Reexamination of the electrophile response element sequences and context reveals a lack of consensus in gene function

Elaiophylin Inhibits Tumorigenesis of Human Lung Adenocarcinoma by Inhibiting Mitophagy via Suppression of SIRT1/Nrf2 Signaling

Lung adenocarcinoma (LADC), the most common type of lung cancer, is still one of the most aggressive and rapidly fatal tumor types, even though achievements in new therapeutic approaches have been developed. Elaiophylin as a C2 symmetrically glycosylated 16 macrolides has been reported to be a late-stage autophagy inhibitor with a potent anti-tumor effect on various cancers. This study investigated the anti-tumor effect of elaiophylin on human LADC for the first time in in vitro and in vivo models. The in vitro study in LADC A549 cells showed that elaiophylin significantly inhibited cell viability and induced cell apoptosis through the suppression of mitophagy and induction of cellular and mitochondrial oxidative stress. Proteomic analysis and molecular docking assay implicated that SIRT1 was likely the direct target of elaiophylin in A549 cells. Further mechanistic study verified that elaiophylin reduced Nrf2 deacetylation, expression, and transcriptional activity as well as cytoplasm translocation by downregulating SIRT1 expression and deacetylase activity. Additionally, SIRT1/Nrf2 activation could attenuate elaiophylin-induced mitophagy inhibition and oxidative stress. The in vivo study in the A549-xenograft mice model showed that the anti-tumor effect of elaiophylin was accompanied by the decreased expressions of SIRT1, Nrf2, Parkin, and PINK1. Thus, the present study reports that elaiophylin has potent anti-tumor properties in LADC, which effect is likely mediated through suppressing the SIRT1/Nrf2 signaling. In conclusion, elaiophylin may be a novel drug candidate for LADC and SIRT1 may be a new therapeutic target for such devastating malignancy.

In Vitro Methodologies to Study the Role of Advanced Glycation End Products (AGEs) in Neurodegeneration

Advanced glycation end products (AGEs) can be present in food or be endogenously produced in biological systems. Their formation has been associated with chronic neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and amyotrophic lateral sclerosis. The implication of AGEs in neurodegeneration is related to their ability to bind to AGE-specific receptors and the ability of their precursors to induce the so-called “dicarbonyl stress”, resulting in cross-linking and protein damage. However, the mode of action underlying their role in neurodegeneration remains unclear. While some research has been carried out in observational clinical studies, further in vitro studies may help elucidate these underlying modes of action. This review presents and discusses in vitro methodologies used in research on the potential role of AGEs in neuroinflammation and neurodegeneration. The overview reveals the main concepts linking AGEs to neurodegeneration, the current findings, and the available and advisable in vitro models to study their role. Moreover, the major questions regarding the role of AGEs in neurodegenerative diseases and the challenges and discrepancies in the research field are discussed.

Sirtuins and Renal Oxidative Stress

Renal failure is a major health problem that is increasing worldwide. To improve clinical outcomes, we need to understand the basic mechanisms of kidney disease. Aging is a risk factor for the development and progression of kidney disease. Cells develop an imbalance of oxidants and antioxidants as they age, resulting in oxidative stress and the development of kidney damage. Calorie restriction (CR) is recognized as a dietary approach that promotes longevity, reduces oxidative stress, and delays the onset of age-related diseases. Sirtuins, a type of nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase, are considered to be anti-aging molecules, and CR induces their expression. The sirtuin family consists of seven enzymes (Sirt1–7) that are involved in processes and functions related to antioxidant and oxidative stress, such as DNA damage repair and metabolism through histone and protein deacetylation. In fact, a role for sirtuins in the regulation of antioxidants and redox substances has been suggested. Therefore, the activation of sirtuins in the kidney may represent a novel therapeutic strategy to enhancing resistance to many causative factors in kidney disease through the reduction of oxidative stress. In this review, we discuss the relationship between sirtuins and oxidative stress in renal disease.

Interdependence of Anti-Inflammatory and Antioxidant Properties of Squalene-Implication for Cardiovascular Health

Cardiovascular diseases (CVD) have been recognized as the leading cause of mortality worldwide, accounting for 31% of global mortality. Among the risk factors of CVD, hyperlipidemia has been established as the most potent risk factor. Statins, a class of drug that reduces lower-density lipoprotein cholesterol (LDL-C), are the preferred medical treatment. However, due to the development of statin-associated muscle symptoms, statins are associated with patients' discontinuation and nonadherence. Other statin-induced side effects, such as hepatotoxicity and gastrointestinal upset, all contribute to patients choosing alternative medicines. Squalene (SQ), an unsaturated hydrocarbon naturally synthesized in plants and animals, could become the alternative treatment or supplementary agent for cardiovascular health. SQ has been shown to exert cardioprotective effect via its antioxidant activity. Oxidative stress and inflammatory responses are closely related to each other, which proposes an interdependence relation between antioxidant and anti-inflammatory. Therefore, this review explores the interdependence between the antioxidant and anti-inflammatory effects of SQ implicated on cardiovascular health.

Does Bach1 & c-Myc dependent redox dysregulation of Nrf2 & adaptive homeostasis decrease cancer risk in ageing?

The Keap1-Nrf2 signal transduction pathway plays a major role in oxidant and electrophile induction of adaptive homeostasis that transiently and reversibly increases cellular and organismal protection from stress. By expanding (and then contracting) the normal homeostatic range of expression of stress-protective genes, Nrf2 allows us to cope with fluctuations in stress levels. Two major inhibitors of Nrf2 are Bach1 and c-Myc which normally serve the important function of turning off adaptation when appropriate. We have found, however, that both Bach1 and c-Myc levels increase substantially with age and that older human cells, worms, flies, and mice loose Nrf2-dependent signaling and adaptive homeostasis. Nrf2 has also been linked with increased risk of cancers, and cancer incidence certainly increases with age. Here we propose that the age-dependent increase in Bach1 and c-Myc may actually cause the age-dependent decline in Nrf2 signaling and adaptive homeostasis, and that this is a coordinated attempt to minimize the age-dependent increase in cancer incidence. In other words, we may trade off adaptive homeostasis for a lower risk of cancer by increasing Bach1 and c-Myc in ageing.

Aging attenuates redox adaptive homeostasis and proteostasis in female mice exposed to traffic-derived nanoparticles ('vehicular smog')

Environmental toxicants are catalysts for protein damage, aggregation, and the aging process. Fortunately, evolution selected adaptive homeostasis as a system to mitigate such damage by expanding the normal capacity to cope with toxic stresses. Little is known about the subcellular degradative responses to proteins oxidatively damaged by air pollution. To better understand the impact of environmental toxicants upon the adaptive homeostatic response, female C57BL/6 mice were exposed for 10 weeks to filtered air or reaerosolized vehicular-derived nano-scale particulate matter (nPM), at which point tissues from young (6 month) and middle-aged (21 month) mice were studied. We found significant increases of proteolytic capacity in lung, liver, and heart. Up to two-fold increases were seen in the 20S Proteasome, the Immunoproteasome, the mitochondrial Lon protease, and NF-E2-related factor 2 (Nrf2), a major transcriptional factor for these and other stress-responsive genes. The responses were equivalent in all organs, despite the indirect input of inhaled particles to heart and liver which are downstream of lung. To our knowledge, this is the first exploration of proteostatic responses to oxidative damage by air pollution. Although, middle-aged mice had higher basal levels, their Nrf2-responsive-genes exhibited no response to nanoparticulate exposure. We also found a parallel age-associated rise in the Nrf2 transcriptional inhibitors, Bach1 and c-Myc which appear to attenuate adaptive responses in older mammals, possibly explaining the 'age-ceiling effect.' This report extends prior findings in male mice by demonstrating the involvement of proteolytic responses to traffic-related air pollution in lung, liver, and heart of female mice, with an age-dependent loss of adaptive homeostasis.

The Role of Sirtuins in Antioxidant and Redox Signaling

SIGNIFICANCE

Antioxidant and redox signaling (ARS) events are regulated by critical molecules that modulate antioxidants, reactive oxygen species (ROS) or reactive nitrogen species (RNS), and/or oxidative stress within the cell. Imbalances in these molecules can disturb cellular functions to become pathogenic. Sirtuins serve as important regulators of ARS in cells. Recent Advances: Sirtuins (SIRTs 1-7) are a family of nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylases with the ability to deacetylate histone and nonhistone targets. Recent studies show that sirtuins modulate the regulation of a variety of cellular processes associated with ARS. SIRT1, SIRT3, and SIRT5 protect the cell from ROS, and SIRT2, SIRT6, and SIRT7 modulate key oxidative stress genes and mechanisms. Interestingly, SIRT4 has been shown to induce ROS production and has antioxidative roles as well.

CRITICAL ISSUES

A complete understanding of the roles of sirtuins in redox homeostasis of the cell is very important to understand the normal functioning as well as pathological manifestations. In this review, we have provided a critical discussion on the role of sirtuins in the regulation of ARS. We have also discussed mechanistic interactions among different sirtuins. Indeed, a complete understanding of sirtuin biology could be critical at multiple fronts.

FUTURE DIRECTIONS

Sirtuins are emerging to be important in normal mammalian physiology and in a variety of oxidative stress-mediated pathological situations. Studies are needed to dissect the mechanisms of sirtuins in maintaining redox homeostasis. Efforts are also required to assess the targetability of sirtuins in the management of redox-regulated diseases. Antioxid. Redox Signal. 28, 643-661.

The Role of Nrf2 in Cardiovascular Function and Disease

Free radicals, reactive oxygen/nitrogen species (ROS/RNS), hydrogen sulphide, and hydrogen peroxide play an important role in both intracellular and intercellular signaling; however, their production and quenching need to be closely regulated to prevent cellular damage. An imbalance, due to exogenous sources of free radicals and chronic upregulation of endogenous production, contributes to many pathological conditions including cardiovascular disease and also more general processes involved in aging. Nuclear factor erythroid 2-like 2 (NFE2L2; commonly known as Nrf2) is a transcription factor that plays a major role in the dynamic regulation of a network of antioxidant and cytoprotective genes, through binding to and activating expression of promoters containing the antioxidant response element (ARE). Nrf2 activity is regulated by many mechanisms, suggesting that tight control is necessary for normal cell function and both hypoactivation and hyperactivation of Nrf2 are indicated in playing a role in different aspects of cardiovascular disease. Targeted activation of Nrf2 or downstream genes may prove to be a useful avenue in developing therapeutics to reduce the impact of cardiovascular disease. We will review the current status of Nrf2 and related signaling in cardiovascular disease and its relevance to current and potential treatment strategies.

Aging-related decline in the induction of Nrf2-regulated antioxidant genes in human bronchial epithelial cells

Evidence from animal studies suggests that stress-induced increases in Nrf2-regulated antioxidant gene expression, a critical mechanism of cellular protection, declines with aging. This study examined whether this also occurs in humans. We measured the basal and inducible levels of Nrf2-regulated antioxidant genes in human bronchial epithelial (HBE) cells from subjects of young adult (21–29 years) and older (60–69 years) non-smokers, and explored factors affecting expresion. The basal expression of three representative Nrf2-regulated genes, the catalytic and modulator subunits of glutamate cysteine ligase (GCLC and GCLM, respectively), and NAD(P)H quinone oxidoreductase 1 (NQO1), was higher in cells from the older donors compared with cells from the young adult donors. Upon exposure to the Nrf2 activator, sulforaphane (SF), the expression of these antioxidant genes was increased in cells from both the young adults and the older donors; however, the induction by SF in older donor cells was significantly less than that seen in young adult cells. In addition, the activation of an EpRE-driven reporter by SF was lower in cells from older donors compared to cells from young adults. The basal expression of Nrf2 protein was also lower in cells from older donors than cells from young adults. Furthermore, we found that the basal expression of both Bach1 and c-Myc, two Nrf2 suppressors, was higher in cells from older adults than from young adult donors. In summary, our data suggest that, as in other species, basal expression of Nrf2-regulated genes increases with aging, while inducibility declines with aging. The increased expression of Nrf2 suppressors such as Bach1 and c-Myc may contribute to the impaired inducibility of the Nrf2-regulated antioxidant genes with aging in human bronchial epithelial cells.

Molecular and cellular basis for the unique functioning of Nrf1, an indispensable transcription factor for maintaining cell homoeostasis and organ integrity

The consensuscis-regulatory AP-1 (activator protein-1)-like AREs (antioxidant-response elements) and/or EpREs (electrophile-response elements) allow for differential recruitment of Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1], Nrf2 and Nrf3, together with each of their heterodimeric partners (e.g. sMaf, c-Jun, JunD or c-Fos), to regulate different sets of cognate genes. Among them, NF-E2 p45 and Nrf3 are subject to tissue-specific expression in haemopoietic and placental cell lineages respectively. By contrast, Nrf1 and Nrf2 are two important transcription factors expressed ubiquitously in various vertebrate tissues and hence may elicit putative combinational or competitive functions. Nevertheless, they have de facto distinct biological activities because knockout of their genes in mice leads to distinguishable phenotypes. Of note, Nrf2 is dispensable during development and growth, albeit it is accepted as a master regulator of antioxidant, detoxification and cytoprotective genes against cellular stress. Relative to the water-soluble Nrf2, less attention has hitherto been drawn to the membrane-bound Nrf1, even though it has been shown to be indispensable for embryonic development and organ integrity. The biological discrepancy between Nrf1 and Nrf2 is determined by differences in both their primary structures and topovectorial subcellular locations, in which they are subjected to distinct post-translational processing so as to mediate differential expression of ARE-driven cytoprotective genes. In the present review, we focus on the molecular and cellular basis for Nrf1 and its isoforms, which together exert its essential functions for maintaining cellular homoeostasis, normal organ development and growth during life processes. Conversely, dysfunction of Nrf1 results in spontaneous development of non-alcoholic steatohepatitis, hepatoma, diabetes and neurodegenerative diseases in animal models.

Oxidative stress response and Nrf2 signaling in aging

Increasing oxidative stress, a major characteristic of aging, has been implicated in a variety of age-related pathologies. In aging, oxidant production from several sources is increased, whereas antioxidant enzymes, the primary lines of defense, are decreased. Repair systems, including the proteasomal degradation of damaged proteins, also decline. Importantly, the adaptive response to oxidative stress declines with aging. Nrf2/EpRE signaling regulates the basal and inducible expression of many antioxidant enzymes and the proteasome. Nrf2/EpRE activity is regulated at several levels, including transcription, posttranslation, and interactions with other proteins. This review summarizes current studies on age-related impairment of Nrf2/EpRE function and discusses the changes in Nrf2 regulatory mechanisms with aging.

The gasotransmitter hydrogen sulfide induces nrf2-target genes by inactivating the keap1 ubiquitin ligase substrate adaptor through formation of a disulfide bond between cys-226 and cys-613

AIMS

The signaling molecule hydrogen sulfide (H2S) protects cells against oxidative stress and activates NF-E2 p45-related factor 2 (Nrf2), a transcription factor that regulates antioxidant genes. We sought to establish whether H2S requires Nrf2 to protect against oxidative stress, and whether activation of Nrf2 by H2S involves antagonism of Kelch-like ECH-associated protein-1 (Keap1), a redox-sensitive ubiquitin ligase substrate adaptor that represses Nrf2 under normal homeostatic conditions.

RESULTS

H2S stabilizes Nrf2 protein and induces Nrf2-target genes via an antioxidant-/electrophile-response element. In mouse embryonic fibroblasts, the ability of H2S to protect against cell death caused by the redox-cycling agent menadione is dependent on Nrf2. Moreover, Nrf2 regulates murine genes involved in the production of H2S (Cystathionine-beta-synthase [Cbs] and Cystathionine-gamma-lyase [Cse]) and the degradation of H2S (Sulfide:quinone reductase-like [yeast] [Sqrdl]). We found that H2S stabilizes Nrf2 through inhibition of Keap1, an event that requires covalent modification of amino acids C226 and C613 in the substrate adaptor. Upregulation of Nrf2 by H2S partially involves the production of H2O2, which inhibits Keap1 by stimulating the formation of an intramolecular disulfide bond between C226 and C613. The Keap1 C226 and C613 residues are also S-sulfhydrated by H2S, and this may entail reduction of the C226-C613 disulfide bridge formed by H2O2.

INNOVATION

Upregulation of Nrf2 by H2S and H2O2 involves inactivation of Keap1 through modification of C226 and C613.

CONCLUSION

Inhibition of Keap1 by H2S leads to Nrf2-mediated induction of cytoprotective genes. Nrf2 controls Cbs, Cse, and Sqrdl, suggesting that a feedback loop exists between Nrf2 and H2S.

The Nrf1 CNC-bZIP protein is regulated by the proteasome and activated by hypoxia

Background

Nrf1 (nuclear factor-erythroid 2 p45 subunit-related factor 1) is a transcription factor mediating cellular responses to xenobiotic and pro-oxidant stress. Nrf1 regulates the transcription of many stress-related genes through the electrophile response elements (EpREs) located in their promoter regions. Despite its potential importance in human health, the mechanisms controlling Nrf1 have not been addressed fully.

Principal Findings

We found that proteasomal inhibitors MG-132 and clasto-lactacystin-β-lactone stabilized the protein expression of full-length Nrf1 in both COS7 and WFF2002 cells. Concomitantly, proteasomal inhibition decreased the expression of a smaller, N-terminal Nrf1 fragment, with an approximate molecular weight of 23 kDa. The EpRE-luciferase reporter assays revealed that proteasomal inhibition markedly inhibited the Nrf1 transactivational activity. These results support earlier hypotheses that the 26 S proteasome processes Nrf1 into its active form by removing its inhibitory N-terminal domain anchoring Nrf1 to the endoplasmic reticulum. Immunoprecipitation demonstrated that Nrf1 is ubiquitinated and that proteasomal inhibition increased the degree of Nrf1 ubiquitination. Furthermore, Nrf1 protein had a half-life of approximately 5 hours in COS7 cells. In contrast, hypoxia (1% O₂) significantly increased the luciferase reporter activity of exogenous Nrf1 protein, while decreasing the protein expression of p65, a shorter form of Nrf1, known to act as a repressor of EpRE-controlled gene expression. Finally, the protein phosphatase inhibitor okadaic acid activated Nrf1 reporter activity, while the latter was repressed by the PKC inhibitor staurosporine.

Conclusions

Collectively, our data suggests that Nrf1 is controlled by several post-translational mechanisms, including ubiquitination, proteolytic processing and proteasomal-mediated degradation as well as by its phosphorylation status.