Oxidative Homeostasis Regulates the Response to Reductive Endoplasmic Reticulum Stress through Translation Control

Metabolic Responses to Redox Stress in Vascular Cells

Significance: Redox stress underlies numerous vascular disease mechanisms. Metabolic adaptability is essential for vascular cells to preserve energy and redox homeostasis. Recent Advances: Single-cell technologies and multiomic studies demonstrate significant metabolic heterogeneity among vascular cells in health and disease. Increasing evidence shows that reductive or oxidative stress can induce metabolic reprogramming of vascular cells. A recent example is intracellular L-2-hydroxyglutarate accumulation in response to hypoxic reductive stress, which attenuates the glucose flux through glycolysis and mitochondrial respiration in pulmonary vascular cells and provides protection against further reductive stress. Critical Issues: Regulation of cellular redox homeostasis is highly compartmentalized and complex. Vascular cells rely on multiple metabolic pathways, but the precise connectivity among these pathways and their regulatory mechanisms is only partially defined. There is also a critical need to understand better the cross-regulatory mechanisms between the redox system and metabolic pathways as perturbations in either systems or their cross talk can be detrimental. Future Directions: Future studies are needed to define further how multiple metabolic pathways are wired in vascular cells individually and as a network of closely intertwined processes given that a perturbation in one metabolic compartment often affects others. There also needs to be a comprehensive understanding of how different types of redox perturbations are sensed by and regulate different cellular metabolic pathways with specific attention to subcellular compartmentalization. Lastly, integration of dynamic changes occurring in multiple metabolic pathways and their cross talk with the redox system is an important goal in this multiomics era.

Sse1, Hsp110 chaperone of yeast, controls the cellular fate during endoplasmic reticulum stress

Sse1 is a cytosolic Hsp110 molecular chaperone of yeast, Saccharomyces cerevisiae. Its multifaceted roles in cellular protein homeostasis as a nucleotide exchange factor (NEF), as a protein-disaggregase and as a chaperone linked to protein synthesis (CLIPS) are well documented. In the current study, we show that SSE1 genetically interacts with IRE1 and HAC1, the endoplasmic reticulum-unfolded protein response (ER-UPR) sensors implicating its role in ER protein homeostasis. Interestingly, the absence of this chaperone imparts unusual resistance to tunicamycin-induced ER stress which depends on the intact Ire1-Hac1 mediated ER-UPR signaling. Furthermore, cells lacking SSE1 show inefficient ER-stress-responsive reorganization of translating ribosomes from polysomes to monosomes that drive uninterrupted protein translation during tunicamycin stress. In consequence, the sse1Δ strain shows prominently faster reversal from ER-UPR activated state indicating quicker restoration of homeostasis, in comparison to the wild-type (WT) cells. Importantly, Sse1 plays a critical role in controlling the ER-stress-mediated cell division arrest, which is escaped in sse1Δ strain during chronic tunicamycin stress. Accordingly, sse1Δ strain shows significantly higher cell viability in comparison to WT yeast imparting the stark fitness following short-term as well as long-term tunicamycin stress. These data, all together, suggest that cytosolic chaperone Sse1 is an important modulator of ER stress response in yeast and it controls stress-induced cell division arrest and cell death during overwhelming ER stress induced by tunicamycin.

Reductive stress and cytotoxicity in the swollen river mussel (Unio tumidus) exposed to microplastics and salinomycin

Multistress effects lead to unpredicted consequences in aquatic ecotoxicology and are extremely concerning. The goal of this study was to trace how specific effects of the antibiotic salinomycin (Sal) and microplastics (MP) on the bivalve molluscs are manifested in the combined environmentally relevant exposures. Unio tumidus specimens were treated with Sal (0.6 μg L-1), MP (1 mg L-1, 2 μm size), and both at 18 °C (Mix) and 25 °C (MixT) for 14 days. The redox stress and apoptotic enzyme responses and the balance of Zn/Cu in the digestive gland were analyzed. The shared signs of stress included a decrease in NAD+/NADH and Zn/Cu ratios and lysosomal integrity and an increase in Zn-metallothioneins and cholinesterase levels. MP caused a decrease in the glutathione (GSH) concentration and redox state, total antioxidant capacity, and Zn levels. MP and Mix induced coordinated apoptotic/autophagy activities, increasing caspase-3 and cathepsin D (CtD) total and extralysosomal levels. Sal activated caspase-3 only and increased by five times Cu level in the tissue. Due to the discriminant analysis, the cumulative effect was evident in the combined exposure at 18 °C. However, under heating, the levels of NAD+, NADH, GSH, GSH/GSSG and metallothionein-related thiols were decreased, and coordination of the cytosolic and lysosomal death stimuli was distorted, confirming that heating and pollution could exert unexpected synergistic effects on aquatic life.

Carbon-Coated Iron Oxide Nanoparticles Promote Reductive Stress-Mediated Cytotoxic Autophagy in Drug-Induced Senescent Breast Cancer Cells

The surface modification of magnetite nanoparticles (Fe3O4 NPs) is a promising approach to obtaining biocompatible and multifunctional nanoplatforms with numerous applications in biomedicine, for example, to fight cancer. However, little is known about the effects of Fe3O4 NP-associated reductive stress against cancer cells, especially against chemotherapy-induced drug-resistant senescent cancer cells. In the present study, Fe3O4 NPs in situ coated by dextran (Fe3O4@Dex) and glucosamine-based amorphous carbon coating (Fe3O4@aC) with potent reductive activity were characterized and tested against drug-induced senescent breast cancer cells (Hs 578T, BT-20, MDA-MB-468, and MDA-MB-175-VII cells). Fe3O4@aC caused a decrease in reactive oxygen species (ROS) production and an increase in the levels of antioxidant proteins FOXO3a, SOD1, and GPX4 that was accompanied by elevated levels of cell cycle inhibitors (p21, p27, and p57), proinflammatory (NFκB, IL-6, and IL-8) and autophagic (BECN1, LC3B) markers, nucleolar stress, and subsequent apoptotic cell death in etoposide-stimulated senescent breast cancer cells. Fe3O4@aC also promoted reductive stress-mediated cytotoxicity in nonsenescent breast cancer cells. We postulate that Fe3O4 NPs, in addition to their well-established hyperthermia and oxidative stress-mediated anticancer effects, can also be considered, if modified using amorphous carbon coating with reductive activity, as stimulators of reductive stress and cytotoxic effects in both senescent and nonsenescent breast cancer cells with different gene mutation statuses.

Roles of Glutathione and AP-1 in the Enhancement of Vitamin D-Induced Differentiation by Activators of the Nrf2 Signaling Pathway in Acute Myeloid Leukemia Cells

Active vitamin D derivatives (VDDs)-1α,25-dihydroxyvitamin D3/D2 and their synthetic analogs-are well-known inducers of cell maturation with the potential for differentiation therapy of acute myeloid leukemia (AML). However, their dose-limiting calcemic activity is a significant obstacle to using VDDs as an anticancer treatment. We have shown that different activators of the NF-E2-related factor-2/Antioxidant Response Element (Nrf2/ARE) signaling pathway, such as the phenolic antioxidant carnosic acid (CA) or the multiple sclerosis drug monomethyl fumarate (MMF), synergistically enhance the antileukemic effects of various VDDs applied at low concentrations in vitro and in vivo. This study aimed to investigate whether glutathione, the major cellular antioxidant and the product of the Nrf2/ARE pathway, can mediate the Nrf2-dependent differentiation-enhancing activity of CA and MMF in HL60 human AML cells. We report that glutathione depletion using L-buthionine sulfoximine attenuated the enhancing effects of both Nrf2 activators concomitant with downregulating vitamin D receptor (VDR) target genes and the activator protein-1 (AP-1) family protein c-Jun levels and phosphorylation. On the other hand, adding reduced glutathione ethyl ester to dominant negative Nrf2-expressing cells restored both the suppressed differentiation responses and the downregulated expression of VDR protein, VDR target genes, as well as c-Jun and P-c-Jun levels. Finally, using the transcription factor decoy strategy, we demonstrated that AP-1 is necessary for the enhancement by CA and MMF of 1α,25-dihydroxyvitamin D3-induced VDR and RXRα protein expression, transactivation of the vitamin D response element, and cell differentiation. Collectively, our findings suggest that glutathione mediates, at least in part, the potentiating effect of Nrf2 activators on VDDs-induced differentiation of AML cells, likely through the positive regulation of AP-1.

Nanosecond Pulsed Electric Fields (nsPEFs) Modulate Electron Transport in the Plasma Membrane and the Mitochondria

Nanosecond pulsed electric fields (nsPEFs) are a pulsed power technology known for ablating tumors, but they also modulate diverse biological mechanisms. Here we show that nsPEFs regulate trans-plasma membrane electron transport (tPMET) rates in the plasma membrane redox system (PMRS) shown as a reduction of the cell-impermeable, WST-8 tetrazolium dye. At lower charging conditions, nsPEFs enhance, and at higher charging conditions inhibit tPMET in H9c2 non-cancerous cardiac myoblasts and 4T1-luc breast cancer cells. This biphasic nsPEF-induced modulation of tPMET is typical of a hormetic stimulus that is beneficial and stress-adaptive at lower levels and damaging at higher levels. NsPEFs also attenuated mitochondrial electron transport system (ETS) activity (O2 consumption) at Complex I when coupled and uncoupled to oxidative phosphorylation. NsPEFs generated more reactive oxygen species (ROS) in mitochondria (mROS) than in the cytosol (cROS) in non-cancer H9c2 heart cells but more cROS than mROS in 4T1-luc cancer cells. Under lower charging conditions, nsPEFs support glycolysis while under higher charging conditions, nsPEFs inhibit electron transport in the PMRS and the mitochondrial ETS producing ROS, ultimately causing cell death. The impact of nsPEF on ETS presents a new paradigm for considering nsPEF modulation of redox functions, including redox homeostasis and metabolism.

Regulation of antioxidants in cancer

Scientists in this field often joke, "If you don't have a mechanism, say it's ROS." Seemingly connected to every biological process ever described, reactive oxygen species (ROS) have numerous pleiotropic roles in physiology and disease. In some contexts, ROS act as secondary messengers, controlling a variety of signaling cascades. In other scenarios, they initiate damage to macromolecules. Finally, in their worst form, ROS are deadly to cells and surrounding tissues. A set of molecules with detoxifying abilities, termed antioxidants, is the direct counterpart to ROS. Notably, antioxidants exist in the public domain, touted as a "cure-all" for diseases. Research has disproved many of these claims and, in some cases, shown the opposite. Of all the diseases, cancer stands out in its paradoxical relationship with antioxidants. Although the field has made numerous strides in understanding the roles of antioxidants in cancer, many questions remain.

Adaptive changes in tumor cells in response to reductive stress

Reductive stress is characterized by an excess of cellular electron donors and can be linked with various human pathologies including cancer. We developed melanoma cell lines resistant to reductive stress agents: rotenone (ROTR), n-acetyl-L-cysteine, (NACR), or dithiothreitol (DTTR). Resistant cells divided more rapidly and had intracellular homeostatic redox-couple ratios that were shifted towards the reduced state. Resistance caused alterations in general cell morphology, but only ROTR cells had significant changes in mitochondrial morphology with higher numbers that were more isolated, fragmented and swollen, with greater membrane depolarization and decreased numbers of networks. These changes were accompanied by lower basal oxygen consumption and maximal respiration rates. Whole cell flux analyses and mitochondrial function assays showed that NACR and DTTR preferentially utilized tricarboxylic acid (TCA) cycle intermediates, while ROTR used ketone body substrates such as D, L-β-hydroxybutyric acid. NACR and DTTR cells had constitutively decreased levels of reactive oxygen species (ROS), although this was accompanied by activation of nuclear factor erythroid 2-related factor 2 (Nrf2), with concomitant increased expression of the downstream gene products such as glutathione S-transferase P (GSTP). Further adaptations included enhanced expression of endoplasmic reticulum proteins controlling the unfolded protein response (UPR). Although expression patterns of these UPR proteins were distinct between the resistant cells, a trend implied that resistance to reductive stress is accompanied by a constitutively increased UPR phenotype in each line. Overall, tumor cells, although tolerant of oxidative stress, can adapt their energy and survival mechanisms in lethal reductive stress conditions.

Thiol reductive stress activates the hypoxia response pathway

Owing to their capability to disrupt the oxidative protein folding environment in the endoplasmic reticulum (ER), thiol antioxidants, such as dithiothreitol (DTT), are used as ER-specific stressors. We recently showed that thiol antioxidants modulate the methionine-homocysteine cycle by upregulating an S-adenosylmethionine-dependent methyltransferase, rips-1, in Caenorhabditis elegans. However, the changes in cellular physiology induced by thiol stress that modulate the methionine-homocysteine cycle remain uncharacterized. Here, using forward genetic screens in C. elegans, we discover that thiol stress enhances rips-1 expression via the hypoxia response pathway. We demonstrate that thiol stress activates the hypoxia response pathway. The activation of the hypoxia response pathway by thiol stress is conserved in human cells. The hypoxia response pathway enhances thiol toxicity via rips-1 expression and confers protection against thiol toxicity via rips-1-independent mechanisms. Finally, we show that DTT might activate the hypoxia response pathway by producing hydrogen sulfide. Our studies reveal an intriguing interaction between thiol-mediated reductive stress and the hypoxia response pathway and challenge the current model that thiol antioxidant DTT disrupts only the ER milieu in the cell.

ER Ca2+ overload activates the IRE1α signaling and promotes cell survival

BACKGROUND

Maintaining homeostasis of Ca²⁺ stores in the endoplasmic reticulum (ER) is crucial for proper Ca²⁺ signaling and key cellular functions. Although Ca²⁺ depletion has been known to cause ER stress which in turn activates the unfolded protein response (UPR), how UPR sensors/transducers respond to excess Ca²⁺ when ER stores are overloaded remain largely unclear.

RESULTS

Here, we report for the first time that overloading of ER Ca²⁺ can directly sensitize the IRE1α-XBP1 axis. The overloaded ER Ca²⁺ in TMCO1-deficient cells can cause BiP dissociation from IRE1α, promote the dimerization and stability of the IRE1α protein, and boost IRE1α activation. Intriguingly, attenuation of the over-activated IRE1α-XBP1s signaling by a IRE1α inhibitor can cause a significant cell death in TMCO1-deficient cells.

CONCLUSIONS

Our data establish a causal link between excess Ca²⁺ in ER stores and the selective activation of IRE1α-XBP1 axis, underscoring an unexpected role of overload of ER Ca²⁺ in IRE1α activation and in preventing cell death.

Reduced Nucleotides, Thiols and O2 in Cellular Redox Balance: A Biochemist's View

In the present review, which is aimed at researchers, teachers and students in life sciences, we try to show how the physicochemical properties of the elements and molecules define the concept of redox balance. Living organism are open systems traversed by fluxes of energy and matter. During catabolic oxidative metabolism, matter-mostly hydrogenated organic molecules-is oxidized and ultimately released as CO₂. Electrons are passed over to coupling molecules, such as NAD+ and FAD, whose reduced forms serve as electrons donors in anabolic reactions. Early photosynthetic activity led to the accumulation of O₂ and the transformation of the reduction to an oxidizing atmosphere, favoring the development of oxidative metabolism in living organisms. We focus on the specific properties of O₂ that provide the chemical energy for the combustion reactions occurring in living cells. We explain the concepts of redox potential and redox balance in complex systems such as living cells, we present the main redox couples involved in cellular redox balance and we discuss the chemical properties underlying their cellular roles and, in particular, their antioxidant properties in the defense against reactive oxygen species (ROS). Finally, we try to provide an integrative view emphasizing the interplay between metabolism, oxidative stress and metabolic compartmentation in mammalian cells.

Distinct Roles of Nrf1 and Nrf2 in Monitoring the Reductive Stress Response to Dithiothreitol (DTT)

Transcription factor Nrf2 (nuclear factor, erythroid 2-like 2, encoded by Nfe2l2) has been accepted as a key player in redox regulatory responses to oxidative or reductive stresses. However, relatively little is known about the potential role of Nrf1 (nuclear factor, erythroid 2-like 1, encoded by Nfe2l1) in the redox responses, particularly to reductive stress, although this ‘fossil-like’ factor is indispensable for cell homeostasis and organ integrity during the life process. Herein, we examine distinct roles of Nrf1 and Nrf2 in monitoring the defense response to 1,4–dithiothreitol (DTT, serving as a reductive stressor), concomitantly with unfolded protein response being induced by this chemical (also defined as an endoplasmic reticulum stressor). The results revealed that intracellular reactive oxygen species (ROS) were modestly increased in DTT-treated wild-type (WT) and Nrf1α^−/− cell lines, but almost unaltered in Nrf2^−/−ΔTA or caNrf2^ΔN cell lines (with a genetic loss of transactivation or N-terminal Keap1-binding domains, respectively). This chemical treatment also enabled the rate of oxidized to reduced glutathione (i.e., GSSG to GSH) to be amplified in WT and Nrf2^−/−ΔTA cells, but diminished in Nrf1α^−/− cells, along with no changes in caNrf2^ΔN cells. Consequently, Nrf1α^−/−, but not Nrf2^−/−ΔTA or caNrf2^ΔN, cell viability was reinforced by DTT against its cytotoxicity, as accompanied by decreased apoptosis. Further experiments unraveled that Nrf1 and Nrf2 differentially, and also synergistically, regulated DTT-inducible expression of critical genes for defending against redox stress and endoplasmic reticulum stress. In addition, we also identified that Cys342 and Cys640 of Nrf1 (as redox-sensing sites within its N-glycodomain and DNA-binding domain, respectively) are required for its protein stability and transcription activity.

NADPH Oxidases Connecting Fatty Liver Disease, Insulin Resistance and Type 2 Diabetes: Current Knowledge and Therapeutic Outlook

Nonalcoholic fatty liver disease (NAFLD), characterized by ectopic fat accumulation in hepatocytes, is closely linked to insulin resistance and is the most frequent complication of type 2 diabetes mellitus (T2DM). One of the features connecting NAFLD, insulin resistance and T2DM is cellular oxidative stress. Oxidative stress refers to a redox imbalance due to an inequity between the capacity of production and the elimination of reactive oxygen species (ROS). One of the major cellular ROS sources is NADPH oxidase enzymes (NOX-es). In physiological conditions, NOX-es produce ROS purposefully in a timely and spatially regulated manner and are crucial regulators of various cellular events linked to metabolism, receptor signal transmission, proliferation and apoptosis. In contrast, dysregulated NOX-derived ROS production is related to the onset of diverse pathologies. This review provides a synopsis of current knowledge concerning NOX enzymes as connective elements between NAFLD, insulin resistance and T2DM and weighs their potential relevance as pharmacological targets to alleviate fatty liver disease.

Molecular relation between biological stress and carcinogenesis

This paper aims to overview different types of stress, including DNA replication stress, oxidative stress, and psychological stress. Understanding the processes that constitute a cellular response to varied types of stress lets us find differences in how normal cells and cancer cells react to the appearance of a particular kind of stressor. The revealed dissimilarities are the key for targeting new molecules and signaling pathways in anticancer treatment. For this reason, molecular mechanisms that underlay DNA replication stress, oxidative stress, and psychological stress have been studied and briefly presented to indicate biochemical points that make stressors contribute to cancer development. What is more, the viewpoint in which cancer constitutes the outcome and the cause of stress has been taken into consideration. In a described way, this paper draws attention to the problem of cancer-related post-traumatic stress disorder and proposes a novel, multidimensional oncological approach, connecting anticancer treatment with psychiatric support.

Endoplasmic reticulum-unfolded protein response pathway modulates the cellular response to mitochondrial proteotoxic stress

Mitochondria and endoplasmic reticulum (ER) remain closely tethered by contact sites to maintain unhindered biosynthetic, metabolic, and signalling functions. Apart from its constituent proteins, contact sites localize ER-unfolded protein response (UPR) sensors like Ire1 and PERK, indicating the importance of ER-mitochondria communication during stress. In the mitochondrial sub-compartment-specific proteotoxic model of yeast, Saccharomyces cerevisiae, we show that an intact ER-UPR pathway is important in stress tolerance of mitochondrial intermembrane space (IMS) proteotoxic stress, while disrupting the pathway is beneficial during matrix stress. Deletion of IRE1 and HAC1 leads to accumulation of misfolding-prone proteins in mitochondrial IMS indicating the importance of intact ER-UPR pathway in enduring mitochondrial IMS proteotoxic stresses. Although localized proteotoxic stress within mitochondrial IMS does not induce ER-UPR, its artificial activation helps cells to better withstand the IMS proteotoxicity. Furthermore, overexpression of individual components of ER-mitochondria contact sites is found to be beneficial for general mitochondrial proteotoxic stress, in an Ire1-Hac1-independent manner.

Reductive stress triggers ANAC017-mediated retrograde signaling to safeguard the endoplasmic reticulum by boosting mitochondrial respiratory capacity

Redox processes are at the heart of universal life processes, such as metabolism, signaling, or folding of secreted proteins. Redox landscapes differ between cell compartments and are strictly controlled to tolerate changing conditions and to avoid cell dysfunction. While a sophisticated antioxidant network counteracts oxidative stress, our understanding of reductive stress responses remains fragmentary. Here, we observed root growth impairment in Arabidopsis thaliana mutants of mitochondrial alternative oxidase 1a (aox1a) in response to the model thiol reductant dithiothreitol (DTT). Mutants of mitochondrial uncoupling protein 1 (ucp1) displayed a similar phenotype indicating that impaired respiratory flexibility led to hypersensitivity. Endoplasmic reticulum (ER) stress was enhanced in the mitochondrial mutants and limiting ER oxidoreductin capacity in the aox1a background led to synergistic root growth impairment by DTT, indicating that mitochondrial respiration alleviates reductive ER stress. The observations that DTT triggered nicotinamide adenine dinucleotide (NAD) reduction in vivo and that the presence of thiols led to electron transport chain activity in isolated mitochondria offer a biochemical framework of mitochondrion-mediated alleviation of thiol-mediated reductive stress. Ablation of transcription factor Arabidopsis NAC domain-containing protein17 (ANAC017) impaired the induction of AOX1a expression by DTT and led to DTT hypersensitivity, revealing that reductive stress tolerance is achieved by adjusting mitochondrial respiratory capacity via retrograde signaling. Our data reveal an unexpected role for mitochondrial respiratory flexibility and retrograde signaling in reductive stress tolerance involving inter-organelle redox crosstalk.

Hydroxy Selenomethionine Alleviates Hepatic Lipid Metabolism Disorder of Pigs Induced by Dietary Oxidative Stress via Relieving the Endoplasmic Reticulum Stress

This study used 40 castrated male pigs to determine the protective effects of a new selenium molecule (hydroxy selenomethionine, OH-SeMet) on dietary oxidative stress (DOS) induced hepatic lipid metabolism disorder, and corresponding response of selenotranscriptome. The pigs were randomly grouped into 5 dietary treatments and fed a basal diet formulated with either normal corn and oils or oxidized diet in which the normal corn and oils were replaced by aged corn and oxidized oils, and supplemented with OH-SeMet at 0.0, 0.3, 0.6 and 0.9 mg Se/kg for a period of 16 weeks (n = 8). The results showed that DOS induced liver damage, increased serum alanine aminotransferase (ALT) and alkaline phosphatase (ALP) levels, decreased serum triacylglycerol (TG) level, suppressed antioxidant capacity in the liver, and changed lipid metabolism enzyme activity, thus causing lipid metabolism disorder in the liver. The DOS-induced lipid metabolism disorder was accompanied with endoplasmic reticulum (ER) stress, changes in lipid metabolism-related genes and selenotranscriptome in the liver. Dietary Se supplementation partially alleviated the negative impact of DOS on the lipid metabolism. These improvements were accompanied by increases in Se concentration, liver index, anti-oxidative capacity, selenotranscriptome especially 11 selenoprotein-encoding genes, and protein abundance of GPX1, GPX4 and SelS in the liver, as well as the decrease in SelF abundance. The Se supplementation also alleviated ER stress, restored liver lipid metabolism enzyme activity, increased the mRNA expression of lipid synthesis-related genes, and decreased the mRNA levels of lipidolysis-related genes. In conclusion, the dietary Se supplementation restored antioxidant capacity and mitigated ER stress induced by DOS, thus resisting hepatic lipid metabolism disorders that are associated with regulation of selenotranscriptome.

Complexity of NAC Action as an Antidiabetic Agent: Opposing Effects of Oxidative and Reductive Stress on Insulin Secretion and Insulin Signaling

Dysregulated redox balance is involved in the pathogenesis of type 2 diabetes. While the benefit of antioxidants in neutralizing oxidative stress is well characterized, the potential harm of antioxidant-induced reductive stress is unclear. The aim of this study was to investigate the dose-dependent effects of the antioxidant N-acetylcysteine (NAC) on various tissues involved in the regulation of blood glucose and the mechanisms underlying its functions. H₂O₂ was used as an oxidizing agent in order to compare the outcomes of oxidative and reductive stress on cellular function. Cellular death in pancreatic islets and diminished insulin secretion were facilitated by H₂O₂-induced oxidative stress but not by NAC. On the other hand, myotubes and adipocytes were negatively affected by NAC-induced reductive stress, as demonstrated by the impaired transmission of insulin signaling and glucose transport, as opposed to H₂O₂-stimulatory action. This was accompanied by redox balance alteration and thiol modifications of proteins. The NAC-induced deterioration of insulin signaling was also observed in healthy mice, while both insulin secretion and insulin signaling were improved in diabetic mice. This study establishes the tissue-specific effects of NAC and the importance of the delicate maintenance of redox balance, emphasizing the challenge of implementing antioxidant therapy in the clinic.

1, 8-cineole protects against ISO-induced heart failure by inhibiting oxidative stress and ER stress in vitro and in vivo

Sugemule-3 is widely adopted in clinical practice to manage cardio-cerebral diseases. 1, 8-cineole is the main ingredient of Sugemule-3, however, the underlying cellular mechanisms for its protective effect are poorly understood. 1, 8-cineole improved the survival of H9C2 cardiomyocytes during isoproterenol (ISO) injury and reduced ISO-induced apoptosis. Compared to the ISO group, 1, 8-cineole highly attenuated the generation of ISO-induced reactive oxygen species (ROS), the depolarization of △ψm, suppression of the Bax/Bcl-2 ratio, and p-caspase 3 expression, in vitro. Furthermore, 1, 8-cineole treatment in H9C2 cardiomyocytes lowered the expressions of 78-kDa glucose-regulated protein (GRP78), p-protein kinase-like ER kinase (PERK), activation of transcription factor (ATF) 4, and ER stress effector protein C/EBP and homologous protein (CHOP). These findings implied that 1, 8-cineole contribute to cardioprotection via the GRP78/CHOP pathways. Using animal models, 1, 8-cineole was revealed to markedly alleviate ISO-induced heart injury, and reduce cardiac hypertrophy, formation of the cytoplasmic vacuole, loss of myofiber, and fibrosis by inhibiting oxidative stress and ER stress. 1, 8-cineole reduces apoptosis by inhibiting signaling pathways related to oxidative stress and ER stress. These findings implicate 1, 8-cineole as a potential therapeutic target for cardiac hypertrophy-related heart diseases, including heart failure.

3,3'-Diselenodipropionic acid (DSePA) induces reductive stress in A549 cells triggering p53-independent apoptosis: A novel mechanism for diselenides

The aim of present study was to investigate the anticancer mechanisms of 3,3'-diselenodipropionic acid (DSePA), a redox-active organodiselenide in human lung cancer cells. DSePA elicited a significant concentration and time-dependent cytotoxicity in human lung cancer cell line A549 than in normal WI38 cells. The cytotoxic effect of DSePA was preceded by an acute decrease in the level of basal reactive oxygen species (ROS) and a concurrent increase in levels of reducing equivalents (like GSH/GSSG and NADH/NAD) within cells. Further, a series of experiments were performed to measure the markers of intrinsic (Bax, cytochrome c and caspase-9), extrinsic (TNFR, FADR and caspase-8) and endoplasmic reticulum (ER) stress (protein ubiquitylation, calcium flux, Bip, CHOP and caspase-12) pathways in DSePA treated cells. DSePA treatment significantly increased the levels of all the above markers. Moreover, DSePA did not alter the expression and phosphorylation (Ser15) of p53 but caused a significant damage to mitochondria. Pharmacological modulation of GSH level by BSO and NAC in DSePA treated cells led to partial abrogation and augmentation of cell kill respectively. This established the role of reductive stress as a trigger for the apoptosis induced by DSePA treatment. Finally, in vitro anticancer activity of DSePA was also corroborated by its in vivo efficacy of suppressing the growth of A549 derived xenograft tumor in SCID mice. In conclusion, above results suggest that DSePA induces apoptosis in a p53 independent manner by involving extrinsic and intrinsic pathways together with ER stress which can an interesting strategy for lung cancer therapy.

Mechanistic analysis of cadmium toxicity in Saccharomyces cerevisiae

As a potentially toxic heavy metal, Cadmium (Cd) can cause endoplasmic reticulum and oxidative stress, and thus lead to cell death. To explore the mechanisms of Cd toxicity, we investigated the UPRE-lacZ expression, the intracellular reactive oxygen species (ROS) and cell death in the 151 Cd-sensitive mutants of Saccharomyces cerevisiae in response to Cd stress. We identified 101 genes regulating UPRE-lacZ expression were involved in preventing ROS production and/or cell death from increasing to high levels, while mutants for 72 genes caused both elevated ROS production and cell death, indicating the Cd-induced ROS production and cell death are mediated by UPR. Genes involved in cell wall integrity (CWI) pathway, vacuolar protein sorting (VPS) and vacuolar transport, calcium/calcineurin pathway and PHO pathways were all required for the Cd-induced UPR, intracellular ROS and cell death. To conclude, this study highlights the importance of Cd-induced UPR, intracellular ROS levels and cell death that may play crucial roles in Cd-induced toxicity.

The Yeast eIF2 Kinase Gcn2 Facilitates H2O2-Mediated Feedback Inhibition of Both Protein Synthesis and Endoplasmic Reticulum Oxidative Folding during Recombinant Protein Production

Recombinant protein production is a known source of oxidative stress. However, knowledge of which reactive oxygen species are involved or the specific growth phase in which stress occurs remains lacking. Using modern, hypersensitive genetic H₂O₂-specific probes, microcultivation, and continuous measurements in batch culture, we observed H₂O₂ accumulation during and following the diauxic shift in engineered Saccharomyces cerevisiae, correlating with peak α-amylase production. In agreement with previous studies supporting a role of the translation initiation factor kinase Gcn2 in the response to H₂O₂, we find that Gcn2-dependent phosphorylation of eIF2α increases alongside translational attenuation in strains engineered to produce large amounts of α-amylase. Gcn2 removal significantly improved α-amylase production in two previously optimized high-producing strains but not in the wild type. Gcn2 deficiency furthermore reduced intracellular H₂O₂ levels and the Hac1 splicing ratio, while expression of antioxidants and the endoplasmic reticulum (ER) disulfide isomerase PDI1 increased. These results suggest protein synthesis and ER oxidative folding are coupled and subject to feedback inhibition by H₂O₂.

IMPORTANCE Recombinant protein production is a multibillion dollar industry. Optimizing the productivity of host cells is, therefore, of great interest. In several hosts, oxidants are produced as an unwanted side product of recombinant protein production. The buildup of oxidants can result in intracellular stress responses that could compromise the productivity of the host cell. Here, we document a novel protein synthesis inhibitory mechanism that is activated by the buildup of a specific oxidant (H₂O₂) in the cytosol of yeast cells upon the production of recombinant proteins. At the center of this inhibitory mechanism lies the protein kinase Gcn2. By removing Gcn2, we observed a doubling of recombinant protein productivity in addition to reduced H₂O₂ levels in the cytosol. In this study, we want to raise awareness of this inhibitory mechanism in eukaryotic cells to further improve protein production and contribute to the development of novel protein-based therapeutic strategies.

Polybrominated diphenyl ethers quinone-induced intracellular protein oxidative damage triggers ubiquitin-proteasome and autophagy-lysosomal system activation in LO2 cells

Polybrominated diphenyl ethers (PBDEs), a kind of flame retardants, were widely used in the furniture, textile and electronics industries. Because of their lipophilic, persistent and bio-accumulative properties, PBDEs were listed on the Stockholm Convention as typical persistent organic pollutants (POPs). We have previously reported that a highly active, quinone-type metabolite of PBDEs (PBDEQ) causes DNA damage and subsequently triggers apoptosis. However, it is remaining unclear whether PBDEQ provokes protein damage and stimulates corresponding signaling cascade. Using human normal liver (LO2) cells as an in vitro model, we demonstrated that PBDEQ causes oxidative protein damage through excess reactive oxygen species (ROS). Consistently, we found PBDEQ exposure causes the depletion of protein thiol group, the appearance of carbonyl group and the accumulation of protein aggregates. Endoplasmic reticulum (ER) stress was involved in the repair of oxidized proteins. Under the scenario of severe damage, LO2 cells degrade oxidized proteins through ubiquitin-proteasome system (UPS) and autophagy. The blockage of these protein degradation pathways aggravates PBDEQ-induced cytotoxicity in LO2 cells, whilst antioxidant N-acetyl-cysteine (NAC) rescues PBDEQ-induced oxidative protein damage conversely. In summary, our current study first demonstrated PBDEQ-induced protein oxidative damage in LO2 cells, which offer a better understanding of the cytotoxicity of PBDEs and corresponding metabolites.

Roles of High Osmolarity Glycerol and Cell Wall Integrity Pathways in Cadmium Toxicity in Saccharomyces cerevisiae

Cadmium is a carcinogen that can induce ER stress, DNA damage, oxidative stress and cell death. The yeast mitogen-activated protein kinase (MAPK) signalling pathways paly crucial roles in response to various stresses. Here, we demonstrate that the unfolded protein response (UPR) pathway, the high osmolarity glycerol (HOG) pathway and the cell wall integrity (CWI) pathway are all essential for yeast cells to defend against the cadmium-induced toxicity, including the elevated ROS and cell death levels induced by cadmium. We show that the UPR pathway is required for the cadmium-induced phosphorylation of HOG_MAPK Hog1 but not for CWI_MAPK Slt2, while Slt2 but not Hog1 is required for the activation of the UPR pathway through the transcription factors of Swi6 and Rlm1. Moreover, deletion of HAC1 and IRE1 could promote the nuclear accumulation of Hog1, and increase the cytosolic and bud neck localisation of Slt2, indicating crucial roles of Hog1 and Slt2 in regulating the cellular process in the absence of UPR pathway. Altogether, our findings highlight the significance of these two MAPK pathways of HOG and CWI and their interrelationship with the UPR pathway in responding to cadmium-induced toxicity in budding yeast.

Calcium as a reliable marker for the quantitative assessment of endoplasmic reticulum stress in live cells

Calcium (Ca²⁺) is an essential mineral of endoplasmic reticulum (ER) luminal biochemistry because of the Ca²⁺ dependence of ER-resident chaperones charged with folding de novo proteins that transit this cellular compartment. ER Ca²⁺ depletion reduces the ability of chaperones to properly fold the proteins entering the ER, thus leading to an accumulation of misfolded proteins and the onset of a state known as ER stress. However, not all conditions that cause ER stress do so in a manner dependent on ER Ca²⁺ depletion. Agents such as tunicamycin inhibit the glycosylation of de novo polypeptides, a key step in the maturation process of newly synthesized proteins. Despite this established effect of tunicamycin, our understanding of how such conditions modulate ER Ca²⁺ levels is still limited. In the present study, we report that a variety of ER stress–inducing agents that have not been known to directly alter ER Ca²⁺ homeostasis can also cause a marked reduction in ER Ca²⁺ levels. Consistent with these observations, protecting against ER stress using small chemical chaperones, such as 4-phenylbutyrate and tauroursodeoxycholic acid, also attenuated ER Ca²⁺ depletion caused by these agents. We also describe a novel high-throughput and low-cost assay for the rapid quantification of ER stress using ER Ca²⁺ levels as a surrogate marker. This report builds on our understanding of ER Ca²⁺ levels in the context of ER stress and also provides the scientific community with a new, reliable tool to study this important cellular process in vitro.

Reductive stress in cancer

Reductive stress is defined as a condition characterized by excess accumulation of reducing equivalents (e.g., NADH, NADPH, GSH), surpassing the activity of endogenous oxidoreductases. Excessive reducing equivalents can perturb cell signaling pathways, change the formation of disulfide bonding in proteins, disturb mitochondrial homeostasis or decrease metabolism. Reductive stress is influenced by cellular antioxidant load, its flux and a subverted homeostasis that paradoxically can result in excess ROS induction. Balanced reducing equivalents and antioxidant enzymes that contribute to reductive stress can be regulated by Nrf2, typically considered as an oxidative stress induced transcription factor. Cancer cells may coordinate distinct pools of redox couples under reductive stress and these may link to biological consequences from both molecular and translational standpoints. In cancer, there is recent interest in understanding how selective induction of reductive stress may influence therapeutic management and disease progression.

Gene Loss Predictably Drives Evolutionary Adaptation

Loss of gene function is common throughout evolution, even though it often leads to reduced fitness. In this study, we systematically evaluated how an organism adapts after deleting genes that are important for growth under oxidative stress. By evolving, sequencing, and phenotyping over 200 yeast lineages, we found that gene loss can enhance an organism’s capacity to evolve and adapt. Although gene loss often led to an immediate decrease in fitness, many mutants rapidly acquired suppressor mutations that restored fitness. Depending on the strain’s genotype, some ultimately even attained higher fitness levels than similarly adapted wild-type cells. Further, cells with deletions in different modules of the genetic network followed distinct and predictable mutational trajectories. Finally, losing highly connected genes increased evolvability by facilitating the emergence of a more diverse array of phenotypes after adaptation. Together, our findings show that loss of specific parts of a genetic network can facilitate adaptation by opening alternative evolutionary paths.

Health disparities: Intracellular consequences of social determinants of health

Health disparities exist dependent on socioeconomic status, living conditions, race/ethnicity, diet, and exposures to environmental pollutants. Herein, the various exposures contributing to a person's exposome are collectively considered social determinants of health (SDOH), and the SDOH-exposome impacts health more than health care. This review discusses the extent of evidence of the physiologic consequences of these exposures at the intracellular level. We consider how the SDOH-exposome, which captures how individuals live, work and age, induces cell processes that modulate a conceptual "redox rheostat." Like an electrical resistor, the SDOH-exposome, along with genetic predisposition and age, regulate reductive and oxidative (redox) stress circuits and thereby stimulate inflammation. Regardless of the source of the SDOH-exposome that induces chronic inflammation and immunosenescence, the outcome influences cardiometabolic diseases, cancers, infections, sepsis, neurodegeneration and autoimmune diseases. The endogenous redox rheostat is connected with regulatory molecules such as NAD⁺/NADH and SIRT1 that drive redox pathways. In addition to these intracellular and mitochondrial processes, we discuss how the SDOH-exposome can influence the balance between metabolism and regulation of immune responsiveness involving the two main molecular drivers of inflammation, the NLRP3 inflammasome and NF-κB induction. Mitochondrial and inflammasome activities play key roles in mediating defenses against pathogens and controlling inflammation before diverse cell death pathways are induced. Specifically, pyroptosis, cell death by inflammation, is intimately associated with common disease outcomes that are influenced by the SDOH-exposome. Redox influences on immunometabolism including protein cysteines and ion fluxes are discussed regarding health outcomes. In summary, this review presents a translational research perspective, with evidence from in vitro and in vivo models as well as clinical and epidemiological studies, to outline the intracellular consequences of the SDOH-exposome that drive health disparities in patients and populations. The relevance of this conceptual and theoretical model considering the SARS-CoV-2 pandemic are highlighted. Finally, the case of asthma is presented as a chronic condition that is modified by adverse SDOH exposures and is manifested through the dysregulation of immune cell redox regulatory processes we highlight in this review.

Endoplasmic Reticulum Homeostasis and Stress Responses in Caenorhabditis elegans

The unfolded protein response (UPR) is an evolutionarily conserved adaptive regulatory pathway that alleviates protein-folding defects in the endoplasmic reticulum (ER). Physiological demands, environmental perturbations and pathological conditions can cause accumulation of unfolded proteins in the ER and the stress signal is transmitted to the nucleus to turn on a series of genes to respond the challenge. In metazoan, the UPR pathways consisted of IRE1/XBP1, PEK-1 and ATF6, which function in parallel and downstream transcriptional activation triggers the proteostasis networks consisting of molecular chaperones, protein degradation machinery and other stress response pathways ((Labbadia J, Morimoto RI, F1000Prime Rep 6:7, 2014); (Shen X, Ellis RE, Lee K, Annu Rev Biochem 28:893-903, 2014)). The integrated responses act on to resolve the ER stress by increasing protein folding capacity, attenuating ER-loading translation, activating ER-associated proteasomal degradation (ERAD), and regulating IRE1-dependent decay of mRNA (RIDD). Therefore, the effective UPR to internal and external causes is linked to the multiple pathophysiological conditions such as aging, immunity, and neurodegenerative diseases. Recent development in the research of the UPR includes cell-nonautonomous features of the UPR, interplay between the UPR and other stress response pathways, unconventional UPR inducers, and noncanonical UPR independent of the three major branches, originated from multiple cellular and molecular machineries in addition to ER. Caenorhabditis elegans model system has critically contributed to these unprecedented aspects of the ER UPR and broadens the possible therapeutic targets to treat the ER-stress associated human disorders and time-dependent physiological deterioration of aging.

Understanding the Role of Exercise in Nonalcoholic Fatty Liver Disease: ERS-Linked Molecular Pathways

Nonalcoholic fatty liver disease (NAFLD) is globally prevalent and characterized by abnormal lipid accumulation in the liver, frequently accompanied by insulin resistance (IR), enhanced hepatic inflammation, and apoptosis. Recent studies showed that endoplasmic reticulum stress (ERS) at the subcellular level underlies these featured pathologies in the development of NAFLD. As an effective treatment, exercise significantly reduces hepatic lipid accumulation and thus alleviates NAFLD. Confusingly, these benefits of exercise are associated with increased or decreased ERS in the liver. Further, the interaction between diet, medication, exercise types, and intensity in ERS regulation is more confusing, though most studies have confirmed the benefits of exercise. In this review, we focus on understanding the role of exercise-modulated ERS in NAFLD and ERS-linked molecular pathways. Moderate ERS is an essential signaling for hepatic lipid homeostasis. Higher ERS may lead to increased inflammation and apoptosis in the liver, while lower ERS may lead to the accumulation of misfolded proteins. Therefore, exercise acts like an igniter or extinguisher to keep ERS at an appropriate level by turning it up or down, which depends on diet, medications, exercise intensity, etc. Exercise not only enhances hepatic tolerance to ERS but also prevents the malignant development of steatosis due to excessive ERS.

Metabolic Responses to Reductive Stress

Significance: Reducing equivalents (NAD(P)H and glutathione [GSH]) are essential for maintaining cellular redox homeostasis and for modulating cellular metabolism. Reductive stress induced by excessive levels of reduced NAD⁺ (NADH), reduced NADP⁺ (NADPH), and GSH is as harmful as oxidative stress and is implicated in many pathological processes. Recent Advances: Reductive stress broadens our view of the importance of cellular redox homeostasis and the influences of an imbalanced redox niche on biological functions, including cell metabolism. Critical Issues: The distribution of cellular NAD(H), NADP(H), and GSH/GSH disulfide is highly compartmentalized. Understanding how cells coordinate different pools of redox couples under unstressed and stressed conditions is critical for a comprehensive view of redox homeostasis and stress. It is also critical to explore the underlying mechanisms of reductive stress and its biological consequences, including effects on energy metabolism. Future Directions: Future studies are needed to investigate how reductive stress affects cell metabolism and how cells adapt their metabolism to reductive stress. Whether or not NADH shuttles and mitochondrial nicotinamide nucleotide transhydrogenase enzyme can regulate hypoxia-induced reductive stress is also a worthy pursuit. Developing strategies (e.g., antireductant approaches) to counteract reductive stress and its related adverse biological consequences also requires extensive future efforts.

Reductive Stress-Induced Mitochondrial Dysfunction and Cardiomyopathy

The goal of this review was to summarize reported studies focusing on cellular reductive stress-induced mitochondrial dysfunction, cardiomyopathy, dithiothreitol- (DTT-) induced reductive stress, and reductive stress-related free radical reactions published in the past five years. Reductive stress is considered to be a double-edged sword in terms of antioxidation and disease induction. As many underlying mechanisms are still unclear, further investigations are obviously warranted. Nonetheless, reductive stress is thought to be caused by elevated levels of cellular reducing power such as NADH, glutathione, and NADPH; and this area of research has attracted increasing attention lately. Albeit, we think there is a need to conduct further studies in identifying more indicators of the risk assessment and prevention of developing heart damage as well as exploring more targets for cardiomyopathy treatment. Hence, it is expected that further investigation of underlying mechanisms of reductive stress-induced mitochondrial dysfunction will provide novel insights into therapeutic approaches for ameliorating reductive stress-induced cardiomyopathy.

Acute sources of mitochondrial NAD+ during respiratory chain dysfunction

It is a textbook definition that in the absence of oxygen or inhibition of the mitochondrial respiratory chain by pharmacologic or genetic means, hyper-reduction of the matrix pyridine nucleotide pool ensues due to impairment of complex I oxidizing NADH, leading to reductive stress. However, even under these conditions, the ketoglutarate dehydrogenase complex (KGDHC) is known to provide succinyl-CoA to succinyl-CoA ligase, thus supporting mitochondrial substrate-level phosphorylation (mSLP). Mindful that KGDHC is dependent on provision of NAD⁺, hereby sources of acute NADH oxidation are reviewed, namely i) mitochondrial diaphorases, ii) reversal of mitochondrial malate dehydrogenase, iii) reversal of the mitochondrial isocitrate dehydrogenase as it occurs under acidic conditions, iv) residual complex I activity and v) reverse operation of the malate-aspartate shuttle. The concept of NAD⁺ import through the inner mitochondrial membrane as well as artificial means of manipulating matrix NAD⁺/NADH are also discussed. Understanding the above mechanisms providing NAD⁺ to KGDHC thus supporting mSLP may assist in dampening mitochondrial dysfunction underlying neurological disorders encompassing impairment of the electron transport chain.

The LAMMER kinase is involved in morphogenesis and response to cell wall- and DNA-damaging stresses in Candida albicans

Dual specificity LAMMER kinase has been reported to be conserved across species ranging from yeasts to animals and has multiple functions. Candida albicans undergoes dimorphic switching between yeast cells and hyphal growth forms as its key virulence factors. Deletion of KNS1, which encodes for LAMMER kinase in C. albicans, led to pseudohyphal growth on YPD media and defects in filamentous growth both on spider and YPD solid media containing 10% serum. These cells exhibited expanded central wrinkled regions and specifically reduced peripheral filaments. Among the several stresses tested, the kns1Δ strains showed sensitivity to cell-wall and DNA-replicative stress. Under fluorescent microscopy, an increase in chitin decomposition was observed near the bud necks and septa in kns1Δ cells. When the expression levels of genes for cell wall integrity (CWI) and the DNA repair mechanism were tested, the kns1 double-deletion cells showed abnormal patterns compared to wild-type cells; The transcript levels of genes for glycosylphosphatidylinositol (GPI)-anchored proteins were increased upon calcofluor white (CFW) treatment. Under DNA replicative stress, the expression of MluI-cell cycle box binding factor (MBF)-targeted genes, which are expressed during the G1/S transition in the cell cycle, was not increased in the kns1 double-deletion cells. This strain showed increased adhesion to the surface of an agar plate and zebrafish embryo. These results demonstrate that Kns1 is involved in dimorphic transition, cell wall integrity, response to DNA replicative stress, and adherence to the host cell surface in C. albicans.

Functional Diversification of ER Stress Responses in Arabidopsis

The endoplasmic reticulum (ER) is responsible for the synthesis of one-third of the cellular proteome and is constantly challenged by physiological and environmental situations that can perturb its homeostasis and lead to the accumulation of misfolded secretory proteins, a condition referred to as ER stress. In response, the ER evokes a set of intracellular signaling processes, collectively known as the unfolded protein response (UPR), which are designed to restore biosynthetic capacity of the ER. As single-cell organisms evolved into multicellular life, the UPR complexity has increased to suit their growth and development. In this review, we discuss recent advances in the understanding of the UPR, emphasizing conserved UPR elements between plants and metazoans and highlighting unique plant-specific features.

MLKL mediates apoptosis via a mutual regulation with PERK/eIF2α pathway in response to reactive oxygen species generation

The pseudokinase mixed lineage kinase domain-like protein (MLKL) is a core effector of necroptosis, and its function in necroptosis is widely studied. However, the function of MLKL in apoptosis remains unclear. In the present study, the role of MLKL in chelerythrine (CHE)-promoted apoptosis was studied. A special band of MLKL (i.e., *MLKL) was observed after treatment with CHE. MLKL and *MLKL were accumulated in the nucleus upon treatment with CHE and MLKL silencing reversed the CHE-induced apoptosis. Blockade of CHE-triggered reactive oxygen species (ROS) generation or inhibition of CHE-activated protein kinase-like endoplasmic reticulum kinase (PERK)-eukaryotic initiation factor 2 α subunit (eIF2α) pathway reversed the apoptosis. A decreased ROS level inhibited CHE-mediated nuclear translocation of MLKL and *MLKL and the activation of eIF2α, whereas MLKL or eIF2α silencing did not affect the CHE-triggered ROS generation. Furthermore, MLKL silencing prevented the CHE-activated eIF2α signal, and eIF2α silencing blocked the CHE-induced nuclear translocation of MLKL and *MLKL. Our studies suggested that CHE possibly induces apoptosis through the nuclear translocation of MLKL and *MLKL, which is promoted by a mutual regulation between MLKL and PERK-eIF2α pathway in response to ROS formation. The present study clarified the new function of MLKL in apoptosis.

Ncl1-mediated metabolic rewiring critical during metabolic stress

Accumulation of cysteine induces translational defects and metabolic rewiring that are abrogated by leucine in a transfer RNA (tRNA) methyltransferase NCL1-dependent manner in yeast.

Nutritional limitation has been vastly studied; however, there is limited knowledge of how cells maintain homeostasis in excess nutrients. In this study, using yeast as a model system, we show that some amino acids are toxic at higher concentrations. With cysteine as a physiologically relevant example, we delineated the pathways/processes that are altered and those that are involved in survival in the presence of elevated levels of this amino acid. Using proteomics and metabolomics approach, we found that cysteine up-regulates proteins involved in amino acid metabolism, alters amino acid levels, and inhibits protein translation—events that are rescued by leucine supplementation. Through a comprehensive genetic screen, we show that leucine-mediated effect depends on a transfer RNA methyltransferase (NCL1), absence of which decouples transcription and translation in the cell, inhibits the conversion of leucine to ketoisocaproate, and leads to tricarboxylic acid cycle block. We therefore propose a role of NCL1 in regulating metabolic homeostasis through translational control.

Dietary restriction improves proteostasis and increases life span through endoplasmic reticulum hormesis

The endoplasmic reticulum (ER) deteriorates with age and fails to mount an effective stress response against misfolded proteins (UPR^ER), leading to protein folding disorders. However, preconditioning the ER using a mild ER stress (ER hormesis) can protect against future insults. We show that dietary restriction, an intervention that protects against protein misfolding disorders and increases life span across species, uses ER hormesis as a mechanism of action. Simply mimicking the ER hormesis in Caenorhabditis elegans by transient treatment with pharmacological reagents leads to delayed age-onset failure of UPR^ER, better capacity to process misfolded proteins, and increased life span. We also show that this process may be conserved in a mammalian cellular model of neurodegenerative disease.

Unfolded protein response (UPR) of the endoplasmic reticulum (UPR^ER) helps maintain proteostasis in the cell. The ability to mount an effective UPR^ER to external stress (iUPR^ER) decreases with age and is linked to the pathophysiology of multiple age-related disorders. Here, we show that a transient pharmacological ER stress, imposed early in development on Caenorhabditis elegans, enhances proteostasis, prevents iUPR^ER decline with age, and increases adult life span. Importantly, dietary restriction (DR), that has a conserved positive effect on life span, employs this mechanism of ER hormesis for longevity assurance. We found that only the IRE-1–XBP-1 branch of UPR^ER is required for the longevity effects, resulting in increased ER-associated degradation (ERAD) gene expression and degradation of ER resident proteins during DR. Further, both ER hormesis and DR protect against polyglutamine aggregation in an IRE-1–dependent manner. We show that the DR-specific FOXA transcription factor PHA-4 transcriptionally regulates the genes required for ER homeostasis and is required for ER preconditioning-induced life span extension. Finally, we show that ER hormesis improves proteostasis and viability in a mammalian cellular model of neurodegenerative disease. Together, our study identifies a mechanism by which DR offers its benefits and opens the possibility of using ER-targeted pharmacological interventions to mimic the prolongevity effects of DR.

Reductive stress imaging in the endoplasmic reticulum by using living cells and zebrafish

Excessive accumulation of reducing agents in the ER leads to a constitutively high UPR. And the co-function of GSH, Cys and HOCl in biological processes is not well understood. To address this, a TP probe, NPCC, was developed for monitoring reductive stress in the ER. It can also distinguish cancer cells from normal cells.

Age-dependent changes and biomarkers of aging in Caenorhabditis elegans

Caenorhabditis elegans is an exceptionally valuable model for aging research because of many advantages, including its genetic tractability, short lifespan, and clear age‐dependent physiological changes. Aged C. elegans display a decline in their anatomical and functional features, including tissue integrity, motility, learning and memory, and immunity. Caenorhabditis elegans also exhibit many age‐associated changes in the expression of microRNAs and stress‐responsive genes and in RNA and protein quality control systems. Many of these age‐associated changes provide information on the health of the animals and serve as valuable biomarkers for aging research. Here, we review the age‐dependent changes in C. elegans and their utility as aging biomarkers indicative of the physiological status of aging.

New approach to modification of poly (l-lactic acid) with nano-hydroxyapatite improving functionality of human adipose-derived stromal cells (hASCs) through increased viability and enhanced mitochondrial activity

The aim of this study was to determine the cytocompatibility of poly (l-lactide) (PLLA) scaffolds fabricated using co-rotating twin screw extrusion technique and functionalized with different concentrations of nano-hydroxyapatite (nHAp). The efforts were aimed on the designing bioactive scaffolds improving the viability and metabolic activity of human adipose-derived multipotent stromal cells (hASCs). The in vitro study was designed to determine the optimal nHAp concentration, based on analysis of hASCs morphology, adhesion rate, as well as metabolic and proliferative potential. Initially, the PLLA filled with three different concentrations of the nHAp were tested i.e. 5%, 10% and 15 wt%. The obtained results indicated that the 10 wt% nHAp in the PLLA (10% nHAp/PLLA) matrices improved the adhesion and proliferation of the hASCs, what was in good agreement with the results of tensile properties of the composites. Further, we performed profound studies regarding the cytotoxicity of 10% nHAp/PLLA. The analysis included the evaluation of the biomaterial influence on viability, apoptosis-related markers expression profile and mitochondrial function. The cytocompatibility of 10% nHAp/PLLA scaffolds toward the hASCs was confirmed. The hASCs propagated on 10% nHAp/PLLA were more viable then those propagated on the plain PLLA. The level of pro-apoptotic markers, i.e. caspase-3 and Bax in cultures on 10% nHAp/PLLA was significantly decreased. Obtained results correlated with higher mitochondrial membrane potential of hASCs in those cultures. The obtained composites may improve therapeutic potential of hASCs via directing their adhesion, enhancing proliferation and viability as well as increasing mitochondrial potential, thus may find potential application in tissue engineering.

Antioxidant treatment induces reductive stress associated with mitochondrial dysfunction in adipocytes

β-Adrenergic stimulation of adipose tissue increases mitochondrial density and activity (browning) that are associated with improved whole-body metabolism. Whereas chronically elevated levels of reactive oxygen species (ROS) in adipose tissue contribute to insulin resistance, transient ROS elevation stimulates physiological processes such as adipogenesis. Here, using a combination of biochemical and cell and molecular biology-based approaches, we studied whether ROS or antioxidant treatment affects β3-adrenergic receptor (β3-AR) stimulation-induced adipose tissue browning. We found that β3-AR stimulation increases ROS levels in cultured adipocytes, but, unexpectedly, pretreatment with different antioxidants (N-acetylcysteine, vitamin E, or GSH ethyl ester) did not prevent this ROS increase. Using fluorescent probes, we discovered that the antioxidant treatments instead enhanced β3-AR stimulation-induced mitochondrial ROS production. This pro-oxidant effect of antioxidants was, even in the absence of β3-AR stimulation, associated with decreased oxygen consumption and increased lactate production in adipocytes. We observed similar antioxidant effects in WT mice: N-acetylcysteine blunted β3-AR stimulation-induced browning of white adipose tissue and reduced mitochondrial activity in brown adipose tissue even in the absence of β3-AR stimulation. Furthermore, N-acetylcysteine increased the levels of peroxiredoxin 3 and superoxide dismutase 2 in adipose tissue, indicating increased mitochondrial oxidative stress. We interpret this negative impact of antioxidants on oxygen consumption in vitro and adipose tissue browning in vivo as essential adaptations that prevent a further increase in mitochondrial ROS production. In summary, these results suggest that chronic antioxidant supplementation can produce a paradoxical increase in oxidative stress associated with mitochondrial dysfunction in adipocytes.

NADPH oxidase activity is required for ER stress survival in plants

In all eukaryotes, the unfolded protein response (UPR) relieves endoplasmic reticulum (ER) stress, which is a potentially lethal condition caused by the accumulation of misfolded proteins in the ER. In mammalian and yeast cells, reactive oxygen species (ROS) generated during ER stress attenuate the UPR, negatively impacting cell survival. In plants, the relationship between the UPR and ROS is less clear. Although ROS develop during ER stress, the sources of ROS linked to ER stress responses and the physiological impact of ROS generation on the survival from proteotoxic stress are yet unknown. Here we show that in Arabidopsis thaliana the respiratory burst oxidase homologs, RBOHD and RBOHF, contribute to the production of ROS during ER stress. We also demonstrate that during ER stress RBOHD and RBOHF are necessary to properly mount the adaptive UPR and overcome temporary and chronic ER stress situations. These results ascribe a cytoprotective role to RBOH-generated ROS in the defense from proteotoxic stress in an essential organelle, and support a plant-specific feature of the UPR management among eukaryotes.

New insights into the cellular temporal response to proteostatic stress

Maintaining a healthy proteome involves all layers of gene expression regulation. By quantifying temporal changes of the transcriptome, translatome, proteome, and RNA-protein interactome in cervical cancer cells, we systematically characterize the molecular landscape in response to proteostatic challenges. We identify shared and specific responses to misfolded proteins and to oxidative stress, two conditions that are tightly linked. We reveal new aspects of the unfolded protein response, including many genes that escape global translation shutdown. A subset of these genes supports rerouting of energy production in the mitochondria. We also find that many genes change at multiple levels, in either the same or opposing directions, and at different time points. We highlight a variety of putative regulatory pathways, including the stress-dependent alternative splicing of aminoacyl-tRNA synthetases, and protein-RNA binding within the 3’ untranslated region of molecular chaperones. These results illustrate the potential of this information-rich resource.

Oxidative stress and cytotoxicity induced by tetrachlorobisphenol A in Saccharomyces cerevisiae cells

Tetrachlorobisphenol A (TCBPA), which is widely used as flame retardant, can be released into various environments, thereby being absorbed by wildlife or human beings through food chain's bio-magnification and causing some adverse influences on wildlife or human beings. However, limited data are currently available on TCBPA-associated cytotoxicity and related mechanisms. Here, the cytotoxicity induced by different concentrations of TCBPA (i.e., 5, 10 and 20 μM) was studied using Saccharomyces cerevisiae, a simple eukaryotic model organism. TCBPA treatment inhibited the growth and survival rate of yeast cell in a dose-dependent manner. Moreover, TCBPA promoted the increasing of intracellular oxidative stress by enhancing accumulation of intracellular reactive oxygen species (ROS). Meanwhile, lipid peroxidation degree (represented by malondialdehyde (MDA) content) and DNA damage degree (represented by 8-hydroxy deoxyguanosine (8-oxodG) content) in yeast cell also increased after TCBPA treatment. However, yeast cell mitochondrial membrane potential (Δψm) decreased after TCBPA treatment. It was noteworthy that there was no significant inhibitory effect on yeast cell growth or survival rate in 5 μM TCBPA-treated cells, but the intracellular MDA content and Δψm level changed significantly, suggesting the potential cell damage secondary to the relative low dose of TCBPA exposure. Results presented here would highlight our knowledge about TCBPA-associated cytotoxicity in organisms.

Use the Protonmotive Force: Mitochondrial Uncoupling and Reactive Oxygen Species

Mitochondrial respiration results in an electrochemical proton gradient, or protonmotive force (pmf), across the mitochondrial inner membrane. The pmf is a form of potential energy consisting of charge (∆ψ_m) and chemical (∆pH) components, that together drive ATP production. In a process called uncoupling, proton leak into the mitochondrial matrix independent of ATP production dissipates the pmf and energy is lost as heat. Other events can directly dissipate the pmf independent of ATP production as well, such as chemical exposure or mechanisms involving regulated mitochondrial membrane electrolyte transport. Uncoupling has defined roles in metabolic plasticity and can be linked through signal transduction to physiologic events. In the latter case, the pmf impacts mitochondrial reactive oxygen species (ROS) production. Although capable of molecular damage, ROS also have signaling properties that depend on the timing, location, and quantity of their production. In this review, we provide a general overview of mitochondrial ROS production, mechanisms of uncoupling, and how these work in tandem to affect physiology and pathologies, including obesity, cardiovascular disease, and immunity. Overall, we highlight that isolated bioenergetic models-mitochondria and cells-only partially recapitulate the complex link between the pmf and ROS signaling that occurs in vivo.

Proteomic profile of 4-PBA treated human neuronal cells during ER stress

Perturbations affecting the homoeostasis of endoplasmic reticulum (ER) activate an adaptive signaling known as the unfolded protein response or UPR. Many studies have reported the association between neurological disorders and ER stress. Decreasing ER stress may therefore aid in therapeutic control of neuronal diseases. Sodium 4-phenylbutyrate (4-PBA), a small molecule, has been shown to alleviate ER stress and various neurological diseases, but the mechanistic basis of its action is not well understood. Using an iTRAQ based LC-MS technique we have delineated the effect of 4-PBA on the proteome of human neuroblastoma cells (SK-N-SH) during Tunicamycin-induced ER stress. The proteomic profile of 4-PBA-treated cells revealed that 4-PBA does not alter the cellular proteome to adapt towards ER stress. However, it can alleviate both the toxicity and proteomic alterations, induced by an ER stress inducer. Hence, the therapeutic effect of 4-PBA is primarily due to its ability to resolve ER stress rather than its ability to alter the expression of proteins required for maintaining ER proteostasis. Thus, we posit here that 4-PBA acts as an authentic chemical chaperone by aiding protein folding in the ER.

Diminished Ost3-dependent N-glycosylation of the BiP nucleotide exchange factor Sil1 is an adaptive response to reductive ER stress

BiP (Kar2 in yeast) is an essential Hsp70 chaperone and master regulator of endoplasmic reticulum (ER) function. BiP's activity is regulated by its intrinsic ATPase activity that can be stimulated by two different nucleotide exchange factors, Sil1 and Lhs1. Both Sil1 and Lhs1 are glycoproteins, but how N-glycosylation regulates their function is not known. Here, we show that N-glycosylation of Sil1, but not of Lhs1, is diminished upon reductive stress. N-glycosylation of Sil1 is predominantly Ost3-dependent and requires a functional Ost3 CxxC thioredoxin motif. N-glycosylation of Lhs1 is largely Ost3-independent and independent of the CxxC motif. Unglycosylated Sil1 is not only functional but is more effective at rescuing loss of Lhs1 activity than N-glycosylated Sil1. Furthermore, substitution of the redox active cysteine pair C52 and C57 in the N terminus of Sil1 results in the Doa10-dependent ERAD of this mutant protein. We propose that reductive stress in the ER inhibits the Ost3-dependent N-glycosylation of Sil1, which regulates specific BiP functions appropriate to the needs of the ER under reductive stress.

Reductive Stress in Inflammation-Associated Diseases and the Pro-Oxidant Effect of Antioxidant Agents

Abstract: Reductive stress (RS) is the counterpart oxidative stress (OS), and can occur in response to conditions that shift the redox balance of important biological redox couples, such as the NAD⁺/NADH, NADP⁺/NADPH, and GSH/GSSG, to a more reducing state. Overexpression of antioxidant enzymatic systems leads to excess reducing equivalents that can deplete reactive oxidative species, driving the cells to RS. A feedback regulation is established in which chronic RS induces OS, which in turn, stimulates again RS. Excess reducing equivalents may regulate cellular signaling pathways, modify transcriptional activity, induce alterations in the formation of disulfide bonds in proteins, reduce mitochondrial function, decrease cellular metabolism, and thus, contribute to the development of some diseases in which NF-κB, a redox-sensitive transcription factor, participates. Here, we described the diseases in which an inflammatory condition is associated to RS, and where delayed folding, disordered transport, failed oxidation, and aggregation are found. Some of these diseases are aggregation protein cardiomyopathy, hypertrophic cardiomyopathy, muscular dystrophy, pulmonary hypertension, rheumatoid arthritis, Alzheimer's disease, and metabolic syndrome, among others. Moreover, chronic consumption of antioxidant supplements, such as vitamins and/or flavonoids, may have pro-oxidant effects that may alter the redox cellular equilibrium and contribute to RS, even diminishing life expectancy.