellular processes. Aberrant regulation of protein glutathionylation/deglutathionylation reactions due to changes in Grx activity can disrupt both apoptotic and survival signaling pathways.

RECENT ADVANCES

Grx is known to regulate the activity of many proteins through reversible glutathionylation, such as Ras, Fas, ASK1, NFκB, and procaspase-3, all of which play important roles in control of apoptosis. Reactive oxygen species and/or reactive nitrogen species mediate oxidative modifications of critical Cys residues on these apoptotic mediators, facilitating protein-SSG formation and thereby altering protein function and apoptotic signaling.

CRITICAL ISSUES

Much of what is known about the regulation of apoptotic mediators by Grx and reversible glutathionylation has been gleaned from in vitro studies of discrete apoptotic pathways. To relate these results to events in vivo it is important to examine changes in protein-SSG status in situ under natural cellular conditions, maintaining relevant GSH:GSSG ratios and using appropriate inducers of apoptosis.

FUTURE DIRECTIONS

Apoptosis is a highly complex, tightly regulated process involving many different checks and balances. The influence of Grx activity on the interconnectivity among these various pathways remains unknown. Knowledge of the effects of Grx is essential for developing novel therapeutic approaches for treating diseases involving dysregulated apoptosis, such as cancer, heart disease, diabetes, and neurodegenerative diseases, where alterations in redox homeostasis are hallmarks for pathogenesis.