O-GlcNAcylation of SKN-1 modulates the lifespan and oxidative stress resistance in Caenorhabditis elegans

Research on the anti-oxidant and anti-aging effects of Polygonatum kingianum saponins in Caenorhabditis elegans

Oxidative stress and its impact on aging are critical areas of research. Natural anti-oxidants, such as saponins found in Polygonatum sibiricum, hold promise as potential clinical interventions against aging. In this study, we utilized the nematode model organism, Caenorhabditis elegans, to investigate the pharmacological effects of Polygonatum sibiricum saponins (PKS) on antioxidation and anti-aging. The results demonstrated a significant anti-aging biological activity associated with PKS. Through experiments involving lifespan and stress, lipofuscin, q-PCR, and ROS measurement, we found that PKS effectively mitigated aging-related processes. Furthermore, the mechanism underlying these anti-aging effects was linked to the SKN-1 signaling pathway. PKS increased the nuclear localization of the SKN-1 transcription factor, leading to the up-regulation of downstream anti-oxidant genes, such as gst-4 and sod-3, and a substantial reduction in intracellular ROS levels within the nematode. In conclusion, our study sheds light on the anti-oxidant and anti-aging properties of PKS in C. elegans. This research not only contributes to understanding the biological mechanisms involved but also highlights the potential therapeutic applications of these natural compounds in combating aging-related processes.

Disrupting the SKN-1 homeostat: mechanistic insights and phenotypic outcomes

The mechanisms that govern maintenance of cellular homeostasis are crucial to the lifespan and healthspan of all living systems. As an organism ages, there is a gradual decline in cellular homeostasis that leads to senescence and death. As an organism lives into advanced age, the cells within will attempt to abate age-related decline by enhancing the activity of cellular stress pathways. The regulation of cellular stress responses by transcription factors SKN-1/Nrf2 is a well characterized pathway in which cellular stress, particularly xenobiotic stress, is abated by SKN-1/Nrf2-mediated transcriptional activation of the Phase II detoxification pathway. However, SKN-1/Nrf2 also regulates a multitude of other processes including development, pathogenic stress responses, proteostasis, and lipid metabolism. While this process is typically tightly regulated, constitutive activation of SKN-1/Nrf2 is detrimental to organismal health, this raises interesting questions surrounding the tradeoff between SKN-1/Nrf2 cryoprotection and cellular health and the ability of cells to deactivate stress response pathways post stress. Recent work has determined that transcriptional programs of SKN-1 can be redirected or suppressed to abate negative health outcomes of constitutive activation. Here we will detail the mechanisms by which SKN-1 is controlled, which are important for our understanding of SKN-1/Nrf2 cytoprotection across the lifespan.

A dicer-related helicase opposes the age-related pathology from SKN-1 activation in ASI neurons

Coordination of cellular responses to stress is essential for health across the lifespan. The transcription factor SKN-1 is an essential homeostat that mediates survival in stress-inducing environments and cellular dysfunction, but constitutive activation of SKN-1 drives premature aging thus revealing the importance of turning off cytoprotective pathways. Here, we identify how SKN-1 activation in two ciliated ASI neurons in Caenorhabditis elegans results in an increase in organismal transcriptional capacity that drives pleiotropic outcomes in peripheral tissues. An increase in the expression of established SKN-1 stress response and lipid metabolism gene classes of RNA in the ASI neurons, in addition to the increased expression of several classes of noncoding RNA, define a molecular signature of animals with constitutive SKN-1 activation and diminished healthspan. We reveal neddylation as a unique regulator of the SKN-1 homeostat that mediates SKN-1 abundance within intestinal cells. Moreover, RNAi-independent activity of the dicer-related DExD/H-box helicase, drh-1, in the intestine, can oppose the effects of aberrant SKN-1 transcriptional activation and delays age-dependent decline in health. Taken together, our results uncover a cell nonautonomous circuit to maintain organism-level homeostasis in response to excessive SKN-1 transcriptional activity in the sensory nervous system.

A dicer-related helicase opposes the age-related pathology from SKN-1 activation in ASI neurons

Coordination of cellular responses to stress are essential for health across the lifespan. The transcription factor SKN-1 is an essential homeostat that mediates survival in stress-inducing environments and cellular dysfunction, but constitutive activation of SKN-1 drives premature aging thus revealing the importance of turning off cytoprotective pathways. Here we identify how SKN-1 activation in two ciliated ASI neurons in C. elegans results in an increase in organismal transcriptional capacity that drives pleiotropic outcomes in peripheral tissues. An increase in the expression of established SKN-1 stress response and lipid metabolism gene classes of RNA in the ASI neurons, in addition to the increased expression of several classes of non-coding RNA, define a molecular signature of animals with constitutive SKN-1 activation and diminished healthspan. We reveal neddylation as a novel regulator of the SKN-1 homeostat that mediates SKN-1 abundance within intestinal cells. Moreover, RNAi-independent activity of the dicer-related DExD/H-box helicase, drh-1 , in the intestine, can oppose the e2ffects of aberrant SKN-1 transcriptional activation and delays age-dependent decline in health. Taken together, our results uncover a cell non-autonomous circuit to maintain organism-level homeostasis in response to excessive SKN-1 transcriptional activity in the sensory nervous system.

SIGNIFICANCE STATEMENT

Unlike activation, an understudied fundamental question across biological systems is how to deactivate a pathway, process, or enzyme after it has been turned on. The irony that the activation of a transcription factor that is meant to be protective can diminish health was first documented by us at the organismal level over a decade ago, but it has long been appreciated that chronic activation of the human ortholog of SKN-1, NRF2, could lead to chemo- and radiation resistance in cancer cells. A colloquial analogy to this biological idea is a sink faucet that has an on valve without a mechanism to shut the water off, which will cause the sink to overflow. Here, we define this off valve.

Phenolic Compounds from Sonchus arvensis Linn. and Hemerocallis citrina Baroni. Inhibit Sucrose and Stearic Acid Induced Damage in Caenorhabditis elegans

Sonchus arvensis Linn. and Hemerocallis citrina Baroni. have been reported to improve body resistance. However, the underlying mechanism is not clear. In this study, Sonchus arvensis Linn. phenolic compounds (SAP) and Hemerocallis citrina Baroni. phenolic compounds (HCP) were extracted and their protective effects in Caenorhabditis elegans evaluated. SAP and HCP showed considerably different phenolic compositions. In the normal C. elegans model, HCP exhibited better effects in promoting growth than SAP. In the sucrose-incubated C. elegans model, both SAP and HCP showed positive effects against the high-sucrose-induced damage. In the stearic acid-incubated C. elegans model, both SAP and HCP improved lifespan, reproductive ability and growth, while HCP had a more evident effect than SAP on reproductive ability. The TGF-β signaling pathway was confirmed to be involved in the protective effects of SAP and HCP. The antioxidant ability of SAP was also found to be related to skn-1. Our study shows that both SAP and HCP have protective effects against high sucrose- or high stearic acid-induced damage.

Regulation and Functions of the ER-Associated Nrf1 Transcription Factor

Nrf1 is a member of the nuclear erythroid 2-like family of transcription factors that regulate stress-responsive gene expression in animals. Newly synthesized Nrf1 is targeted to the endoplasmic reticulum (ER) where it is N-glycosylated. N-glycosylated Nrf1 is trafficked to the cytosol by the ER-associated degradation (ERAD) machinery and is subject to rapid proteasomal degradation. When proteasome function is impaired, Nrf1 escapes degradation and undergoes proteolytic cleavage and deglycosylation. Deglycosylation results in deamidation of N-glycosylated asparagine residues to edit the protein sequence encoded by the genome. This truncated and "sequence-edited" form of Nrf1 enters the nucleus where it induces up-regulation of proteasome subunit genes. Thus, Nrf1 drives compensatory proteasome biogenesis in cells exposed to proteasome inhibitor drugs and other proteotoxic insults. In addition to its role in proteasome homeostasis, Nrf1 is implicated in responses to oxidative stress, and maintaining lipid and cholesterol homeostasis. Here, we describe the conserved and complex mechanism by which Nrf1 is regulated and highlight emerging evidence linking this unusual transcription factor to development, aging, and disease.

O-GlcNAc transferase plays a non-catalytic role in C. elegans male fertility

Animal behavior is influenced by the competing drives to maintain energy and to reproduce. The balance between these evolutionary pressures and how nutrient signaling pathways intersect with mating remains unclear. The nutrient sensor O-GlcNAc transferase, which post-translationally modifies intracellular proteins with a single monosaccharide, is responsive to cellular nutrient status and regulates diverse biological processes. Though essential in most metazoans, O-GlcNAc transferase (ogt-1) is dispensable in Caenorhabditis elegans, allowing genetic analysis of its physiological roles. Compared to control, ogt-1 males had a four-fold reduction in mean offspring, with nearly two thirds producing zero progeny. Interestingly, we found that ogt-1 males transferred sperm less often, and virgin males had reduced sperm count. ogt-1 males were also less likely to engage in mate-searching and mate-response behaviors. Surprisingly, we found normal fertility for males with hypodermal expression of ogt-1 and for ogt-1 strains with catalytic-dead mutations. This suggests OGT-1 serves a non-catalytic function in the hypodermis impacting male fertility and mating behavior. This study builds upon research on the nutrient sensor O-GlcNAc transferase and demonstrates a role it plays in the interplay between the evolutionary drives for reproduction and survival.

MEKK-3 Acts Cooperatively with NSY-1 in SKN-1-Dependent Manner against Oxidative Stress and Aging in Caenorhabditis elegans

Oxidative stress resulting from reactive oxygen species and other toxic metabolites is involved in human diseases, and it plays an important role in aging. In Caenorhabditis elegans, SKN-1 is required for protection against oxidative stress and aging. As p38 mitogen-activated protein kinase signaling is activated in response to oxidative stress, SKN-1 accumulates in intestinal nuclei and induces phase II detoxification genes. However, NSY-1, a well-known mitogen-activated protein kinase kinase kinase (MAPKKK) of C. elegans, acts as a partial regulator of the SKN-1-induced oxidative stress signaling pathway, suggesting that the regulator for optimal activation of SKN-1 remains unknown. Here, we report a MAPKKK, MEKK-3, as a new regulator required for full activation of SKN-1-mediated resistance against oxidative stress and aging. In RNA-interference-based screening, we found that the simultaneous knockdown of mekk-3 and nsy-1 significantly decreased the oxidative stress resistance and survival of SKN-1 transgenic worms. MEKK-3 was induced in response to oxidative stress. Mechanistic analysis revealed that double knockdown of mekk-3 and nsy-1 completely suppressed the nuclear localization of SKN-1. These results were reproduced in mutant worms in which SKN-1 is constitutively localized to intestinal nuclei. In addition, mekk-3 and nsy-1 were required for optimal induction of SKN-1 target genes such as gcs-1 and trx-1. These data indicate that MEKK-3 plays an essential role in the SKN-1-dependent signaling pathway involved in oxidative stress resistance and longevity by cooperating with NSY-1.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Methods for Studying Site-Specific O-GlcNAc Modifications: Successes, Limitations, and Important Future Goals

O-GlcNAcylation is a dynamic post-translational modification which affects myriad proteins, cellular functions, and disease states. Its presence or absence modulates protein function via differential protein- and site-specific mechanisms, necessitating innovative techniques to probe the modification in highly selective manners. To this end, a variety of biological and chemical methods have been developed to study specific O-GlcNAc modification events both in vitro and in vivo, each with their own respective strengths and shortcomings. Together, they comprise a potent chemical biology toolbox for the analysis of O-GlcNAcylation (and, in theory, other post-translational modifications) while highlighting the need and space for more facile, generalizable, and biologically authentic techniques.

Roles of CNC Transcription Factors NRF1 and NRF2 in Cancer

Simple Summary

Although NRF1 (nuclear factor erythroid 2-like 1; NFE2L1) and NRF2 (nuclear factor erythroid 2-like 2; NFE2L2) belong to the CNC (cap‘n’collar) transcription factor family and share DNA recognition elements, their functions in vivo are substantially different. In cancer cells, while NRF2 confers therapeutic resistance via increasing antioxidant capacity and modulating glucose and glutamine metabolism, NRF1 confers therapeutic resistance via triggering proteasome bounce back response. Proteasome inhibition activates NRF1, and NRF1, in turn, activates the proteasome by inducing the transcriptional activation of proteasome subunit genes. One of the oncometabolites, UDP-GlcNAc (uridine diphosphate N-acetylglucosamine), has been found to be a key to the NRF1-mediated proteasome bounce back response. In this review, we introduce the roles of NRF1 in the cancer malignancy in comparison with NRF2.

Abstract

Cancer cells exhibit unique metabolic features and take advantage of them to enhance their survival and proliferation. While the activation of NRF2 (nuclear factor erythroid 2-like 2; NFE2L2), a CNC (cap‘n’collar) family transcription factor, is effective for the prevention and alleviation of various diseases, NRF2 contributes to cancer malignancy by promoting aggressive tumorigenesis and conferring therapeutic resistance. NRF2-mediated metabolic reprogramming and increased antioxidant capacity underlie the malignant behaviors of NRF2-activated cancer cells. Another member of the CNC family, NRF1, plays a key role in the therapeutic resistance of cancers. Since NRF1 maintains proteasome activity by inducing proteasome subunit genes in response to proteasome inhibitors, NRF1 protects cancer cells from proteotoxicity induced by anticancer proteasome inhibitors. An important metabolite that activates NRF1 is UDP-GlcNAc (uridine diphosphate N-acetylglucosamine), which is abundantly generated in many cancer cells from glucose and glutamine via the hexosamine pathway. Thus, the metabolic signatures of cancer cells are closely related to the oncogenic and tumor-promoting functions of CNC family members. In this review, we provide a brief overview of NRF2-mediated cancer malignancy and elaborate on NRF1-mediated drug resistance affected by an oncometabolite UDP-GlcNAc.

Ginsenoside extract from ginseng extends lifespan and health span in Caenorhabditis elegans

Nutrition intervention has become a potential strategy to improve healthspan and prolong lifespan. Ginseng has been used for thousands of years and developed as a functional food to provide various protective effects to humans. An extract of total ginsenosides (TGS), a mixture of the main active ginsenosides from ginseng, has wide biological activities and health benefits for age-related diseases, including antioxidation and improvements in mitochondrial function. However, the molecular mechanism of TGS for prolonging lifespan and improving fitness and how exactly this is achieved under normal and stress conditions remain largely unclear. In this study, wild-type and mutant C. elegans strains are used to investigate the role and molecular mechanism of TGS-mediated longevity, health benefits, and stress resistance. The results showed that treatment with TGS at 0.2 mg mL-1 from the stage of day four to death significantly extended the lifespan of worms by 14.02% without effects on bacterial metabolism and food intake. Furthermore, TGS treatment obviously improved age-associated mobility, muscle fiber organization, lipofuscin accumulation, and enhanced resistance under oxidative stress. Importantly, these effects of TGS were achieved by activating the signaling pathways of anti-oxidant regulation and longevity, including the NRF2/SKN-1, SIRT1/SIR 2.1, and FOXO/DAF-16 signaling pathways. Finally, it was found that Rg1, Re, and Rb1 were the major effective components of anti-oxidative activity and longevity. Collectively, the protective effect of ginsenoside extract in healthy aging and stress responses provides new insights for the development and practical application of ginseng functional products.

Stressful development: integrating endoderm development, stress, and longevity

In addition to performing digestion and nutrient absorption, the intestine serves as one of the first barriers to the external environment, crucial for protecting the host from environmental toxins, pathogenic invaders, and other stress inducers. The gene regulatory network (GRN) governing embryonic development of the endoderm and subsequent differentiation and maintenance of the intestine has been well-documented in C. elegans. A key regulatory input that initiates activation of the embryonic GRN for endoderm and mesoderm in this animal is the maternally provided SKN-1 transcription factor, an ortholog of the vertebrate Nrf1 and 2, which, like C. elegans SKN-1, perform conserved regulatory roles in mediating a variety of stress responses across metazoan phylogeny. Other key regulatory factors in early gut development also participate in stress response as well as in innate immunity and aging and longevity. In this review, we discuss the intersection between genetic nodes that mediate endoderm/intestine differentiation and regulation of stress and homeostasis. We also consider how direct signaling from the intestine to the germline, in some cases involving SKN-1, facilitates heritable epigenetic changes, allowing transmission of adaptive stress responses across multiple generations. These connections between regulation of endoderm/intestine development and stress response mechanisms suggest that varying selective pressure exerted on the stress response pathways may influence the architecture of the endoderm GRN, thereby leading to genetic and epigenetic variation in early embryonic GRN regulatory events.

Functional Analysis of O-GlcNAcylation in Cancer Metastasis

One common and reversible type of post-translational modification (PTM) is the addition of O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), and its dynamic balance is controlled by O-GlcNAc transferase (OGT) and glycoside hydrolase O-GlcNAcase (OGA) through the addition or removal of O-GlcNAc groups. A large amount of research data confirms that proteins regulated by O-GlcNAcylation play a pivotal role in cells. In particularly, imbalanced levels of OGT and O-GlcNAcylation have been found in various types of cancers. Recently, increasing evidence shows that imbalanced O-GlcNAcylation directly or indirectly impacts the process of cancer metastasis. This review summarizes the current understanding of the influence of O-GlcNAc-proteins on the regulation of cancer metastasis. It will provide a theoretical basis to further elucidate of the molecular mechanisms underlying cancer emergence and progression.

The O-GlcNAc transferase OGT is a conserved and essential regulator of the cellular and organismal response to hypertonic stress

The conserved O-GlcNAc transferase OGT O-GlcNAcylates serine and threonine residues of intracellular proteins to regulate their function. OGT is required for viability in mammalian cells, but its specific roles in cellular physiology are poorly understood. Here we describe a conserved requirement for OGT in an essential aspect of cell physiology: the hypertonic stress response. Through a forward genetic screen in Caenorhabditis elegans, we discovered OGT is acutely required for osmoprotective protein expression and adaptation to hypertonic stress. Gene expression analysis shows that ogt-1 functions through a post-transcriptional mechanism. Human OGT partially rescues the C. elegans phenotypes, suggesting that the osmoregulatory functions of OGT are ancient. Intriguingly, expression of O-GlcNAcylation-deficient forms of human or worm OGT rescue the hypertonic stress response phenotype. However, expression of an OGT protein lacking the tetracopeptide repeat (TPR) domain does not rescue. Our findings are among the first to demonstrate a specific physiological role for OGT at the organismal level and demonstrate that OGT engages in important molecular functions outside of its well described roles in post-translational O-GlcNAcylation of intracellular proteins.

Effects of excess sugars and lipids on the growth and development of Caenorhabditis elegans

Background

Excessive intake of carbohydrates and fats causes over-nutrition, leading to a variety of diseases and complications. Here, we characterized the effects of different types of sugar and lipids on the growth and development of Caenorhabditis elegans.

Methods

We measured the lifespan, reproductive capacity, and length of nematodes after sugars and lipids treatment alone and co-treatment of sugars and lipids. Furthermore, we studied the mechanisms underlying the damage caused by high-sucrose and high-stearic acid on C.elegans by using transcriptome sequencing technology.

Results

The results showed that a certain concentration of sugar and lipid promoted the growth and development of nematodes. However, excessive sugars and lipids shortened the lifespan and length of nematodes and destroyed their reproductive capacity. Based on the results of the orthogonal test, we selected 400 mmol/L sucrose and 500 μg/mL stearic acid to model a high-sugar and high-lipid diet for C. elegans.

Conclusion

High-sugar and high-lipid intake altered the expression of genes involved in biofilm synthesis, genes that catalyze the synthesis and degradation of endogenous substances, and genes involved in innate immunity, resulting in physiological damage. Furthermore, we explored the protective effect of resveratrol on high-sugar and high-lipid damage to nematodes. Resveratrol plays a role in repairing by participating in the metabolism of foreign substances and reducing cellular oxidative stress.

AAK-2 and SKN-1 Are Involved in Chicoric-Acid-Induced Lifespan Extension in Caenorhabditis elegans

Chicoric acid is a dicaffeoyl ester with many bioactivities, including antioxidation, antidiabetes, and anti-inflammation. A previous study reported that chicoric acid extended the lifespan in Caenorhabditis elegans; however, the mechanism behind the effect of chicoric acid on the extended lifespan remains unknown. Consistent with the previous report, chicoric acid (25 and 50 μM) extended the maximum lifespan compared to the control (17.5 ± 3.3 and 15.6 ± 5%, respectively; p < 0.001 for both). The declines of the pumping rate and locomotive activity, two indicators of aging, were delayed by chicoric acid. Moreover, chicoric acid enhanced resistance to oxidative stress in C. elegans. It was further determined that the extended lifespan by chicoric acid was in part via aak-2 [a homologue of adenosine monophosphate (AMP)-activated protein kinase] and skn-1 (a homologue of nuclear factor erythroid 2-related factor 2). The current findings suggest that chicoric acid has the potential to be used as an anti-aging bioactive compound.

CBP-1/p300 acetyltransferase regulates SKN-1/Nrf cellular levels, nuclear localization, and activity in C. elegans

SKN-1/Nrf transcription factors regulate diverse biological processes essentially stress defense, detoxification, and longevity. Studies in model organisms have identified a broad range of regulatory processes and mechanisms that profoundly influence SKN-1/Nrf functions. Defining the mechanisms how SKN-1 is regulated will provide insight how cells defend against diverse stressors contributing to aging and disease. In this study, we demonstrate a crucial role for the acetyltransferase CBP-1, the C. elegans homolog of mammalian CREB-binding protein CBP/p300 in the activation of SKN-1. cbp-1 is essential for tolerance of oxidative stress and normal lifespan. CBP-1 directly interacts with SKN-1 and increases SKN-1 protein abundance. In particular CBP-1 modulates SKN-1 nuclear translocation under basal conditions and in response to stress and promotes SKN-1-dependent transcription of protective genes. Moreover, CBP-1 is required for SKN-1 nuclear recruitment, transcriptional activity, and longevity due to reduced insulin/IGF-1-like signaling, mTOR-, and GSK-3 signaling. Our findings establish the acetyltransferase CBP-1 as a critical activator of SKN-1 that directly modulates SKN-1 protein stability, nuclear localization, and function to ascertain normal stress response and lifespan.

Muscle-Specific Histone H3K36 Dimethyltransferase SET-18 Shortens Lifespan of Caenorhabditis elegans by Repressing daf-16a Expression

Mounting evidence shows that histone methylation, a typical epigenetic mark, is crucial for gene expression regulation during aging. Decreased trimethylation of Lys 36 on histone H3 (H3K36me3) in worms and yeast is reported to shorten lifespan. The function of H3K36me2 in aging remains unclear. In this study, we identified Caenorhabditis elegans SET-18 as a histone H3K36 dimethyltransferase. SET-18 deletion extended lifespan and increased oxidative stress resistance, dependent on daf-16 activity in the insulin/IGF pathway. In set-18 mutants, transcription of daf-16 isoform a (daf-16a) was specifically upregulated. Accordingly, a decrease in H3K36me2 on daf-16a promoter was observed. Muscle-specific expression of SET-18 increased in aged worms (day 7 and day 11), attributable to elevation of global H3K36me2 and inhibition of daf-16a expression. Consequently, longevity was shortened. These findings suggested that chromatic repression mediated by tissue-specific H3K36 dimethyltransferase might be detrimental to lifespan and may have implications in human age-related diseases.

Acute increases in O-GlcNAc indirectly impair mitochondrial bioenergetics through dysregulation of LonP1-mediated mitochondrial protein complex turnover

The attachment of O-linked β-N-acetylglucosamine (O-GlcNAc) to the serine and threonine residues of proteins in distinct cellular compartments is increasingly recognized as an important mechanism regulating cellular function. Importantly, the O-GlcNAc modification of mitochondrial proteins has been identified as a potential mechanism to modulate metabolism under stress with both potentially beneficial and detrimental effects. This suggests that temporal and dose-dependent changes in O-GlcNAcylation may have different effects on mitochondrial function. In the current study, we found that acutely augmenting O-GlcNAc levels by inhibiting O-GlcNAcase with Thiamet-G for up to 6 h resulted in a time-dependent decrease in cellular bioenergetics and decreased mitochondrial complex I, II, and IV activities. Under these conditions, mitochondrial number was unchanged, whereas an increase in the protein levels of the subunits of several electron transport complex proteins was observed. However, the observed bioenergetic changes appeared not to be due to direct increased O-GlcNAc modification of complex subunit proteins. Increases in O-GlcNAc were also associated with an accumulation of mitochondrial ubiquitinated proteins; phosphatase and tensin homolog induced kinase 1 (PINK1) and p62 protein levels were also significantly increased. Interestingly, the increase in O-GlcNAc levels was associated with a decrease in the protein levels of the mitochondrial Lon protease homolog 1 (LonP1), which is known to target complex IV subunits and PINK1, in addition to other mitochondrial proteins. These data suggest that impaired bioenergetics associated with short-term increases in O-GlcNAc levels could be due to impaired, LonP1-dependent, mitochondrial complex protein turnover.

Nutrient regulation of signaling and transcription

In the early 1980s, while using purified glycosyltransferases to probe glycan structures on surfaces of living cells in the murine immune system, we discovered a novel form of serine/threonine protein glycosylation (O-linked β-GlcNAc; O-GlcNAc) that occurs on thousands of proteins within the nucleus, cytoplasm, and mitochondria. Prior to this discovery, it was dogma that protein glycosylation was restricted to the luminal compartments of the secretory pathway and on extracellular domains of membrane and secretory proteins. Work in the last 3 decades from several laboratories has shown that O-GlcNAc cycling serves as a nutrient sensor to regulate signaling, transcription, mitochondrial activity, and cytoskeletal functions. O-GlcNAc also has extensive cross-talk with phosphorylation, not only at the same or proximal sites on polypeptides, but also by regulating each other's enzymes that catalyze cycling of the modifications. O-GlcNAc is generally not elongated or modified. It cycles on and off polypeptides in a time scale similar to phosphorylation, and both the enzyme that adds O-GlcNAc, the O-GlcNAc transferase (OGT), and the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), are highly conserved from C. elegans to humans. Both O-GlcNAc cycling enzymes are essential in mammals and plants. Due to O-GlcNAc's fundamental roles as a nutrient and stress sensor, it plays an important role in the etiologies of chronic diseases of aging, including diabetes, cancer, and neurodegenerative disease. This review will present an overview of our current understanding of O-GlcNAc's regulation, functions, and roles in chronic diseases of aging.

Arginine methylation of SKN-1 promotes oxidative stress resistance in Caenorhabditis elegans

Caenorhabditis elegans NRF (NF-E2-related factor)/CNC (Cap'n'collar) transcription factor, Skinhead-1 (SKN-1), is conservatively critical for promoting phase II detoxification gene expressions in response to oxidative stress. SKN-1 activity is controlled by well-known phosphorylation and recently-reported O-GlcNAcylation. Whether other kinds of posttanslational modifications of SKN-1 occur and influence its function remains elusive. Here, we found arginines 484 and 516 (R484/R516) of SKN-1 were asymmetrically dimethylated by PRMT-1. Oxidative stress enhanced the binding of PRMT-1 to SKN-1. Consequently, asymmetrical dimethylation of arginines on SKN-1 was elevated. Loss of prmt-1 or disruption of R484/R516 dimethylation decreased the enrichment of SKN-1 on the promoters of SKN-1-driven phase II detoxification genes, including gamma-glutamine cysteine synthetase gcs-1, glutathione S-transferases gst-7 and gst-4, which resulted in reduced ability of worms to defense against oxidative stress. These findings have important implications for investigating the physiological and pathological functions of arginine methylation on conserved NRF/CNC transcription factors in human diseases related to oxidative stress response.

•

Arg 484/516 of SKN-1 are asymmetrically dimethylated by PRMT-1 in C. elegans.

•

Oxidative stress enhances the binding of PRMT-1 to SKN-1.

•

Oxidative stress elevates asymmetrical dimethylation of arginines on SKN-1.

•

Arg 484/516 methylation increases SKN-1 binding to detoxification gene promoters.

•

Arg 484/516 methylation promotes function of SKN-1 in oxidative stress resistance.

Functional crosstalk among oxidative stress and O-GlcNAc signaling pathways

In metazoans, thousands of intracellular proteins are modified with O-linked β-N-acetylglucosamine (O-GlcNAc) in response to a wide range of stimuli and stresses. In particular, a complex and evolutionarily conserved interplay between O-GlcNAcylation and oxidative stress has emerged in recent years. Here, we review the current literature on the connections between O-GlcNAc and oxidative stress, with a particular emphasis on major signaling pathways, such as KEAP1/NRF2, FOXO, NFκB, p53 and cell metabolism. Taken together, this work sheds important light on the signaling functions of protein glycosylation and the mechanisms of stress responses alike and illuminates how the two are integrated in animal cell physiology.

O-GlcNAc in cancer: An Oncometabolism-fueled vicious cycle

Cancer cells exhibit unregulated growth, altered metabolism, enhanced metastatic potential and altered cell surface glycans. Fueled by oncometabolism and elevated uptake of glucose and glutamine, the hexosamine biosynthetic pathway (HBP) sustains glycosylation in the endomembrane system. In addition, the elevated pools of UDP-GlcNAc drives the O-GlcNAc modification of key targets in the cytoplasm, nucleus and mitochondrion. These targets include transcription factors, kinases, key cytoplasmic enzymes of intermediary metabolism, and electron transport chain complexes. O-GlcNAcylation can thereby alter epigenetics, transcription, signaling, proteostasis, and bioenergetics, key 'hallmarks of cancer'. In this review, we summarize accumulating evidence that many cancer hallmarks are linked to dysregulation of O-GlcNAc cycling on cancer-relevant targets. We argue that onconutrient and oncometabolite-fueled elevation increases HBP flux and triggers O-GlcNAcylation of key regulatory enzymes in glycolysis, Kreb's cycle, pentose-phosphate pathway, and the HBP itself. The resulting rerouting of glucose metabolites leads to elevated O-GlcNAcylation of oncogenes and tumor suppressors further escalating elevation in HBP flux creating a 'vicious cycle'. Downstream, elevated O-GlcNAcylation alters DNA repair and cellular stress pathways which influence oncogenesis. The elevated steady-state levels of O-GlcNAcylated targets found in many cancers may also provide these cells with a selective advantage for sustained growth, enhanced metastatic potential, and immune evasion in the tumor microenvironment.

Nuclear factor-erythroid-2 related transcription factor-1 (Nrf1) is regulated by O-GlcNAc transferase

The Nrf1 (Nuclear factor E2-related factor 1) transcription factor performs a critical role in regulating cellular homeostasis. Using a proteomic approach, we identified Host Cell Factor-1 (HCF1), a co-regulator of transcription, and O-GlcNAc transferase (OGT), the enzyme that mediates protein O-GlcNAcylation, as cellular partners of Nrf1a, an isoform of Nrf1. Nrf1a directly interacts with HCF1 through the HCF1 binding motif (HBM), while interaction with OGT is mediated through HCF1. Overexpression of HCF1 and OGT leads to increased Nrf1a protein stability. Addition of O-GlcNAc decreases ubiquitination and degradation of Nrf1a. Transcriptional activation by Nrf1a is increased by OGT overexpression and treatment with PUGNAc. Together, these data suggest that OGT can act as a regulator of Nrf1a.

Glycosylation of KEAP1 links nutrient sensing to redox stress signaling

O-GlcNAcylation is an essential, nutrient-sensitive post-translational modification, but its biochemical and phenotypic effects remain incompletely understood. To address this question, we investigated the global transcriptional response to perturbations in O-GlcNAcylation. Unexpectedly, many transcriptional effects of O-GlcNAc transferase (OGT) inhibition were due to the activation of NRF2, the master regulator of redox stress tolerance. Moreover, we found that a signature of low OGT activity strongly correlates with NRF2 activation in multiple tumor expression datasets. Guided by this information, we identified KEAP1 (also known as KLHL19), the primary negative regulator of NRF2, as a direct substrate of OGT We show that O-GlcNAcylation of KEAP1 at serine 104 is required for the efficient ubiquitination and degradation of NRF2. Interestingly, O-GlcNAc levels and NRF2 activation co-vary in response to glucose fluctuations, indicating that KEAP1 O-GlcNAcylation links nutrient sensing to downstream stress resistance. Our results reveal a novel regulatory connection between nutrient-sensitive glycosylation and NRF2 signaling and provide a blueprint for future approaches to discover functionally important O-GlcNAcylation events on other KLHL family proteins in various experimental and disease contexts.

Sustained O-GlcNAcylation reprograms mitochondrial function to regulate energy metabolism

Dysfunctional mitochondria and generation of reactive oxygen species (ROS) promote chronic diseases, which have spurred interest in the molecular mechanisms underlying these conditions. Previously, we have demonstrated that disruption of post-translational modification of proteins with β-linked N-acetylglucosamine (O-GlcNAcylation) via overexpression of the O-GlcNAc-regulating enzymes O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA) impairs mitochondrial function. Here, we report that sustained alterations in O-GlcNAcylation either by pharmacological or genetic manipulation also alter metabolic function. Sustained O-GlcNAc elevation in SH-SY5Y neuroblastoma cells increased OGA expression and reduced cellular respiration and ROS generation. Cells with elevated O-GlcNAc levels had elongated mitochondria and increased mitochondrial membrane potential, and RNA-sequencing analysis indicated transcriptome reprogramming and down-regulation of the NRF2-mediated antioxidant response. Sustained O-GlcNAcylation in mouse brain and liver validated the metabolic phenotypes observed in the cells, and OGT knockdown in the liver elevated ROS levels, impaired respiration, and increased the NRF2 antioxidant response. Moreover, elevated O-GlcNAc levels promoted weight loss and lowered respiration in mice and skewed the mice toward carbohydrate-dependent metabolism as determined by indirect calorimetry. In summary, sustained elevation in O-GlcNAcylation coupled with increased OGA expression reprograms energy metabolism, a finding that has potential implications for the etiology, development, and management of metabolic diseases.