Direct interaction between Nrf2 and p21(Cip1/WAF1) upregulates the Nrf2-mediated antioxidant response

Frequent alteration of the protein synthesis of enzymes for glucose metabolism in hepatocellular carcinomas

BACKGROUND

Cancer cells show enhanced glycolysis and inhibition of oxidative phosphorylation, even in the presence of sufficient oxygen (aerobic glycolysis). Glycolysis is much less efficient for energy production than oxidative phosphorylation, and the reason why cancer cells selectively use glycolysis remains unclear.

METHODS

Biospecimens were collected from 45 hepatocellular carcinoma patients. Protein samples were prepared through subcellular localization or whole-cell lysis. Protein synthesis was measured by SDS-PAGE and immunoblotting. mRNA transcription was measured using quantitative RT-PCR. Statistical correlation among immunoblotting data and clinicolaboratory factors were analyzed using SPSS.

RESULTS

Enzymes for oxidative phosphorylation (SDHA and SDHB) were frequently decreased (56 and 48 % of patients, respectively) in hepatocellular carcinomas. The lowered amount of the SDH protein complex was rarely accompanied by stabilization of HIF1α and subsequent activation of the hypoxia response. On the other hand, protein synthesis of G6PD and TKT, enzymes critical for pentose phosphate pathway (PPP), was increased (in 45 and 55 % of patients, respectively), while that of ALDOA, an enzyme for mainstream glycolysis, was eliminated (in 55 % of patients). Alteration of protein synthesis was correlated with gene expression for G6PD and TKT, but not for TKTL1, ALDOA, SDHA or SDHB. Augmented transcription and synthesis of PPP enzymes were accompanied by nuclear accumulation of NRF2.

CONCLUSION

Hepatocellular carcinomas divert glucose metabolism to the anabolic shunt by activating transcription factor NRF2.

How do nutritional antioxidants really work: nucleophilic tone and para-hormesis versus free radical scavenging in vivo

We present arguments for an evolution in our understanding of how antioxidants in fruits and vegetables exert their health-protective effects. There is much epidemiological evidence for disease prevention by dietary antioxidants and chemical evidence that such compounds react in one-electron reactions with free radicals in vitro. Nonetheless, kinetic constraints indicate that in vivo scavenging of radicals is ineffective in antioxidant defense. Instead, enzymatic removal of nonradical electrophiles, such as hydroperoxides, in two-electron redox reactions is the major antioxidant mechanism. Furthermore, we propose that a major mechanism of action for nutritional antioxidants is the paradoxical oxidative activation of the Nrf2 (NF-E2-related factor 2) signaling pathway, which maintains protective oxidoreductases and their nucleophilic substrates. This maintenance of "nucleophilic tone," by a mechanism that can be called "para-hormesis," provides a means for regulating physiological nontoxic concentrations of the nonradical oxidant electrophiles that boost antioxidant enzymes, and damage removal and repair systems (for proteins, lipids, and DNA), at the optimal levels consistent with good health.

The emerging role of the Nrf2-Keap1 signaling pathway in cancer

The Nrf2–Keap1 signaling pathway is key to cell defense and survival pathways. Nrf2 can protect cells and tissues from toxicants and carcinogens, and several Nrf2 activators are currently being tested as chemopreventive compounds. However, several studies also suggest that Nrf2 may protect cancer cells from chemotherapeutic agents and promote cancer cell proliferation. Here, Jaramillo and Zhang provide an overview of the Nrf2–Keap1 signaling pathway in cancer. They discuss the dual role of Nrf2 in cancer and the challenges in developing Nrf2-based drugs for chemoprevention and therapy.

The Nrf2 (nuclear factor erythroid 2 [NF-E2]-related factor 2 [Nrf2])–Keap1 (Kelch-like erythroid cell-derived protein with CNC homology [ECH]-associated protein 1) signaling pathway is one of the most important cell defense and survival pathways. Nrf2 can protect cells and tissues from a variety of toxicants and carcinogens by increasing the expression of a number of cytoprotective genes. As a result, several Nrf2 activators are currently being tested as chemopreventive compounds in clinical trials. Just as Nrf2 protects normal cells, studies have shown that Nrf2 may also protect cancer cells from chemotherapeutic agents and facilitate cancer progression. Nrf2 is aberrantly accumulated in many types of cancer, and its expression is associated with a poor prognosis in patients. In addition, Nrf2 expression is induced during the course of drug resistance. Collectively, these studies suggest that Nrf2 contributes to both intrinsic and acquired chemoresistance. This discovery has opened up a broad spectrum of research geared toward a better understanding of the role of Nrf2 in cancer. This review provides an overview of (1) the Nrf2–Keap1 signaling pathway, (2) the dual role of Nrf2 in cancer, (3) the molecular basis of Nrf2 activation in cancer cells, and (4) the challenges in the development of Nrf2-based drugs for chemoprevention and chemotherapy.

Administration of low dose methamphetamine 12 h after a severe traumatic brain injury prevents neurological dysfunction and cognitive impairment in rats

We recently published data that showed low dose of methamphetamine is neuroprotective when delivered 3 h after a severe traumatic brain injury (TBI). In the current study, we further characterized the neuroprotective potential of methamphetamine by determining the lowest effective dose, maximum therapeutic window, pharmacokinetic profile and gene expression changes associated with treatment. Graded doses of methamphetamine were administered to rats beginning 8 h after severe TBI. We assessed neuroprotection based on neurological severity scores, foot fault assessments, cognitive performance in the Morris water maze, and histopathology. We defined 0.250 mg/kg/h as the lowest effective dose and treatment at 12 h as the therapeutic window following severe TBI. We examined gene expression changes following TBI and methamphetamine treatment to further define the potential molecular mechanisms of neuroprotection and determined that methamphetamine significantly reduced the expression of key pro-inflammatory signals. Pharmacokinetic analysis revealed that a 24-hour intravenous infusion of methamphetamine at a dose of 0.500 mg/kg/h produced a plasma Cmax value of 25.9 ng/ml and a total exposure of 544 ng/ml over a 32 hour time frame. This represents almost half the 24-hour total exposure predicted for a daily oral dose of 25mg in a 70 kg adult human. Thus, we have demonstrated that methamphetamine is neuroprotective when delivered up to 12 h after injury at doses that are compatible with current FDA approved levels.

Cancer-derived mutations in KEAP1 impair NRF2 degradation but not ubiquitination

NRF2 is a transcription factor that mediates stress responses. Oncogenic mutations in NRF2 localize to one of its two binding interfaces with KEAP1, an E3 ubiquitin ligase that promotes proteasome-dependent degradation of NRF2. Somatic mutations in KEAP1 occur commonly in human cancer, where KEAP1 may function as a tumor suppressor. These mutations distribute throughout the KEAP1 protein but little is known about their functional impact. In this study, we characterized 18 KEAP1 mutations defined in a lung squamous cell carcinoma tumor set. Four mutations behaved as wild-type KEAP1, thus are likely passenger events. R554Q, W544C, N469fs, P318fs, and G333C mutations attenuated binding and suppression of NRF2 activity. The remaining mutations exhibited hypomorphic suppression of NRF2, binding both NRF2 and CUL3. Proteomic analysis revealed that the R320Q, R470C, G423V, D422N, G186R, S243C, and V155F mutations augmented the binding of KEAP1 and NRF2. Intriguingly, these "super-binder" mutants exhibited reduced degradation of NRF2. Cell-based and in vitro biochemical analyses demonstrated that despite its inability to suppress NRF2 activity, the R320Q "superbinder" mutant maintained the ability to ubiquitinate NRF2. These data strengthen the genetic interactions between KEAP1 and NRF2 in cancer and provide new insight into KEAP1 mechanics.

A p21-ZEB1 complex inhibits epithelial-mesenchymal transition through the microRNA 183-96-182 cluster

The tumor suppressor p21 acts as a cell cycle inhibitor and has also been shown to regulate gene expression by functioning as a transcription corepressor. Here, we identified p21-regulated microRNAs (miRNAs) by sequencing small RNAs from isogenic p21(+/+) and p21(-/-) cells. Three abundant miRNA clusters, miR-200b-200a-429, miR-200c-141, and miR-183-96-182, were downregulated in p21-deficient cells. Consistent with the known function of the miR-200 family and p21 in inhibition of the epithelial-mesenchymal transition (EMT), we observed EMT upon loss of p21 in multiple model systems. To explore a role of the miR-183-96-182 cluster in EMT, we identified its genome-wide targets and found that miR-183 and miR-96 repressed common targets, including SLUG, ZEB1, ITGB1, and KLF4. Reintroduction of miR-200, miR-183, or miR-96 in p21(-/-) cells inhibited EMT, cell migration, and invasion. Conversely, antagonizing miR-200 and miR-183-96-182 cluster miRNAs in p21(+/+) cells increased invasion and elevated the levels of VIM, ZEB1, and SLUG mRNAs. Furthermore, we found that p21 forms a complex with ZEB1 at the miR-183-96-182 cluster promoter to inhibit transcriptional repression of this cluster by ZEB1, suggesting a reciprocal feedback loop.

Metabolic regulation by p53 family members

The function of p53 is best understood in response to genotoxic stress, but increasing evidence suggests that p53 also plays a key role in the regulation of metabolic homeostasis. p53 and its family members directly influence various metabolic pathways, enabling cells to respond to metabolic stress. These functions are likely to be important for restraining the development of cancer but could also have a profound effect on the development of metabolic diseases, including diabetes. A better understanding of the metabolic functions of p53 family members may aid in the identification of therapeutic targets and reveal novel uses for p53-modulating drugs.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Oncogene PKCε controls INrf2-Nrf2 interaction in normal and cancer cells through phosphorylation of INrf2

The INrf2 (Keap1)-Nrf2 cell sensor complex has a crucial role in protection against chemical- and radiation-induced oxidative stress and cellular transformation. INrf2, in association with Cul3-Rbx1, ubiquitylates and degrades Nrf2. Exposure to stressors leads to stabilization of Nrf2 and the coordinated activation of cytoprotective proteins and cellular protection. However, the molecular signal(s) that regulate control of Nrf2 by INrf2 remain elusive. In this report, we demonstrate that phosphorylation of INrf2 at Ser599 and Ser602 by the oncoprotein PKCε is essential for INrf2-Nrf2 interaction, and the subsequent ubiquitylation and degradation of Nrf2. Inhibition of PKCε, knockdown of PKCε and the INrf2S602A mutant all failed to phosphorylate INrf2, leading to loss of the INrf2-Nrf2 interaction, Nrf2 degradation and enhanced cytoprotection and drug resistance. Molecular modeling analyses revealed that phosphorylation of S599 exposes the deeply buried S602 for phosphorylation and enhanced INrf2-Nrf2 interaction. Analysis of human lung and liver tumor protein arrays showed lower PKCε and higher Nrf2 levels, which presumably promoted cancer cell survival and drug resistance. In conclusion, phosphorylation of INrf2 by PKCε leads to regulation of Nrf2, with significant implications for the survival of cancer cells, which often express lower levels of PKCε.

Keap1-Nrf2 signaling pathway: mechanisms of regulation and role in protection of cells against toxicity caused by xenobiotics and electrophiles

The transcription factor Nrf2 governs the expression of a considerable group of genes involved in cell protection against oxidants, electrophiles, and genotoxic compounds. The activity of Nrf2 is sensitive to xenobiotics and endogenous electrophiles. Nrf2 is negatively regulated by specific suppressor protein Keap1, which is also a receptor of electrophiles and adapter for Cul3 ubiquitin ligase. Electrophiles react with critical thiol groups of Keap1 leading to the loss of its ability to inhibit Nrf2. The Keap1-Nrf2 signaling pathway also down-regulates NF-κB transcriptional activity and attenuates cytokine-mediated induction of proinflammatory genes. Pharmacological activation of the Keap1-Nrf2 pathway can be used for treatment and prevention of many diseases. Widely known natural Keap1-Nrf2 activators include curcumin, quercetin, resveratrol, and sulforaphane. The most effective Keap1-Nrf2 activators are synthetic oleanane triterpenoids.

Nrf2 is a potential therapeutic target in radioresistance in human cancer

Radiation therapy can effectively kill cancer cells through ROS generation. Cancer cells with upregulated antioxidant systems can develop high radioresistance ability, and the transcription factor NF-E2-related factor 2 (Nrf2) is a key regulator of the antioxidant system. Currently, there are numerous data indicating the important role of Nrf2 in cancer radioresistance. In this review, we summarize the aberrant regulation of Nrf2 in radioresistant cells and discuss the effects and underlying mechanism of Nrf2 in promoting radioresistance. These findings suggest that Nrf2 might be a potential therapeutic target in cancer radiation resistance or a promising radioprotector for normal organs during radiation therapy in the future.

Regulatory flexibility in the Nrf2-mediated stress response is conferred by conformational cycling of the Keap1-Nrf2 protein complex

The transcription factor NF-E2 p45-related factor 2 (Nrf2), a master regulator of cytoprotective genes, is controlled by dimeric Kelch-like ECH associated protein 1 (Keap1), a substrate adaptor protein for Cullin3/RING-box protein 1 ubiquitin ligase, which normally targets Nrf2 for ubiquitination and degradation but loses this ability in response to electrophiles and oxidants (inducers). By using recombinant proteins and populations of cells, some of the general features of the regulation of Nrf2 by Keap1 have been outlined. However, how the two proteins interact at a single-cell level is presently unknown. We now report the development of a quantitative Förster resonance energy transfer-based system using multiphoton fluorescence lifetime imaging microscopy and its application for investigating the interaction between Nrf2 and Keap1 in single live cells. By using this approach, we found that under homeostatic conditions, the interaction between Keap1 and Nrf2 follows a cycle in which the complex sequentially adopts two distinct conformations: "open," in which Nrf2 interacts with a single molecule of Keap1, followed by "closed," in which Nrf2 binds to both members of the Keap1 dimer. Inducers disrupt this cycle by causing accumulation of the complex in the closed conformation without release of Nrf2. As a consequence, free Keap1 is not regenerated, and newly synthesized Nrf2 is stabilized. On the basis of these findings, we propose a model we have named the "cyclic sequential attachment and regeneration model of Keap1-mediated degradation of Nrf2." This previously unanticipated dynamism allows rapid transcriptional responses to environmental changes and can accommodate multiple modes of regulation.

Epigenetic mechanisms in cerebral ischemia

Treatment efficacy for ischemic stroke represents a major challenge. Despite fundamental advances in the understanding of stroke etiology, therapeutic options to improve functional recovery remain limited. However, growing knowledge in the field of epigenetics has dramatically changed our understanding of gene regulation in the last few decades. According to the knowledge gained from animal models, the manipulation of epigenetic players emerges as a highly promising possibility to target diverse neurologic pathologies, including ischemia. By altering transcriptional regulation, epigenetic modifiers can exert influence on all known pathways involved in the complex course of ischemic disease development. Beneficial transcriptional effects range from attenuation of cell death, suppression of inflammatory processes, and enhanced blood flow, to the stimulation of repair mechanisms and increased plasticity. Most striking are the results obtained from pharmacological inhibition of histone deacetylation in animal models of stroke. Multiple studies suggest high remedial qualities even upon late administration of histone deacetylase inhibitors (HDACi). In this review, the role of epigenetic mechanisms, including histone modifications as well as DNA methylation, is discussed in the context of known ischemic pathways of damage, protection, and regeneration.

NRF2 and p53: Januses in cancer?

The transcription factor nuclear factor (erythroid-derived 2)-like 2, also known as NFE2L2 or NRF2, is a master regulator of the anti-oxidative stress response and positively controls the expression of a battery of anti-oxidative stress response proteins and enzymes implicated in detoxification and glutathione generation. Although its detoxifying activity is important in cancer prevention, it has recently been shown that cancer cells also exploit its protective functions to thrive and resist chemotherapy. NRF2 was also shown to the pentose phosphate pathway and glutaminolysis, which promotes purine synthesis for supporting rapid proliferation and glutathione for providing anti-oxidative stress protection. Evidence obtained from cancer patients and cell lines suggest that NRF2 is highly active in a variety of human cancers and is associated with aggressiveness. p53 is a tumor suppressor that also promotes an anti-oxidative stress metabolic program and glutaminolysis. Here we will discuss the similarities between NRF2 and p53 and review evidence that p53 might be exploited by cancer cells to gain protection against oxidative stress, as is the case for NRF2. We discuss findings of co-regulation between these transcription factors and propose possible therapeutic strategies that can be used for treatment of cancers that harbor WT p53 and express high levels of NRF2.

Role of nrf2 in oxidative stress and toxicity

Organismal life encounters reactive oxidants from internal metabolism and environmental toxicant exposure. Reactive oxygen and nitrogen species cause oxidative stress and are traditionally viewed as being harmful. On the other hand, controlled production of oxidants in normal cells serves useful purposes to regulate signaling pathways. Reactive oxidants are counterbalanced by complex antioxidant defense systems regulated by a web of pathways to ensure that the response to oxidants is adequate for the body's needs. A recurrent theme in oxidant signaling and antioxidant defense is reactive cysteine thiol-based redox signaling. The nuclear factor erythroid 2-related factor 2 (Nrf2) is an emerging regulator of cellular resistance to oxidants. Nrf2 controls the basal and induced expression of an array of antioxidant response element-dependent genes to regulate the physiological and pathophysiological outcomes of oxidant exposure. This review discusses the impact of Nrf2 on oxidative stress and toxicity and how Nrf2 senses oxidants and regulates antioxidant defense.

BRCA1 interacts with Nrf2 to regulate antioxidant signaling and cell survival

BRCA1 deficiency results in impaired Nrf2-mediated antioxidant responses followed by cell death, with estradiol rescuing the effect by inducing Nrf2 stabilization.

Oxidative stress plays an important role in cancer development and treatment. Recent data implicate the tumor suppressor BRCA1 in regulating oxidative stress, but the molecular mechanism and the impact in BRCA1-associated tumorigenesis remain unclear. Here, we show that BRCA1 regulates Nrf2-dependent antioxidant signaling by physically interacting with Nrf2 and promoting its stability and activation. BRCA1-deficient mouse primary mammary epithelial cells show low expression of Nrf2-regulated antioxidant enzymes and accumulate reactive oxygen species (ROS) that impair survival in vivo. Increased Nrf2 activation rescues survival and ROS levels in BRCA1-null cells. Interestingly, 53BP1 inactivation, which has been shown to alleviate several defects associated with BRCA1 loss, rescues survival of BRCA1-null cells without restoring ROS levels. We demonstrate that estrogen treatment partially restores Nrf2 levels in the absence of BRCA1. Our data suggest that Nrf2-regulated antioxidant response plays a crucial role in controlling survival downstream of BRCA1 loss. The ability of estrogen to induce Nrf2 posits an involvement of an estrogen-Nrf2 connection in BRCA1 tumor suppression. Lastly, BRCA1-mutated tumors retain a defective antioxidant response that increases the sensitivity to oxidative stress. In conclusion, the role of BRCA1 in regulating Nrf2 activity suggests important implications for both the etiology and treatment of BRCA1-related cancers.

Identification and functional studies of a new Nrf2 partner IQGAP1: a critical role in the stability and transactivation of Nrf2

AIMS

Nuclear factor-erythroid-related factor 2 (Nrf2) is a critical transcriptional factor that is used in regulating cellular defense against oxidative stress. This study is aimed at investigating new interacting protein partners of Nrf2 using One-strep tag pull-down coupled with LTQ Orbitrap LC/MS/MS, and at examining the impact on Nr2 signaling by the newly identified IQ motif containing GTPase activating protein 1 (IQGAP1).

RESULTS

Using the One-strep tag pull-down and LTQ Orbitrap LC/MS/MS, we identified IQGAP1 as a new Nrf2 interacting partner. Direct interactions between IQGAP1 and Nrf2 proteins were verified using in vitro glutathione S-transferase (GST) pull-down, transcription/translation assays, and in vivo utilizing Nrf2 overexpressing cells. Coexpression of Dsredmono-IQGAP1 and eGFP-Nrf2 increased the stability of eGFP-Nrf2 and enhanced the expression of Nrf2-target gene heme oxygenase-1 (HO-1). To confirm the functional role of IQGAP1 on Nrf2, knock-downed IQGAP1 using siIQGAP1 attenuated the expression of endogenous Nrf2, HO-1 proteins, and Nrf2-target genes GSTpi, GCLC, and

NAD(P)H

quinone oxidoreductase 1 (NQO-1). Furthermore, the stability of Nrf2 was dramatically decreased in IQGAP1-deficient mouse embryonic fibroblast (MEF) cells. Since IQGAP1 signaling could be mediated by calcium, treating the cells with calcium showed the translocation of IQGAP1/Nrf2 complex into the nucleus, suggesting that IQGAP1 may play a critical role in Nrf2 stability. Interestingly, consistent with calcium signaling for IQGAP1, treating the cells with calcium functionally enhanced Nrf2-mediated antioxidant responsive element-transcription activity and enhanced the expression of the endogenous Nrf2-target gene HO-1.

INNOVATION

In the aggregate, our current study identifies and functionally characterizes a new Nrf2 partner protein IQGAP1, which may contribute to Nrf2's regulation of antioxidant enzymes such as HO-1.

CONCLUSION

IQGAP1 may play a critical role in the stability and transactivation of Nrf2.

Sulforaphane inhibits PDGF-induced proliferation of rat aortic vascular smooth muscle cell by up-regulation of p53 leading to G1/S cell cycle arrest

Vascular diseases such as atherosclerosis and restenosis artery angioplasty are associated with vascular smooth muscle cell (VSMC) proliferation and intimal thickening arterial walls. In the present study, we investigated the inhibitory effects of sulforaphane, an isothiocyanate produced in cruciferous vegetables, on VSMC proliferation and neointimal formation in a rat carotid artery injury model. Sulforaphane at the concentrations of 0.5, 1.0, and 2.0 μM significantly inhibited platelet-derived growth factor (PDGF)-BB-induced VSMC proliferation in a concentration-dependent manner, determined by cell count. The IC50 value of sulforaphane-inhibited VSMC proliferation was 0.8 μM. Sulforaphane increased the cyclin-dependent kinase inhibitor p21 and p53 levels, while it decreased CDK2 and cyclin E expression. The effects of sulforaphane on vascular thickening were determined 14 days after the injury to the rat carotid artery. The angiographic mean luminary diameters of the group treated with 2 and 4 μM sulforaphane were 0.25±0.1 and 0.09±0.1 mm², respectively, while the value of the control groups was 0.40±0.1 mm², indicating that sulforaphane may inhibit neointimal formation. The expression of PCNA, maker for cell cycle arrest, was decreased, while that of p53 and p21 was increased, which showed the same pattern as one in in-vitro study. These results suggest that sulforaphane-inhibited VSMC proliferation may occur through the G1/S cell cycle arrest by up-regulation of p53 signaling pathway, and then lead to the decreased neointimal hyperplasia thickening. Thus, sulforaphane may be a promising candidate for the therapy of atherosclerosis and post-angiography restenosis.

USP15 negatively regulates Nrf2 through deubiquitination of Keap1

Nrf2 is a master regulator of the antioxidant response. Under basal conditions, Nrf2 is polyubiquitinated by the Keap1-Cul3 E3 ligase and degraded by the 26S proteasome. In response to Nrf2 inducers there is a switch in polyubiquitination from Nrf2 to Keap1. Currently, regulation of the Nrf2-Keap1 pathway by ubiquitination is largely understood. However, the mechanism responsible for removal of ubiquitin conjugated to Nrf2 or Keap1 remains unknown. Here we report that the deubiquitinating enzyme, USP15, specifically deubiquitinates Keap1, which suppresses the Nrf2 pathway. We demonstrated that deubiquitinated Keap1 incorporates into the Keap1-Cul3-E3 ligase complex more efficiently, enhancing the complex stability and enzymatic activity. Consequently, there is an increase in Nrf2 protein degradation and a reduction in Nrf2 target gene expression. Furthermore, USP15-siRNA enhances chemoresistance of cells through upregulation of Nrf2. These findings further our understanding of how the Nrf2-Keap1 pathway is regulated, which is imperative in targeting this pathway for chemoprevention or chemotherapy.

Toward clinical application of the Keap1-Nrf2 pathway

The Keap1-Nrf2 pathway plays a crucial role in determining the sensitivity of cells to chemical and/or oxidative insults by regulating the basal and inducible expression of detoxification and antioxidant enzymes, ABC transporters, and other stress response enzymes and/or proteins. Increasing attention has been focused on the roles that the Keap1-Nrf2 pathway plays in the protection of our body against drug toxicity and stress-induced diseases. Simultaneously, Nrf2 has been recognized to promote oncogenesis and resistance to chemotherapeutic drugs. Cancer cells hijack Nrf2 activity to support their malignant growth and thus Nrf2 has emerged as a therapeutic target. Translational studies of the Keap1-Nrf2 system, from mechanistic understanding to clinical applications, are now important to improve human health.

RXRα inhibits the NRF2-ARE signaling pathway through a direct interaction with the Neh7 domain of NRF2

The transcription factor NRF2 (NFE2L2) is a pivotal activator of genes encoding cytoprotective and detoxifying enzymes that limit the action of cytotoxic therapies in cancer. NRF2 acts by binding antioxidant response elements (ARE) in its target genes, but there is relatively limited knowledge about how it is negatively controlled. Here, we report that retinoic X receptor alpha (RXRα) is a hitherto unrecognized repressor of NRF2. RNAi-mediated knockdown of RXRα increased basal ARE-driven gene expression and induction of ARE-driven genes by the NRF2 activator tert-butylhydroquinone (tBHQ). Conversely, overexpression of RXRα decreased ARE-driven gene expression. Biochemical investigations showed that RXRα interacts physically with NRF2 in cancer cells and in murine small intestine and liver tissues. Furthermore, RXRα bound to ARE sequences in the promoters of NRF2-regulated genes. RXRα loading onto AREs was concomitant with the presence of NRF2, supporting the hypothesis that a direct interaction between the two proteins on gene promoters accounts for the antagonism of ARE-driven gene expression. Mutation analyses revealed that interaction between the two transcription factors involves the DNA-binding domain of RXRα and a region comprising amino acids 209-316 in human NRF2 that had not been defined functionally, but that we now designate as the NRF2-ECH homology (Neh) 7 domain. In non-small cell lung cancer cells where NRF2 levels are elevated, RXRα expression downregulated NRF2 and sensitized cells to the cytotoxic effects of therapeutic drugs. In summary, our findings show that RXRα diminishes cytoprotection by NRF2 by binding directly to the newly defined Neh7 domain in NRF2.

Concurrent regulation of the transcription factors Nrf2 and ATF4 mediates the enhancement of glutathione levels by the flavonoid fisetin

Glutathione (GSH) and GSH-associated metabolism provide the major line of defense for the protection of cells from various forms of toxic stress. GSH also plays a key role in regulating the intracellular redox environment. Thus, maintenance of GSH levels is developing into an important therapeutic objective for the treatment of a variety of diseases. Among the transcription factors that play critical roles in GSH metabolism are NF-E2-related factor 2 (Nrf2) and activating transcription factor 4 (ATF4). Thus, compounds that can upregulate these transcription factors may be particularly useful as treatment options through their effects on GSH metabolism. We previously showed that the flavonoid fisetin not only increases basal levels of GSH but also maintains GSH levels under oxidative stress conditions. However, the mechanisms underlying these effects have remained unknown until now. Here we show that fisetin rapidly increases the levels of both Nrf2 and ATF4 as well as Nrf2- and ATF4-dependent gene transcription via distinct mechanisms. Although fisetin greatly increases the stability of both Nrf2 and ATF4, only the effect on ATF4 is dependent on protein kinase activity. Using siRNA we found that ATF4, but not Nrf2, is important for fisetin's ability to increase GSH levels under basal conditions whereas both ATF4 and Nrf2 appear to cooperate to increase GSH levels under oxidative stress conditions. Based upon these results, we hypothesize that compounds able to increase GSH levels via multiple mechanisms, such as fisetin, will be particularly effective for maintaining GSH levels under a variety of different stresses.

Multiple functions of Kip-related protein5 connect endoreduplication and cell elongation

Despite considerable progress in our knowledge regarding the cell cycle inhibitor of the Kip-related protein (KRP) family in plants, less is known about the coordination of endoreduplication and cell differentiation. In animals, the role of cyclin-dependent kinase (CDK) inhibitors as multifunctional factors coordinating cell cycle regulation and cell differentiation is well documented and involves not only the inhibition of CDK/cyclin complexes but also other mechanisms, among them the regulation of transcription. Interestingly, several plant KRPs have a punctuated distribution in the nucleus, suggesting that they are associated with heterochromatin. Here, one of these chromatin-bound KRPs, KRP5, has been studied in Arabidopsis (Arabidopsis thaliana). KRP5 is expressed in endoreduplicating cells, and loss of KRP5 function decreases endoreduplication, indicating that KRP5 is a positive regulator of endoreduplication. This regulation relies on several mechanisms: in addition to its role in cyclin/CDK kinase inhibition previously described, chromatin immunoprecipitation sequencing data combined with transcript quantification provide evidence that KRP5 regulates the transcription of genes involved in cell wall organization. Furthermore, KRP5 overexpression increases chromocenter decondensation and endoreduplication in the Arabidopsis trithorax-related protein5 (atxr5) atxr6 double mutant, which is deficient for the deposition of heterochromatin marks. Hence, KRP5 could bind chromatin to coordinately control endoreduplication and chromatin structure and allow the expression of genes required for cell elongation.

Proteomic analysis of ubiquitin ligase KEAP1 reveals associated proteins that inhibit NRF2 ubiquitination

Somatic mutations in the KEAP1 ubiquitin ligase or its substrate NRF2 (NFE2L2) commonly occur in human cancer, resulting in constitutive NRF2-mediated transcription of cytoprotective genes. However, many tumors display high NRF2 activity in the absence of mutation, supporting the hypothesis that alternative mechanisms of pathway activation exist. Previously, we and others discovered that via a competitive binding mechanism, the proteins WTX (AMER1), PALB2, and SQSTM1 bind KEAP1 to activate NRF2. Proteomic analysis of the KEAP1 protein interaction network revealed a significant enrichment of associated proteins containing an ETGE amino acid motif, which matches the KEAP1 interaction motif found in NRF2. Like WTX, PALB2, and SQSTM1, we found that the dipeptidyl peptidase 3 (DPP3) protein binds KEAP1 via an "ETGE" motif to displace NRF2, thus inhibiting NRF2 ubiquitination and driving NRF2-dependent transcription. Comparing the spectrum of KEAP1-interacting proteins with the genomic profile of 178 squamous cell lung carcinomas characterized by The Cancer Genome Atlas revealed amplification and mRNA overexpression of the DPP3 gene in tumors with high NRF2 activity but lacking NRF2 stabilizing mutations. We further show that tumor-derived mutations in KEAP1 are hypomorphic with respect to NRF2 inhibition and that DPP3 overexpression in the presence of these mutants further promotes NRF2 activation. Collectively, our findings further support the competition model of NRF2 activation and suggest that "ETGE"-containing proteins such as DPP3 contribute to NRF2 activity in cancer.

The regulation of cellular metabolism by tumor suppressor p53

As a hallmark of tumor cells, metabolic alterations play a critical role in tumor development and could be targeted for tumor therapy. Tumor suppressor p53 plays a central role in tumor prevention. As a transcription factor, p53 mainly exerts its function in tumor suppression through its transcriptional regulation of its target genes to initiate various cellular responses. Cell cycle arrest, apoptosis and senescence are most well-understood functions of p53, and are traditionally accepted as the major mechanisms for p53 in tumor suppression. Recent studies have revealed a novel function of p53 in regulation of cellular metabolism. p53 regulates mitochondrial oxidative phosphorylation, glycolysis, glutamine metabolism, lipid metabolism, and antioxidant defense. Through the regulation of these metabolic processes, p53 maintains the homeostasis of cellular metabolism and redox balance in cells, which contributes significantly to the role of p53 as a tumor suppressor. Further understanding of the role and molecular mechanism of p53 in cellular metabolism could lead to the identification of novel targets and development of novel strategies for tumor therapy.

Reactive oxygen species regulate FSH-induced expression of vascular endothelial growth factor via Nrf2 and HIF1α signaling in human epithelial ovarian cancer

Follicle-stimulating hormone (FSH) and the FSH receptor contribute to tumor angiogenesis and are acknowledged risk factors for ovarian epithelial cancer (OEC). Accumulating evidence suggests that FSH can induce vascular endothelial growth factor (VEGF) and hypoxia inducible factor 1α (HIF1α) expression. We previously demonstrated that FSH induces reactive oxygen species (ROS) production and activates Nrf2 signaling. This study was performed to investigate whether FSH induces VEGF expression via a ROS-mediated Nrf2 signaling pathway. In the current study, OET cells were treated with FSH; dichlorofluorescein staining was used to determine ROS generation, western blotting was used to quantify Nrf2 expression and VEGF expression was measured using an ELISA. Nrf2 and HIF1α were knocked down using siRNAs to investigate the role of the Nrf2 and HIF1α signaling pathways in FSH-induced VEGF expression. The chromatin immunoprecipitation assay (ChIP) was used to determine HIF1α binding to the VEGF promoter. Finally, it was found that FSH induced ROS production and activated Nrf2 signaling; elimination of ROS or knockdown of Nrf2 blocked FSH-induced VEGF expression. Knockdown of Nrf2 impaired HIF1α signaling activation. Blockage of the FSH-ROS-Nrf2-HIF1α signaling pathway attenuated FSH-induced binding of HIF1α to the VEGF promoter. Collectively, this study indicates that ROS and aberrant expression of Nrf2 play an important role in FSH-induced angiogenesis in OEC, and provides insight into the mechanisms of FSH-induced VEGF expression. Elimination of ROS or inhibition of Nrf2 may represent potential therapeutic targets for the treatment of ovarian cancer.

The transcription factor NF-E2-related factor 2 (Nrf2): a protooncogene?

The transcription factor Nrf2 is responsible for regulating a battery of antioxidant and cellular protective genes, primarily in response to oxidative stress. A member of the cap 'n' collar family of transcription factors, Nrf2 activation is tightly controlled by a series of signaling events. These events can be separated into the basal state, a preinduction response, gene induction, and finally a postinduction response, culminating in the restoration of redox homeostasis. However, despite the immensely intricate level of control the cellular environment imposes on Nrf2 activity, there are many opportunities for perturbations to arise in the signaling events that favor carcinogenesis and, therefore, implicate Nrf2 as both a tumor suppressor and a protooncogene. Herein, we highlight the ways in which Nrf2 is regulated, and discuss some of the Nrf2-inducible antioxidant (NQO1, NQO2, HO-1, GCLC), antiapoptotic (Bcl-2), metabolic (G6PD, TKT, PPARγ), and drug efflux transporter (ABCG2, MRP3, MRP4) genes. In addition, we focus on how Nrf2 functions as a tumor suppressor under normal conditions and how its ability to detoxify the cellular environment makes it an attractive target for other oncogenes either via stabilization or degradation of the transcription factor. Finally, we discuss some of the ways in which Nrf2 is being considered as a therapeutic target for cancer treatment.

Sestrins activate Nrf2 by promoting p62-dependent autophagic degradation of Keap1 and prevent oxidative liver damage

Sestrins (Sesns) protect cells from oxidative stress. The mechanism underlying the antioxidant effect of Sesns has remained unknown, however. The Nrf2-Keap1 pathway provides cellular defense against oxidative stress by controlling the expression of antioxidant enzymes. We now show that Sesn1 and Sesn2 interact with the Nrf2 suppressor Keap1, the autophagy substrate p62, and the ubiquitin ligase Rbx1 and that the antioxidant function of Sesns is mediated through activation of Nrf2 in a manner reliant on p62-dependent autophagic degradation of Keap1. Sesn2 was upregulated in the liver of mice subjected to fasting or subsequent refeeding with a high-carbohydrate, fat-free diet, whereas only refeeding promoted Keap1 degradation and Nrf2 activation, because only refeeding induced p62 expression. Ablation of Sesn2 blocked Keap1 degradation and Nrf2 activation induced by refeeding and thereby increased the susceptibility of the liver to oxidative damage resulting from the acute stimulation of lipogenesis associated with refeeding.

Dehydrozingerone, a structural analogue of curcumin, induces cell-cycle arrest at the G2/M phase and accumulates intracellular ROS in HT-29 human colon cancer cells

Dehydrozingerone (1) is a pungent constituent present in the rhizomes of ginger (Zingiber officinale) and belongs structurally to the vanillyl ketone class. It is a representative of half the chemical structure of curcumin (2), which is an antioxidative yellow pigment obtained from the rhizomes of turmeric (Curcuma longa). Numerous studies have suggested that 2 is a promising phytochemical for the inhibition of malignant tumors, including colon cancer. On the other hand, there have been few studies on the potential antineoplastic properties of 1, and its mode of action based on a molecular mechanism is little known. Therefore, the antiproliferative effects of 1 were evaluated against HT-29 human colon cancer cells, and it was found that 1 dose-dependently inhibited growth at the G2/M phase with up-regulation of p21. Dehydrozingerone additionally led to the accumulation of intracellular ROS, although most radical scavengers could not clearly repress the cell-cycle arrest at the G2/M phase. Furthermore, two synthetic isomers of 1 (iso-dehydrozingerone, 3, and ortho-dehydrozingerone, 4) were also examined. On comparing of their activities, accumulation of intracellular ROS was found to be interrelated with growth-inhibitory effects. These results suggest that analogues of 1 may be potential chemotherapeutic agents for colon cancer.

The Keap1-Nrf2 system in cancers: stress response and anabolic metabolism

The Keap1–Nrf2 [Kelch-like ECH-associated protein 1–nuclear factor (erythroid-derived 2)-like 2] pathway plays a central role in the protection of cells against oxidative and xenobiotic stresses. Nrf2 is a potent transcription activator that recognizes a unique DNA sequence known as the antioxidant response element (ARE). Under normal conditions, Nrf2 binds to Keap1 in the cytoplasm, resulting in proteasomal degradation. Following exposure to electrophiles or reactive oxygen species, Nrf2 becomes stabilized, translocates into the nucleus, and activates the transcription of various cytoprotective genes. Increasing attention has been paid to the role of Nrf2 in cancer cells because the constitutive stabilization of Nrf2 has been observed in many human cancers with poor prognosis. Recent studies have shown that the antioxidant and detoxification activities of Nrf2 confer chemo- and radio-resistance to cancer cells. In this review, we provide an overview of the Keap1–Nrf2 system and discuss its role under physiological and pathological conditions, including cancers. We also introduce the results of our recent study describing Nrf2 function in the metabolism of cancer cells. Nrf2 likely confers a growth advantage to cancer cells through enhancing cytoprotection and anabolism. Finally, we discuss the possible impact of Nrf2 inhibitors on cancer therapy.

Does Nrf2 contribute to p53-mediated control of cell survival and death?

In response to oxidative stress, the transcription factor Nrf2 is upregulated and controls activation of many genes that work in concert to defend cells from damages and to maintain cellular redox homeostasis. p53 has been regarded as the guardian of the genome through its pro-oxidant and antioxidant functions. Under low levels of reactive oxygen species (ROS), "normal" amounts of p53 upregulates expression of antioxidant genes, protecting macromolecules from ROS-induced damage. However, at high levels or extended exposure of ROS, p53 expression is enhanced, activating pro-oxidant genes and resulting in p53-dependent apoptosis. We observed a two-phase Nrf2 expression controlled by p53. (i) The induction phase: when p53 expression is relatively low, p53 enhances the protein level of Nrf2 and its target genes to promote cell survival in a p21-dependent manner. (ii) The repression phase: when p53 expression is high, the Nrf2-mediated survival response is inhibited by p53. Our observation leads to the hypothesis that the p53-mediated biphasic regulation of Nrf2 may be key for the tumor-suppressor function of p53 by coordinating cell survival and death pathways.

The Nrf2 cell defence pathway: Keap1-dependent and -independent mechanisms of regulation

The transcription factor Nrf2 (NF-E2-related factor 2) plays a vital role in maintaining cellular homeostasis, especially upon the exposure of cells to chemical or oxidative stress, through its ability to regulate the basal and inducible expression of a multitude of antioxidant proteins, detoxification enzymes and xenobiotic transporters. In addition, Nrf2 contributes to diverse cellular functions including differentiation, proliferation, inflammation and lipid synthesis and there is an increasing association of aberrant expression and/or function of Nrf2 with pathologies including cancer, neurodegeneration and cardiovascular disease. The activity of Nrf2 is primarily regulated via its interaction with Keap1 (Kelch-like ECH-associated protein 1), which directs the transcription factor for proteasomal degradation. Although it is generally accepted that modification (e.g. chemical adduction, oxidation, nitrosylation or glutathionylation) of one or more critical cysteine residues in Keap1 represents a likely chemico-biological trigger for the activation of Nrf2, unequivocal evidence for such a phenomenon remains elusive. An increasing body of literature has revealed alternative mechanisms of Nrf2 regulation, including phosphorylation of Nrf2 by various protein kinases (PKC, PI3K/Akt, GSK-3β, JNK), interaction with other protein partners (p21, caveolin-1) and epigenetic factors (micro-RNAs -144, -28 and -200a, and promoter methylation). These and other processes are potentially important determinants of Nrf2 activity, and therefore may contribute to the maintenance of cellular homeostasis. Here, we dissect evidence supporting these Keap1-dependent and -independent mechanisms of Nrf2 regulation. Furthermore, we highlight key knowledge gaps in this important field of biology, and suggest how these may be addressed experimentally.

A novel small molecule, N-(4-(2-pyridyl)(1,3-thiazol-2-yl))-2-(2,4,6-trimethylphenoxy) acetamide, selectively protects against oxidative stress-induced cell death by activating the Nrf2-ARE pathway: therapeutic implications for ALS

Antioxidant defense is crucial in restoring cellular redox homeostasis. Recent findings have suggested that oxidative stress plays pivotal roles in the pathogenesis of many neurodegenerative diseases. Thus, an anti-oxidative stress remedy might be a promising means for the treatment of such disorders. In this study, we employed a novel ligand-based virtual screening system and identified a novel small molecule, N-(4-(2-pyridyl)(1,3-thiazol-2-yl))-2-(2,4,6-trimethylphenoxy) acetamide (CPN-9), which selectively suppressed oxidative stress-induced cell death in a cell-type-independent manner. CPN-9 upregulates NF-E2-related factor 2 (Nrf2), a key transcriptional regulator of the expression of phase II detoxification enzymes and antioxidant proteins, and Nrf2-regulated factors such as heme oxygenase-1 (HO-1), NAD(P)H quinone oxidoreductase 1 (NQO1), and glutamate-cysteine ligase modifier subunit (GCLM). The CPN-9-mediated upregulation of HO-1, NQO1, and GCLM was abolished by Nrf2 knockdown. Moreover, the antioxidant N-acetylcysteine reduced the protective effect of CPN-9 against oxidative stress-induced cell death with concomitant diminishing of Nrf2 nuclear translocation. These results indicate that CPN-9 exerts its activity via the reactive oxygen species-dependent activation of the Nrf2 signaling pathway in cultured cells. It is noteworthy that the postonset systemic administration of CPN-9 to a transgenic ALS mouse model carrying the H46R mutation in the human Cu/Zn superoxide dismutase (SOD1) gene sustained motor functions and delayed disease progression after onset. Collectively, CPN-9 is a novel Nrf2 activator and a neuroprotective candidate for the treatment of neurodegenerative diseases, including ALS.

Familial melanoma-associated mutations in p16 uncouple its tumor-suppressor functions

Familial melanoma is associated with point mutations in the cyclin-dependent kinase (CDK) inhibitor p16^INK4A (p16). We recently reported that p16 regulates intracellular oxidative stress in a cell cycle-independent manner. Here, we constructed 12 different familial melanoma-associated point mutants spanning the p16 coding region and analyzed their capacity to regulate cell-cycle phase and suppress reactive oxygen species (ROS). Compared to wild-type p16 which fully restored both functions in p16-deficient fibroblasts, various p16 mutants differed in their capacity to normalize ROS and cell cycle profiles. While some mutations did not impair either function, others impaired both. Interestingly, several impaired cell-cycle (R24Q, R99P, V126D) or oxidative function (A36P, A57V, P114S) selectively, indicating that these two functions of p16 can be uncoupled. Similar activities were confirmed with selected mutants in human melanoma cells. Many mutations impairing both cell-cycle and oxidative functions, or only cell cycle function, localize to the third ankyrin repeat of the p16 molecule. Alternatively, most mutations impairing oxidative but not cell-cycle function, or those not impairing either function, lie outside this region. These results demonstrate that particular familial melanoma-associated mutations in p16 can selectively compromise these two independent tumor-suppressor functions, which may be mediated by distinct regions of the protein.

Regulation of phosphatidylinositol-5-phosphate signaling by Pin1 determines sensitivity to oxidative stress

Oxidative signaling and oxidative stress contribute to aging, cancer, and diseases resulting from neurodegeneration. Pin1 is a proline isomerase that recognizes phosphorylated substrates and regulates the localization and conformation of its targets. Pin1(-/-) mice show phenotypes associated with premature aging, yet mouse embryonic fibroblasts (MEFs) from these mice are resistant to hydrogen peroxide (H(2)O(2))-induced cell death. We found that the abundance of phosphatidylinositol-5-phosphate (PtdIns5P) was increased in response to H(2)O(2), an effect that was enhanced in Pin1(-/-) MEFs. Reduction of H(2)O(2)-induced PtdIns5P compromised cell viability in response to oxidative stress, suggesting that PtdIns5P contributed to the enhanced cell viability of Pin1(-/-) MEFs exposed to oxidative stress. The increased PtdIns5P in the Pin1(-/-) MEFs stimulated the expression of genes involved in defense against oxidative stress and reduced the accumulation of reactive oxygen species. Pin1 and PtdIns5P 4-kinases (PIP4Ks), enzymes that phosphorylate and thereby reduce the amount of PtdIns5P, interacted in a manner dependent on the phosphorylation of PIP4K. Although reintroduction of Pin1 into the Pin1(-/-) MEFs reduced the amount of PtdIns5P produced in response to H(2)O(2), in vitro assays indicated that the isomerase activity of Pin1 inhibited PIP4K activity. Whether this isomerise-mediated inhibition of PIP4K occurs in cells remains an open question, but the data suggest that the regulation of PIP4K by Pin1 may be complex.

The Warburg effect: insights from the past decade

Several decades ago, Otto Warburg discovered that cancer cells produce energy predominantly by glycolysis; a phenomenon now termed "Warburg effect". Warburg linked mitochondrial respiratory defects in cancer cells to aerobic glycolysis; this theory of his gradually lost its importance with the lack of conclusive evidence confirming the presence of mitochondrial defects in cancer cells. Scientists began to believe that this altered mechanism of energy production in cancer cells was more of an effect than the cause. More than 50 years later, the clinical use of FDG-PET imaging in the diagnosis and monitoring of cancers rekindled the interest of the scientific community in Warburg's hypothesis. In the last ten years considerable progress in the field has advanced our understanding of the Warburg effect. However, it still remains unclear if the Warburg effect plays a causal role in cancers or it is an epiphenomenon in tumorigenesis. In this review we aim to discuss the molecular mechanisms associated with the Warburg effect with emphasis on recent advances in the field including the role of epigenetic changes, miRNAs and post-translational modification of proteins. In addition, we also discuss emerging therapeutic strategies that target the dependence of cancer cells on altered energy processing through aerobic glycolysis.

Methionine sulfoxide reductase A attenuates heme oxygenase-1 induction through inhibition of Nrf2 activation

Methionine sulfoxide reductase A (MsrA) functions as a protein repair enzyme by catalyzing the stereospecific reduction of methionine-S-sulfoxide to methionine. We previously identified that MsrA deficiency inhibits normal cell growth via activation of the p53-p21 pathway. In this study, we report a critical role of MsrA in expression of heme oxygenase-1 (HO-1), a highly inducible enzyme that has an anti-proliferative effect mediated by up-regulation of p21. Down-regulation of MsrA induced HO-1 expression in mammalian cells with increased p21 levels, but MsrA overexpression did not affect HO-1 expression. MsrA depletion activated Nrf2 by increasing its expression and nuclear translocation. Nrf2 activation was associated with increased reactive oxygen species production. MsrA overexpression in MsrA-depleted cells led to the reduction of increased HO-1 expression, and suppressed nuclear accumulation of Nrf2. Taken together, the data suggest that MsrA attenuates HO-1 induction by inhibiting Nrf2 activation.

Emerging roles of Nrf2 and phase II antioxidant enzymes in neuroprotection

Phase II metabolic enzymes are a battery of critical proteins that detoxify xenobiotics by increasing their hydrophilicity and enhancing their disposal. These enzymes have long been studied for their preventative and protective effects against mutagens and carcinogens and for their regulation via the Keap1 (Kelch-like ECH associated protein 1)/Nrf2 (Nuclear factor erythroid 2 related factor 2)/ARE (antioxidant response elements) pathway. Recently, a series of studies have reported the altered expression of phase II genes in postmortem tissue of patients with various neurological diseases. These observations hint at a role for phase II enzymes in the evolution of such conditions. Furthermore, promising findings reveal that overexpression of phase II genes, either by genetic or chemical approaches, confers neuroprotection in vitro and in vivo. Therefore, there is a need to summarize the current literature on phase II genes in the central nervous system (CNS). This should help guide future studies on phase II genes as therapeutic targets in neurological diseases. In this review, we first briefly introduce the concept of phase I, II and III enzymes, with a special focus on phase II enzymes. We then discuss their expression regulation, their inducers and executors. Following this background, we expand our discussion to the neuroprotective effects of phase II enzymes and the potential application of Nrf2 inducers to the treatment of neurological diseases.

Molecular basis of electrophilic and oxidative defense: promises and perils of Nrf2

Induction of drug-metabolizing enzymes through the antioxidant response element (ARE)-dependent transcription was initially implicated in chemoprevention against cancer by antioxidants. Recent progress in understanding the biology and mechanism of induction revealed a critical role of induction in cellular defense against electrophilic and oxidative stress. Induction is mediated through a novel signaling pathway via two regulatory proteins, the nuclear factor erythroid 2-related factor 2 (Nrf2) and the Kelch-like erythroid cell-derived protein with CNC homology-associated protein 1 (Keap1). Nrf2 binds to Keap1 at a two site-binding interface and is ubiquitinated by the Keap1/cullin 3/ring box protein-1-ubiquitin ligase, resulting in a rapid turnover of Nrf2 protein. Electrophiles and oxidants modify critical cysteine thiols of Keap1 and Nrf2 to inhibit Nrf2 ubiquitination, leading to Nrf2 activation and induction. Induction increases stress resistance critical for cell survival, because knockout of Nrf2 in mice increased susceptibility to a variety of toxicity and disease processes. Collateral to diverse functions of Nrf2, genome-wide search has led to the identification of a plethora of ARE-dependent genes regulated by Nrf2 in an inducer-, tissue-, and disease-dependent manner to control drug metabolism, antioxidant defense, stress response, proteasomal degradation, and cell proliferation. The protective nature of Nrf2 could also be hijacked in a number of pathological conditions by means of somatic mutation, epigenetic alteration, and accumulation of disruptor proteins, promoting drug resistance in cancer and pathologic liver features in autophagy deficiency. The repertoire of ARE inducers has expanded enormously; the therapeutic potential of the inducers has been examined beyond cancer prevention. Developing potent and specific ARE inducers and Nrf2 inhibitors holds certain new promise for the prevention and therapy against cancer, chronic disease, and toxicity.

Regulation of transforming growth factor β1-dependent aldose reductase expression by the Nrf2 signal pathway in human mesangial cells

Aldose reductase (AR) is a key enzyme in the alternative glucose metabolism pathway, the polyol pathway. To date, AR is known to be involved in several secondary complications of diabetes and various kidney diseases. The goal of this study was to elucidate how the Nrf2-anti-oxidant response element (ARE) signal pathway plays a role in TGFβ1's regulation of AR expression in human renal mesangial cells (HRMCs). As an in vitro model system, HRMCs were used to investigate AR mRNA by qPCR, protein by Western blot and enzymatic activity by spectrophotometric assay. The ability of TGFβ1 to induce reactive oxygen species (ROS) in cells was measured by electron-spin resonance (ESR) trapping method. Reporter assays were used to test the activity of the AR promoter region, and ChIP was employed to test the direct binding of Nrf2 with the endogenous AR promoter. Treatment of HRMCs with TGFβ1 up-regulated the expression of AR mRNA, protein, and activity level. Additionally, TGFβ1 rapidly increased cellular ROS levels, which in turn activated the Nrf2-ARE pathway. Either inhibition of ROS production or knockdown of Nrf2 in HRMCs decreased the TGFβ1-induction of AR expression. Nrf2 regulated AR luciferase activity specifically via two AREs within the AR promoter, and bound directly to the endogenous AR promoter. Furthermore, the TGFβ1-mediated expression of AR required Nrf2 and was significantly abrogated in Nrf2-/- cells. These data show the regulation of AR by TGFβ1 is induced by TGFβ1 stimulation of ROS, which activates the Nrf2-ARE pathway allowing Nrf2 to directly increase AR expression in HRMCs.

The critical role of catalase in prooxidant and antioxidant function of p53

The tumor suppressor p53 is an important regulator of intracellular reactive oxygen species (ROS) levels, although downstream mediators of p53 remain to be elucidated. Here, we show that p53 and its downstream targets, p53-inducible ribonucleotide reductase (p53R2) and p53-inducible gene 3 (PIG3), physically and functionally interact with catalase for efficient regulation of intracellular ROS, depending on stress intensity. Under physiological conditions, the antioxidant functions of p53 are mediated by p53R2, which maintains increased catalase activity and thereby protects against endogenous ROS. After genotoxic stress, high levels of p53 and PIG3 cooperate to inhibit catalase activity, leading to a shift in the oxidant/antioxidant balance toward an oxidative status, which could augment apoptotic cell death. These results highlight the essential role of catalase in p53-mediated ROS regulation and suggest that the p53/p53R2-catalase and p53/PIG3-catalase pathways are critically involved in intracellular ROS regulation under physiological conditions and during the response to DNA damage, respectively.

Differential transcriptional profiles mediated by exposure to the cannabinoids cannabidiol and Δ9-tetrahydrocannabinol in BV-2 microglial cells

BACKGROUND AND PURPOSE

Apart from their effects on mood and reward, cannabinoids exert beneficial actions such as neuroprotection and attenuation of inflammation. The immunosuppressive activity of cannabinoids has been well established. However, the underlying mechanisms are largely unknown. We previously showed that the psychoactive cannabinoid Δ(9) -tetrahydrocannabinol (THC) and the non-psychoactive cannabidiol (CBD) differ in their anti-inflammatory signalling pathways.

EXPERIMENTAL APPROACH

To characterize the transcriptional effects of CBD and THC, we treated BV-2 microglial cells with these compounds and performed comparative microarray analysis using the Illumina MouseRef-8 BeadChip platform. Ingenuity Pathway Analysis was performed to identify functional subsets of genes and networks regulated by CBD and/or THC.

KEY RESULTS

Overall, CBD altered the expression of many more genes; from the 1298 transcripts found to be differentially regulated by the treatments, 680 gene probe sets were up-regulated by CBD and 58 by THC, and 524 gene products were down-regulated by CBD and only 36 by THC. CBD-specific gene expression profile showed changes associated with oxidative stress and glutathione depletion, normally occurring under nutrient limiting conditions or proteasome inhibition and involving the GCN2/eIF2α/p8/ATF4/CHOP-TRIB3 pathway. Furthermore, CBD-stimulated genes were shown to be controlled by nuclear factors known to be involved in the regulation of stress response and inflammation, mainly via the (EpRE/ARE)-Nrf2/ATF4 system and the Nrf2/Hmox1 axis.

CONCLUSIONS AND IMPLICATIONS

These observations indicated that CBD, but much less than THC, induced a cellular stress response in microglial cells and suggested that this effect could underlie its anti-inflammatory activity.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

The canonical NF-κB pathway differentially protects normal and human tumor cells from ROS-induced DNA damage

DNA damage responses (DDR) invoke senescence or apoptosis depending on stimulus intensity and the degree of activation of the p53-p21(Cip1/Waf1) axis; but the functional impact of NF-κB signaling on these different outcomes in normal vs. human cancer cells remains poorly understood. We investigated the NF-κB-dependent effects and mechanism underlying reactive oxygen species (ROS)-mediated DDR outcomes of normal human lung fibroblasts (HDFs) and A549 human lung cancer epithelial cells. To activate DDR, ROS accumulation was induced by different doses of H(2)O(2). The effect of ROS induction caused a G2 or G2-M phase cell cycle arrest of both human cell types. However, ROS-mediated DDR eventually culminated in different end points with HDFs undergoing premature senescence and A549 cancer cells succumbing to apoptosis. NF-κB p65/RelA nuclear translocation and Ser536 phosphorylation were induced in response to H(2)O(2)-mediated ROS accumulation. Importantly, blocking the activities of canonical NF-κB subunits with an IκBα super-repressor or suppressing canonical NF-κB signaling by IKKβ knock-down accelerated HDF premature senescence by up-regulating the p53-p21(Cip1/Waf1) axis; but inhibiting the canonical NF-κB pathway exacerbated H(2)O(2)-induced A549 cell apoptosis. HDF premature aging occurred in conjunction with γ-H2AX chromatin deposition, senescence-associated heterochromatic foci and beta-galactosidase staining. p53 knock-down abrogated H(2)O(2)-induced premature senescence of vector control- and IκBαSR-expressing HDFs functionally linking canonical NF-κB-dependent control of p53 levels to ROS-induced HDF senescence. We conclude that IKKβ-driven canonical NF-κB signaling has different functional roles for the outcome of ROS responses in the contexts of normal vs. human tumor cells by respectively protecting them against DDR-dependent premature senescence and apoptosis.

p21 as a transcriptional co-repressor of S-phase and mitotic control genes

It has been previously described that p21 functions not only as a CDK inhibitor but also as a transcriptional co-repressor in some systems. To investigate the roles of p21 in transcriptional control, we studied the gene expression changes in two human cell systems. Using a human leukemia cell line (K562) with inducible p21 expression and human primary keratinocytes with adenoviral-mediated p21 expression, we carried out microarray-based gene expression profiling. We found that p21 rapidly and strongly repressed the mRNA levels of a number of genes involved in cell cycle and mitosis. One of the most strongly down-regulated genes was CCNE2 (cyclin E2 gene). Mutational analysis in K562 cells showed that the N-terminal region of p21 is required for repression of gene expression of CCNE2 and other genes. Chromatin immunoprecipitation assays indicated that p21 was bound to human CCNE2 and other p21-repressed genes gene in the vicinity of the transcription start site. Moreover, p21 repressed human CCNE2 promoter-luciferase constructs in K562 cells. Bioinformatic analysis revealed that the CDE motif is present in most of the promoters of the p21-regulated genes. Altogether, the results suggest that p21 exerts a repressive effect on a relevant number of genes controlling S phase and mitosis. Thus, p21 activity as inhibitor of cell cycle progression would be mediated not only by the inhibition of CDKs but also by the transcriptional down-regulation of key genes.

The yin and yang of nrf2-regulated selenoproteins in carcinogenesis

The NF-E2-related factor-2 (Nrf2) is a transcription factor which regulates the major cellular defense systems and thereby contributes to the prevention of many diseases including cancer. Selenium deficiency is associated with a higher cancer risk making also this essential trace element a promising candidate for cancer prevention. Two selenoproteins, thioredoxin reductase-1 (TrxR1) and glutathione peroxidase-2 (GPx2), are targets for Nrf2. Selenium deficiency activates Nrf2 as does a TrxR1 knockout making a synergism between both systems plausible. Although this might hold true for healthy cells, the interplay may turn into the opposite in cancer cells. The induction of the detoxifying and antioxidant enzymes by Nrf2 will make cancer cells chemoresistant and will protect them against oxidative damage. The essential role of TrxR1 in maintaining proliferation makes its upregulation in cancer cells detrimental. The anti-inflammatory potential of GPx2 will help to inhibit cancer initiation and inflammation-triggered promotion, but its growth supporting potential will also support tumor growth. This paper considers beneficial and adverse consequences of the activation of Nrf2 and the selenoproteins which appear to depend on the cancer stage.

Heme oxygenase, inflammation, and fibrosis: the good, the bad, and the ugly?

Upon injury, prolonged inflammation and oxidative stress may cause pathological wound healing and fibrosis, leading to formation of excessive scar tissue. Fibrogenesis can occur in most organs and tissues and may ultimately lead to organ dysfunction and failure. The underlying mechanisms of pathological wound healing still remain unclear, and are considered to be multifactorial, but so far, no efficient anti-fibrotic therapies exist. Extra- and intracellular levels of free heme may be increased in a variety of pathological conditions due to release from hemoproteins. Free heme possesses pro-inflammatory and oxidative properties, and may act as a danger signal. Effects of free heme may be counteracted by heme-binding proteins or by heme degradation. Heme is degraded by heme oxygenase (HO) that exists as two isoforms: inducible HO-1 and constitutively expressed HO-2. HO generates the effector molecules biliverdin/bilirubin, carbon monoxide, and free iron/ferritin. HO deficiency in mouse and man leads to exaggerated inflammation following mild insults, and accumulating epidemiological and preclinical studies support the widely recognized notion of the cytoprotective, anti-oxidative, and anti-inflammatory effects of the activity of the HO system and its effector molecules. In this review, we address the potential effects of targeted HO-1 induction or administration of HO-effector molecules as therapeutic targets in fibrotic conditions to counteract inflammatory and oxidative insults. This is exemplified by various clinically relevant conditions, such as hypertrophic scarring, chronic inflammatory liver disease, chronic pancreatitis, and chronic graft rejection in transplantation.