Hypoxia, oxidative stress, and the interplay of HIFs and NRF2 signaling in cancer

Oxygen is crucial for life and acts as the final electron acceptor in mitochondrial energy production. Cells adapt to varying oxygen levels through intricate response systems. Hypoxia-inducible factors (HIFs), including HIF-1α and HIF-2α, orchestrate the cellular hypoxic response, activating genes to increase the oxygen supply and reduce expenditure. Under conditions of excess oxygen and resulting oxidative stress, nuclear factor erythroid 2-related factor 2 (NRF2) activates hundreds of genes for oxidant removal and adaptive cell survival. Hypoxia and oxidative stress are core hallmarks of solid tumors and activated HIFs and NRF2 play pivotal roles in tumor growth and progression. The complex interplay between hypoxia and oxidative stress within the tumor microenvironment adds another layer of intricacy to the HIF and NRF2 signaling systems. This review aimed to elucidate the dynamic changes and functions of the HIF and NRF2 signaling pathways in response to conditions of hypoxia and oxidative stress, emphasizing their implications within the tumor milieu. Additionally, this review explored the elaborate interplay between HIFs and NRF2, providing insights into the significance of these interactions for the development of novel cancer treatment strategies.

Emerging Role of NRF2 Signaling in Cancer Stem Cell Phenotype

Cancer stem cells (CSCs) are a small population of tumor cells characterized by self-renewal and differentiation capacity. CSCs are currently postulated as the driving force that induces intra-tumor heterogeneity leading to tumor initiation, metastasis, and eventually tumor relapse. Notably, CSCs are inherently resistant to environmental stress, chemotherapy, and radiotherapy due to high levels of antioxidant systems and drug efflux transporters. In this context, a therapeutic strategy targeting the CSC-specific pathway holds a promising cure for cancer. NRF2 (nuclear factor erythroid 2-like 2; NFE2L2) is a master transcription factor that regulates an array of genes involved in the detoxification of reactive oxygen species/electrophiles. Accumulating evidence suggests that persistent NRF2 activation, observed in multiple types of cancer, supports tumor growth, aggressive malignancy, and therapy resistance. Herein, we describe the core properties of CSCs, focusing on treatment resistance, and review the evidence that demonstrates the roles of NRF2 signaling in conferring unique properties of CSCs and the associated signaling pathways.

High Levels of Hyaluronic Acid Synthase-2 Mediate NRF2-Driven Chemoresistance in Breast Cancer Cells

Hyaluronic acid (HA), a ligand of CD44, accumulates in some types of tumors and is responsible for tumor progression. The nuclear factor erythroid 2-like 2 (NRF2) regulates cytoprotective genes and drug transporters, which promotes therapy resistance in tumors. Previously, we showed that high levels of CD44 are associated with NRF2 activation in cancer stem like-cells. Herein, we demonstrate that HA production was increased in doxorubicin-resistant breast cancer MCF7 cells (MCF7-DR) via the upregulation of HA synthase-2 (HAS2). HA incubation increased NRF2, aldo-keto reductase 1C1 (AKR1C1), and multidrug resistance gene 1 (MDR1) levels. Silencing of HAS2 or CD44 suppressed NRF2 signaling in MCF7-DR, which was accompanied by increased doxorubicin sensitivity. The treatment with a HAS2 inhibitor, 4-methylumbelliferone (4-MU), decreased NRF2, AKR1C1, and MDR1 levels in MCF7-DR. Subsequently, 4-MU treatment inhibited sphere formation and doxorubicin resistance in MCF7-DR. The Cancer Genome Atlas (TCGA) data analysis across 32 types of tumors indicates the amplification of HAS2 gene is a common genetic alteration and is negatively correlated with the overall survival rate. In addition, high HAS2 mRNA levels are associated with increased NRF2 signaling and poor clinical outcome in breast cancer patients. Collectively, these indicate that HAS2 elevation contributes to chemoresistance and sphere formation capacity of drug-resistant MCF7 cells by activating CD44/NRF2 signaling, suggesting a potential benefit of HAS2 inhibition.

Role of CD133/NRF2 Axis in the Development of Colon Cancer Stem Cell-Like Properties

Cancer stem cells (CSCs) exhibit intrinsic therapy/stress resistance, which often cause cancer recurrence after therapy. In this study, we investigated the potential relationship between the cluster of differentiation (CD)-133, a CSC marker of colon cancer, and nuclear factor erythroid 2-like 2 (NFE2L2; NRF2), a master transcription factor for the regulation of multiple antioxidant genes. In the first model of CSC, a sphere culture of the colorectal cell line HCT116, showed increased levels of CD133 and NRF2. Silencing of CD133 reduced the levels of CSC markers, such as Kruppel-like factor 4 (KLF4) and ATP-binding cassette subfamily G member 2 (ABCG2), and further suppressed the expression levels of NRF2 and its target genes. As a potential molecular link, CD133-mediated activation of phosphoinositide 3-kinase/serine-threonine kinase (PI3K/AKT) signaling appears to increase the NRF2 protein levels via phosphorylation and the consequent inhibition of glycogen synthase kinase (GSK)-3β. Additionally, NRF2-silenced HCT116 cells showed attenuated sphere formation capacity and reduced CSC markers expression, indicating the critical role of the NRF2 pathway in the development of CSC-like properties. As a second model of CSC, the CD133^high cell population was isolated from HCT116 cells. CSC-like properties, including sphere formation, motility, migration, colony formation, and anticancer resistance, were enhanced in the CD133^high population compared to CD133^low HCT116 cells. Levels of NRF2, which were elevated in CD133^high HCT116, were suppressed by CD133-silencing. In line with these, the analysis of The Cancer Genome Atlas (TCGA) database showed that high levels of CD133 expression are correlated with increased NRF2 signaling, and alterations in CD133 gene or expression are associated with unfavorable clinical outcome in colorectal carcinoma patients. These results indicate that the CD133/NRF2 axis contributes to the development of CSC-like properties in colon cancer cells, and that PI3K/AKT signaling activation is involved in CD133-mediated NRF2 activation.

The multifaceted role of NRF2 in cancer progression and cancer stem cells maintenance

The transcription factor nuclear factor erythroid 2-like 2 (NEF2L2; NRF2) plays crucial roles in the defense system against electrophilic or oxidative stress by upregulating an array of genes encoding antioxidant proteins, electrophile/reactive oxygen species (ROS) detoxifying enzymes, and drug efflux transporters. In contrast to the protective roles in normal cells, the multifaceted role of NRF2 in tumor growth and progression, resistance to therapy and intratumoral stress, and metabolic adaptation is rapidly expanding, and the complex association of NRF2 with cancer signaling networks is being unveiled. In particular, the implication of NRF2 signaling in cancer stem cells (CSCs), a small population of tumor cells responsible for therapy resistance and tumor relapse, is emerging. Here, we described the dark side of NRF2 signaling in cancers discovered so far. A particular focus was put on the role of NRF2 in CSCs maintenance and therapy resistance, showing that low ROS levels and refractory drug response of CSCs are mediated by the activation of NRF2 signaling. A better understanding of the roles of the NRF2 pathway in CSCs will allow us to develop a novel therapeutic approach to control tumor relapse after therapy.

Negative correlation of urinary miR-199a-3p level with ameliorating effects of sarpogrelate and cilostazol in hypertensive diabetic nephropathy

The prevalence of chronic kidney disease is increasing globally; however, effective therapeutic options are limited. In this study, we aimed to identify urinary miRNAs reflecting the effect of therapeutic intervention in rats with comorbid hypertension and diabetes. Additionally, the potential beneficial effects of anti-platelet sarpogrelate and cilostazol were investigated. Nephropathy progression in streptozotocin (STZ)-treated spontaneously hypertensive rats (SHRs), including albuminuria, collagen deposition, and histopathological changes, was alleviated by sarpogrelate and antihypertensive agent telmisartan. Global analysis of urinary miRNAs identified that miR-199a-3p was commonly reduced by sarpogrelate and telmisartan treatment. In vitro analysis suggested CD151 as a target gene of miR-199a-3p: miR-199a-3p overexpression repressed CD151 levels and miR-199a-3p interacted with the 3'-untranslated region of the CD151 gene. In addition, we demonstrated that the miR-199a-3p/CD151 axis is associated with the transforming growth factor-β1 (TGF-β1)-induced fibrogenic pathway. TGF-β1 treatment led to miR-199a-3p elevation and CD151 suppression, and miR-199a-3p overexpression or CD151-silencing enhanced TGF-β1-inducible collagen IV and α-smooth muscle actin (α-SMA) levels. In vivo analysis showed that the decrease in CD151 and the increase in collagen IV and α-SMA in the kidney from STZ-treated SHR were restored by sarpogrelate and telmisartan administration. In an additional animal experiment using cilostazol and telmisartan, there was a correlation between urinary miR-199a-3p reduction and the ameliorating effects of cilostazol or combination with telmisartan. Collectively, these results indicate that urinary miR-199a-3p might be utilized as a marker for nephropathy treatment. We also provide evidence of the benefits of antiplatelet sarpogrelate and cilostazol in nephropathy progression.

NRF2 level is negatively correlated with TGF-β1-induced lung cancer motility and migration via NOX4-ROS signaling

Transforming growth factor-β1 (TGF-β1) is a multifaceted factor in cancer biology that regulates cell proliferation and migration. Overactivation of nuclear factor erythroid 2-like 2 (NFE2L2; NRF2) in cancers has been associated with facilitated tumor growth and therapy resistance; however, role in cancer migration has not been clearly explained yet. In this study, we investigated the role of NRF2 on TGF-β1-induced cell motility/migration. In NRF2-silenced lung cancer A549 cells, both basal and TGF-β1-inducible cell motility/migration increased compared to those in A549. SMAD transcription activity and phosphorylated SMAD2/3 levels were higher in TGF-β1-treated NRF2-low A549 cells than those in A549. Notably, the levels of reactive oxygen species (ROS) that were elevated by TGF-β1 treatment were higher in the NRF2-low A549 than those in control cells, and treatment with ROS scavenger blocked TGF-β1-induced cell motility. As an underlying molecular link, NADPH oxidase 4 (NOX4) was associated with higher ROS elevation and cell motility of NRF2-low A549. NOX4 and TGF-β1-inducible NOX4 levels were higher in NRF2-low A549 cells than those in A549. Moreover, the pharmacological inhibition of NOX4 blocked the TGF-β1-induced motility of NRF2-low A549 cells. Collectively, these results indicate that TGF-β1-induced cell motility/migration is facilitated in NRF2-inhibited lung cancer cells and that high levels of NOX4/ROS are associated with enhanced motility/migration.

Abstract B079: Inhibitory role of NFE2L2/NRF2-silencing in hypoxia-induced metabolic adaptation of cancer cells

The nuclear factor erythroid 2-like-2 (NFE2L2/NRF2) pathway contributes to the environmental resistance of cancers by enhancing the antioxidant capacity. Hypoxia, a common element in the tumor environment, leads to hypoxia-inducible factor-1α (HIF-1α) stabilization to induce the adaptive response to hypoxic condition, including metabolic reprogramming and blood vessel formation. Our global miRNA expression analysis of HT29 and HCT116 human colon cancer cells identified that NRF2 silencing upregulated miR-181c, and this increase was associated with the reduction in mitochondria-encoded cytochrome c oxidase subunit-1 (MT-CO1), a catalytic core subunit of the complex IV of the electron transport chain. Therefore, NRF2-silenced cancer cells exhibited the decreases in the mitochondrial membrane potential, oxygen consumption rate, and cellular ATP contents. Notably, as a result of low oxygen consumption rate, NRF2-inhibited cancer cells failed to accumulate HIF-1a protein under hypoxic conditions. This phenomenon led to differential responses of NRF2-inhibted cancer cells to hypoxic condition when compared to control cells. First, stable knockdown of NRF2 in human colon cancer cells suppressed tumor growth in mouse xenograft settings with a concomitant reduction in blood vessel formation and vascular endothelial growth factor expression. Second, NRF2-silencing in breast cancer cells inhibited metabolic adaptation upon hypoxia, and hypoxic-inducible levels of metabolites involved in glycolysis and pentose phosphate pathway were reduced. Collectively, targeting NRF2/miR-181c could be an effective strategy to counteract HIF-1α-orchestrated adaptation of cancer cells to hypoxic microenvironment.

Citation Format: Mi-Kyoung Kwak, Sujin Lee, Steffanus Pranoto Hallis. Inhibitory role of NFE2L2/NRF2-silencing in hypoxia-induced metabolic adaptation of cancer cells [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B079. doi:10.1158/1535-7163.TARG-19-B079

Correction to: CD4 T cell count is positively associated with lumbar spine bone mass in HIV-infected men under the age of 50 years

Two sentences in the Discussion section were incorrect.

Bardoxolone ameliorates TGF-β1-associated renal fibrosis through Nrf2/Smad7 elevation

Transforming growth factor-β (TGF-β) is a potent pathogenic factor of renal injury through the upregulation of extracellular matrix (ECM) expression and facilitation of renal fibrosis. Nuclear factor erythroid 2-like 2 (Nfe2l2; Nrf2), a master regulator of antioxidant and detoxifying systems, is mainly controlled by the binding with cytosolic protein Kelch-like ECH-associated protein 1 (Keap1) and subsequent proteasomal degradation. The protective effect of Nrf2 on renal injury has been attributed to its antioxidant role, where it aids in coping with oxidative stress-associated progression of renal disease. In this study, we investigated the effect of Nrf2 activation on ECM production and TGF-β/Smad signaling using Keap1-silenced MES-13 cells (a genetic glomerular mesangial cell model with Nrf2 overexpression). The TGF-β1-inducible expression of fibronectin and α-smooth muscle actin (α-Sma) was suppressed and Smad2/3 phosphorylation was blocked in Nrf2-high mesangial cells as compared with that in control cells. Notably, in these Nrf2-high mesangial cells, levels of TGF-β1 receptor 1 (TβR1) were substantially diminished, and the protein levels of Smad7, an inhibitor TGF-β1/Smad signaling, were increased. Nrf2-mediated Smad7 elevation and its anti-fibrotic role in Keap1-silenced cells were confirmed by studies with Nrf2-or Smad7-silencing. As a molecular link for Smad7 elevation in Nrf2-high cells, the reduction of Smad-ubiquitination-regulatory factor 1 (Smurf1), an E3 ubiquitin ligase for Smad7, was notable. Silencing of Smurf1 increased Smad7 in the control mesangial cells; however, forced expression of Smurf1 repressed Smad7 levels in Keap1-silenced cells. Additionally, we demonstrate that bardoxolone (BARD; CDDO-methyl), a pharmacological activator of Nrf2, increased Smad7 levels and attenuated TGF-β/Smad/ECM expression in MES-13. Moreover, in an aristolochic acid (AA)-mediated nephropathy mouse model, the renal expression of Nrf2 and Smad7 was elevated by BARD treatment, and AA-induced tubular necrosis and interstitial fibrosis were substantially ameliorated by BARD. Collectively, these results indicate that the Nrf2-Smad7 axis plays a key role in the protection of TGF-β-induced renal fibrosis, and further suggest a novel molecular mechanism of beneficial effect of BARD on renal disease.

CD4 T cell count is inversely associated with lumbar spine bone mass in HIV-infected men under the age of 50 years

HIV-infected men under the age of 50 years had a lower bone mass compared to that of HIV-uninfected men. Lower CD4 T cell counts, independent of whether antiretroviral therapy (ART) was used, were associated with lower BMD. HIV-infected patients with low CD4 T cell counts may need follow-up and intervention regarding bone health, including younger patients.

INTRODUCTION

HIV-infected patients have a low bone mineral density (BMD) owing to multifactorial interaction between common osteoporosis risk factors and HIV-related factors, including chronic inflammation and ART. Although HIV infection and ART might affect bone metabolism, little data is available for patients aged under 50 years. We aimed to investigate the association of HIV infection-induced low CD4 T cell counts and ART with BMD in men aged under 50 years.

METHODS

We performed an age- and body mass index-matched case-control study. BMD values of HIV-infected and HIV-uninfected men (< 50 years) were compared, and HIV-infected men were stratified by CD4 T cell counts and ART use.

RESULTS

After adjusting confounders, HIV-infected men with CD4 T cell counts ≥ 500 cells/μL (n = 28) and < 500 cells/μL (n = 139) had lower BMD at the femoral neck (FN, p < 0.001) and total hip (TH, p < 0.001) than HIV-uninfected men (n = 167). HIV-infected men with CD4 T cell counts < 500/μL had lower BMD at the lumbar spine (LS, p = 0.034) than those with counts of ≥ 500 cells/μL, but not at FN and TH. The CD4 T cell count (γ = 0.169, p = 0.031) was positively correlated with BMD at LS. There was no significant difference in the BMD (p = 0.499-> 0.999) between the ART-naïve (n = 75) and ART-user group (n = 92).

CONCLUSIONS

Despite their relatively younger age, HIV-infected men had a lower BMD than HIV-uninfected men. Lower CD4 T cell counts, irrespective of ART, might result in lower bone mass.

Impairment of HIF-1α-mediated metabolic adaption by NRF2-silencing in breast cancer cells

Hypoxia, a common element in the tumor environment, leads to Hypoxia-Inducible Factor-1α (HIF-1α) stabilization to modulate cellular metabolism as an adaptive response. In a previous study, we showed that inhibition of the nuclear factor erythroid 2-like-2 (NFE2L2; NRF2), a master regulator of many genes coping with electrophilic and oxidative stress, elevated the level of miR-181c and induced mitochondrial dysfunction in colon cancer cells. In this study, we demonstrate that NRF2-silencing hindered HIF-1α accumulation in hypoxic breast cancer cells and subsequently suppressed hypoxia-inducible expression of glycolysis-associated glucose transporter-1, hexokinase-2, pyruvate dehydrogenase kinase-1, and lactate dehydrogenase A. HIF-1α dysregulation in NRF2-silenced cancer cells was associated with miR-181c elevation. Overexpression of miR-181c in breast cancer cells blocked HIF-1α accumulation and diminished hypoxia-inducible levels of glycolysis enzymes, whereas the inhibition of miR-181c in NRF2-silenced cells restored HIF-1α accumulation. In a subsequent metabolomic analysis, hypoxic incubation increased the levels of metabolites involved in glycolysis and activated the pentose phosphate pathway (PPP) in control cells. However, these elevations were less pronounced in NRF2-silenced cells. In particular, hypoxic incubation increased the levels of amino acids, which implies a shift to catabolic metabolism, and the increased levels were higher in control cells than in NRF2-silenced cells. Concurrently, hypoxia activated BCL2 interacting protein 3 (BNIP3)-mediated autophagy in the control cells and miR-181c was found to be involved in this autophagy activation. Taken together, these results show that hypoxia-induced metabolic changes to glycolysis, the PPP, and autophagy are inhibited by NRF2-silencing through miR-181c-mediated HIF-1α dysregulation. Therefore, targeting NRF2/miR-181c could be an effective strategy to counteract HIF-1α-orchestrated metabolic adaptation of hypoxic cancer cells.

Association of blood n-3 fatty acid with bone mass and bone marrow TRAP-5b in the elderly with and without hip fracture

The plasma n-3 fatty acid level was 26.2% lower in patients with osteoporotic hip fracture than in those with osteoarthritis. In all patients, n-3 fatty acid was positively associated with bone mineral density and inversely associated with tartrate-resistant acid phosphatase-5b level in bone marrow aspirates, reflecting the bone microenvironment.

INTRODUCTION

Despite the potential beneficial role of n-3 fatty acid (FA) on bone metabolism, the specific mechanisms underlying these effects in humans remain unclear. Here, we assessed whether the plasma n-3 level, as an objective indicator of its status, is associated with osteoporosis-related phenotypes and bone-related markers in human bone marrow (BM) samples.

METHODS

This was a case-control and cross-sectional study conducted in a clinical unit. n-3 FA in the blood and bone biochemical markers in the BM aspirates were measured by gas chromatography/mass spectrometry and immunoassay, respectively. BM fluids were collected from 72 patients who underwent hip surgery because of either osteoporotic hip fracture (HF; n = 28) or osteoarthritis (n = 44).

RESULTS

After adjusting for confounders, patients with HF had 26.2% lower plasma n-3 levels than those with osteoarthritis (P = 0.006), and each standard deviation increment in plasma n-3 was associated with a multivariate-adjusted odds ratio of 0.40 for osteoporotic HF (P = 0.010). In multivariate analyses including all patients, a higher plasma n-3 level was associated with higher bone mass at the lumbar spine (β = 0.615, P = 0.002) and total femur (β = 0.244, P = 0.045). Interestingly, the plasma n-3 level was inversely associated with the tartrate-resistant acid phosphatase-5b level (β = - 0.633, P = 0.023), but not with the bone-specific alkaline phosphatase level, in BM aspirates.

CONCLUSIONS

These findings provide clinical evidence that n-3 FA is a potential inhibitor of osteoclastogenesis that favors human bone health.

Transfer printing of vertical-type microscale light-emitting diode array onto flexible substrate using biomimetic stamp

We report the transfer printing of GaN-based microscale vertical-type light-emitting diodes (μ-VLEDs) using a functional layer and a biomimetic stamp. An oxide-based functional layer is inserted onto the structure of a μ-VLED and used to separate the chip from the μ-VLED wafer by absorbing the pulse of a UV pulse laser during pick-up of the transfer printing process. Polydimethylsiloxane (PDMS)-based biomimetic stamps have been fabricated to mimic the gecko lizard cilia for improved adhesion and repeatability. The biomimetic stamp has an adhesion force of 25.6 N/cm², which is 12 times the adhesion of a flat stamp; an adhesion force of 10 N/cm² or more was maintained after 100,000 repeated adhesion tests. A flexible 10 × 10 prototype array on a polyimide substrate was fabricated, and its bending test results indicated that the strain effect on the forward voltage and the output power was less than 1%. The stable bending test results of the prototype indicate that μ-VLEDs using biomimetic stamps allow the necessary stability for practical transfer printing.

The association of cortisol and adrenal androgen with trabecular bone score in patients with adrenal incidentaloma with and without autonomous cortisol secretion

Despite ethnic differences in cortisol sensitivity, only one study in Caucasians has assessed trabecular bone score (TBS) in patients with subclinical hypercortisolism (SH). We showed that both subtle cortisol excess and reduced adrenal androgen may contribute to impaired bone quality in Asian women with SH.

INTRODUCTION

One study in Caucasians has assessed trabecular bone score (TBS), an index of bone microstructure, in adrenal incidentaloma (AI) patients with subclinical hypercortisolism (SH). There are ethnic differences in cortisol sensitivities between Caucasian and Asian populations. We investigated the associations of cortisol and the adrenal androgen dehydroepiandrosterone-sulfate (DHEA-S) with TBS in AI patients with SH, adrenal Cushing's syndrome (CS), and nonfunctional AI (NFAI).

METHODS

We measured TBS, cortisol levels after the overnight 1 mg dexamethasone suppression test (1 mg DST), and cortisol/DHEA-S in 61 patients with SH (30 men; 31 women), 19 with adrenal CS (4 men; 15 women), and 355 with NFAI (213 men; 142 women).

RESULTS

After adjusting for confounders, the serum cortisol level after 1 mg DST was inversely correlated with TBS in men (β = -0.133, P = 0.045) and women (β = - 0.140, P = 0.048). Higher cortisol/DHEA-S ratio was associated with lower TBS in women (β = - 0.252, P < 0.001), but not men. This inverse association of cortisol/DHEA-S ratio in women remained statistically significant after adjusting for the serum cortisol level after 1 mg DST (β = - 0.221, P = 0.008). Compared with women with NFAI, women with SH had 2.2% lower TBS (P = 0.040). Deteriorated bone microstructure (TBS < 1.230) was associated with the serum cortisol level after 1 mg DST (odds ratio [OR], 2.18; 95% confidence interval [CI], 1.04-4.53) and cortisol/DHEA-S ratio (OR, 2.05; 95% CI, 1.03-4.08).

CONCLUSIONS

Subtle cortisol excess in both genders and reduced DHEA-S, especially in women, may contribute to impaired bone quality in Asian patients with SH.

Regulatory crosstalk between the oxidative stress-related transcription factor Nfe2l2/Nrf2 and mitochondria

Mitochondria play essential roles in cellular bioenergetics, biosynthesis, and apoptosis. During the process of respiration and oxidative phosphorylation, mitochondria utilize oxygen to generate ATP, and at the same time, there is an inevitable generation of reactive oxygen species (ROS). As excess ROS create oxidative stress and damage cells, the proper function of the antioxidant defense system is critical for eukaryotic cell survival under aerobic conditions. Nuclear factor, erythroid 2-like 2 (Nfe2l2/Nrf2) is a master transcription factor for regulating basal as well as inducible expression of multiple antioxidant proteins. Nrf2 has been involved in maintaining mitochondrial redox homeostasis by providing reduced forms of glutathione (GSH); the reducing cofactor NADPH; and mitochondrial antioxidant enzymes such as GSH peroxidase 1, superoxide dismutase 2, and peroxiredoxin 3/5. In addition, recent research advances suggest that Nrf2 contributes to mitochondrial regulation through more divergent intermolecular linkages. Nrf2 has been positively associated with mitochondrial biogenesis through the direct upregulation of mitochondrial transcription factors and is involved in the mitochondrial quality control system through mitophagy activation. Moreover, several mitochondrial proteins participate in regulating Nrf2 to form a reciprocal regulatory loop between mitochondria and Nrf2. Additionally, Nrf2 modulation in cancer cells leads to changes in the mitochondrial respiration system and cancer bioenergetics that overall affect cancer metabolism. In this review, we describe recent experimental observations on the relationship between Nrf2 and mitochondria, and further discuss the effects of Nrf2 on cancer mitochondria and metabolism.

Overexpression of CD44 Standard Isoform Upregulates HIF-1α Signaling in Hypoxic Breast Cancer Cells

Cluster of differentiation 44 (CD44), a cell surface receptor for hyaluronic acid (HA), is involved in aggressive cancer phenotypes. Herein, we investigated the role of the CD44 standard isoform (CD44s) in hypoxia-inducible factor-1α (HIF-1α) regulation using MCF7 overexpressing CD44s (pCD44s-MCF7). When pCD44s-MCF7 was incubated under hypoxia, levels of HIF-1α, vascular endothelial growth factor, and the HIF-1α response element-derived luciferase activity were significantly increased compared to those in the control MCF7. Incubation of pCD44s-MCF7 cells with HA further increased HIF-1α accumulation, and the silencing of CD44s attenuated HIF-1α elevation, which verifies the role of CD44s in HIF-1α regulation. In addition, the levels of phosphorylated extracellular signal-regulated kinase (ERK) was higher in hypoxic pCD44s-MCF7 cells, and HIF-1α accumulation was diminished by the pharmacological inhibitors of ERK. CD44s-mediated HIF-1α augmentation resulted in two functional outcomes. First, pCD44s-MCF7 cells showed facilitated cell motility under hypoxia via the upregulation of proteins associated with epithelial-mesenchymal transition, such as SNAIL1 and ZEB1. Second, pCD44s-MCF7 cells exhibited higher levels of glycolytic proteins, such as glucose transporter-1, and produced higher levels of lactate under hypoxa. As a consequence of the enhanced glycolytic adaptation to hypoxia, pCD44s-MCF7 cells exhibited a higher rate of cell survival under hypoxia than that of the control MCF7, and glucose deprivation abolished these differential responses of the two cell lines. Taken together, these results suggest that CD44s activates hypoxia-inducible HIF-1α signaling via ERK pathway, and the CD44s-ERK-HIF-1α pathway is involved in facilitated cancer cell viability and motility under hypoxic conditions.

High CD44 expression mediates p62-associated NFE2L2/NRF2 activation in breast cancer stem cell-like cells: Implications for cancer stem cell resistance

Cluster of differentiation 44 (CD44) is the most common cancer stem cell (CSC) marker and high CD44 expression has been associated with anticancer drug resistance, tumor recurrence, and metastasis. In this study, we aimed to investigate the molecular mechanism by which CD44 and nuclear factor erythroid 2-like 2 (NFE2L2; NRF2), a key regulator of antioxidant genes, are linked to CSC resistance using CD44high breast CSC-like cells. NRF2 expression was higher in CD44high cell populations isolated from doxorubicin-resistant MCF7 (ADR), as well as MCF7, MDA-MB231, and A549 cells, than in corresponding CD44low cells. High NRF2 expression in the CD44highCD24low CSC population (ADR44P) established from ADR cells depended on standard isoform of CD44. Silencing of CD44 or overexpression of CD44 resulted in the reduction or elevation of NRF2, respectively, and treatment with hyaluronic acid, a CD44 ligand, augmented NRF2 activation. As functional implications, NRF2 silencing rendered ADR44P cells to retain higher levels of reactive oxygen species and to be sensitive to anticancer drug toxicity. Moreover, NRF2-silenced ADR44P cells displayed tumor growth retardation and reduced colony/sphere formation and invasion capacity. In line with these, CD44 significantly colocalized with NRF2 in breast tumor clinical samples. The molecular mechanism of CD44-mediated NRF2 activation was found to involve high p62 expression. CD44 elevation led to an increase in p62, and inhibition of p62 resulted in NRF2 suppression in ADR44P. Collectively, our results showed that high CD44 led to p62-associated NRF2 activation in CD44high breast CSC-like cells. NRF2 activation contributed to the aggressive phenotype, tumor growth, and anticancer drug resistance of CD44high CSCs. Therefore, the CD44-NRF2 axis might be a promising therapeutic target for the control of stress resistance and survival of CD44high CSC population within breast tumors.

NFE2L2/NRF2 Activity Is Linked to Mitochondria and AMP-Activated Protein Kinase Signaling in Cancers Through miR-181c/Mitochondria-Encoded Cytochrome c Oxidase Regulation

AIMS

The nuclear factor (erythroid-derived 2)-like 2 (NFE2L2; NFE2L2/NRF2) pathway contributes to the environmental resistance of cancers by enhancing the antioxidant capacity. Here, we explored the potential connection between NFE2L2/NRF2 and mitochondrial function in cancers.

RESULTS

Global miRNA expression analysis of HT29 and HCT116 human colon cancer cells identified that NFE2L2/NRF2 silencing upregulated miR-181c through nuclear factor-κB signaling, and this increase was associated with the reduction in mitochondria-encoded cytochrome c oxidase subunit-1 (MT-CO1), a catalytic core subunit of the complex IV of the electron transport chain (ETC). As a result of ETC dysfunction, NFE2L2/NRF2-silenced cancer cells exhibited the decreases in the mitochondrial membrane potential, oxygen consumption rate, and cellular adenosine triphosphate (ATP) contents. Notably, these changes induced adenosine monophosphate (AMP)-activated protein kinase-α (AMPKα) activation and subsequent metabolic adaptation signaling, including the inhibition of fatty acid and sterol biosynthesis enzymes. As supportive evidence of AMPKα-driven adaption, NFE2L2/NRF2-silenced cells were more vulnerable to AMPKα inhibition-induced growth suppression. Similarly, mouse tumor xenografts derived from NFE2L2/NRF2-silenced HT29 exhibited MT-CO1 reduction and AMPKα activation, thereby increasing responsiveness to the AMPK inhibitor treatment. The association of NFE2L2/NRF2 with MT-CO1 and AMPKα was confirmed in breast cancer cells.

INNOVATION

We demonstrated the significance of NFE2L2/NRF2 in cancer mitochondria by elucidating the involvement of miR-181c/MT-CO1 as underlying molecular events. We also provide evidence of the crosstalk between NFE2L2/NRF2 and AMPKα as an adaptive link in cancers.

CONCLUSION

Therefore, it may be an effective strategy to inhibit both NFE2L2/NRF2 and AMPKα signaling to overcome adaptive behaviors of cancer. Antioxid. Redox Signal. 27, 945-961.

Abstract 82: MiR-206-mediated c-MET suppression modulates BCRP/ABCG2 levels in NRF2-silenced cancer cells

Breast cancer resistance protein (BCRP/ABCG2), a xenobiotic efflux transporter, is responsible for anticancer resistance in tumors. NF-E2-related factor 2 (NRF2) is a critical transcription factor in cellular defense system by regulating the expression of antioxidant and detoxifying enzymes; however, its overexpression is often associated with tumor resistance to chemotherapy. In the present study, we demonstrate that NRF2 modulation affects the expression of hepatocyte growth factor receptor (HGFR/c-MET) and consequently suppresses BCRP/ABCG2 activity in cancer cells. Interfering RNA-mediated stable inhibition of NRF2 in both ovarian carcinoma SKOV3 and renal carcinoma A498 cell lines reduced the expression of c-MET and this was accompanied by BCRP/ABCG2 down-regulation. The treatment of cells with pharmacological or genetic inhibitor of c-MET decreased BCRP/ABCG2 level and subsequently increased intracellular accumulation of doxorubicin and Hoechst 33342, indicating a link between c-MET and BCRP/ABCG2 activity. As a potential molecular mechanism of altered c-MET expression, miR-206 was identified as a predictive regulator of c-MET. As experimental evidence, transient and stable expressions of miR-206 in SKOV3 and A498 cells repressed c-MET and BCRP/ABCG2 levels. In addition, NRF2-knockdown cancer cells expressed higher levels of miR-206 compared to the control cells, and the treatment of NRF2 knockdown cells with the miR-206 inhibitor could restore c-MET and BCRP/ABCG levels. Collectively, our results showed that the NRF2 silencing-mediated miR-206 regulation could suppress BCRP/ABCG2 levels through c-MET modulation, which is providing an additional evidence of chemosensitization of tumor cells by NRF2 inhibition.

Citation Format: Bo-hyun Choi, In-geun Ryoo, Donghyeok Kim, Sujin Lee, Mi-Kyoung Kwak. MiR-206-mediated c-MET suppression modulates BCRP/ABCG2 levels in NRF2-silenced cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 82. doi:10.1158/1538-7445.AM2017-82

Abstract 1502: The link between NRF2 and mitochondria through the regulation of miR-181c/mitochondria-encoded cytochrome c oxidase subunit-1

The nuclear factor erythroid 2-related factor 2 (NRF2) pathway is involved in the environmental resistance of cancers by enhancing the antioxidant capacity. In the current study, we investigated the potential link between NRF2 and mitochondrial function in cancer cells. Global miRNA expression analysis of HT29 and HCT116 identified miR-181c as an NRF2-silencing-inducible miRNA, and miR-181c elevation was associated with the decrease in mitochondria-encoded cytochrome c oxidase subunit-1 (MT-CO1), a mitochondrial genome-encoded complex IV subunit of the electron transport chain. As a result of decreased complex IV activity, NRF2-silenced cancer cells exhibited reduced levels of mitochondrial membrane potential (MMP), oxygen consumption rate, and ATP production. Notably, these changes induced adaptive activation of AMP-activated protein kinase-α (AMPKα) and thereby, NRF2-silenced cells were more vulnerable to AMPK inhibitor-induced growth suppression. Similarly, mouse tumor xenografts derived from NRF2-silenced HT29 exhibited MT-CO1 reduction and AMPKα activation, thereby increasing responsiveness to the AMPK inhibitor treatment. Collectively, we showed a novel link of NRF2 to cancer mitochondria and AMPK signaling by elucidating miR-181c/MT-CO1 signaling as an underlying molecular mechanism. These results also suggest that it may an effective strategy to inhibit both NRF2 and AMPK to control adaptive tumor responses.

Citation Format: Mi-Kyoung Kwak, Kyeong-Ah Jung, Donghyeok Kim, Sujin Lee. The link between NRF2 and mitochondria through the regulation of miR-181c/mitochondria-encoded cytochrome c oxidase subunit-1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1502. doi:10.1158/1538-7445.AM2017-1502

Tempo-spatial Activation of Sequential Quadruple Stimuli for High Gene Expression of Polymeric Gene Nanocomplexes

The clinical application of intracellular gene delivery via nanosized carriers is hindered by intracellular multistep barriers that limit high levels of gene expression. To solve these issues, four different intracellular or external stimuli that can efficiently activate a gene carrier, a gene, or a photosensitizer (pheophorbide A [PhA]) were assessed in this study. The designed nanosized polymeric gene complexes were composed of PhA-loaded thiol-degradable polycation (PhA@RPC) and cytomegalovirus (CMV) promoter-equipped pDNA. After cellular internalization of the resulting PhA@RPC/pDNA complexes, the complexes escaped endosomal sequestration, owing to the endosomal pH-induced endosomolytic activity of RPC in PhA@RPC. Subsequently, intracellular thiol-mediated polycation degradation triggered the release of PhA and pDNA from the complexes. Late exposure to light (for example, 12 h post-treatment) activated the released PhA and resulted in the production of reactive oxygen species (ROS). Intracellular ROS successively activated NF-κB, which then reactivated the CMV promoter in the pDNA. These sequential, stimuli-responsive chemical and biological reactions resulted in high gene expression. In particular, the time-point of light exposure was very significant to tune efficient gene expression as well as negligible cytotoxicity: early light treatment induced photochemical internalization but high cytotoxicity, whereas late light treatment influenced the reactivation of silent pDNA via PhA-generated ROS and activation of NF-κB. In conclusion, the quadruple triggers, such as pH, thiol, light, and ROS, successively influenced a gene carrier (RPC), a photosensitizer, and a genetic therapeutic, and the tempo-spatial activation of the designed quadruple stimuli-activatable nanosized gene complexes could be potential in gene delivery applications.

Amelioration of high fat diet-induced nephropathy by cilostazol and rosuvastatin

Multiple comorbidities of metabolic disorders are associated with facilitated chronic kidney disease progression. Anti-platelet cilostazol is used for the treatment of peripheral artery disease. In this study, we investigated the potential beneficial effects of cilostazol and rosuvastatin on metabolic disorder-induced renal dysfunctions. C57BL/6 mice that received high fat diet (HFD) for 22 weeks and a low dose of streptozotocin (STZ, 40 mg/kg) developed albuminuria and had increased urinary cystatin C excretion, and cilostazol treatment (13 weeks) improved these markers. Histopathological changes, including glomerular mesangial expansion, tubular vacuolization, apoptosis, and lipid accumulation were ameliorated by cilostazol treatment. Tubulointerstitial fibrosis that was indicated by the increases in collagen and transforming growth factor-β1 subsided by cilostazol. Renoprotective effects were also observed in rosuvastatin-treated mice, and combinatorial treatment with cilostazol and rosuvastatin demonstrated enhanced ameliorative effects in histopathological evaluations. Notably, repressed renal heme oxygenase-1 (Ho-1) level in HFD/STZ mice was restored in cilostazol group. Further, we demonstrated that cilostazol enhanced Nrf2/Ho-1 signaling in cultured proximal tubular epithelial cells. Collectively, these results suggest the potential advantageous use of cilostazol as an adjunctive therapy with statins for the amelioration of metabolic disorder-associated renal injury.

Involvement of NRF2 Signaling in Doxorubicin Resistance of Cancer Stem Cell-Enriched Colonospheres

Cancer stem cells (CSCs) are a subset of tumor cells, which are characterized by resistance against chemotherapy and environmental stress, and are known to cause tumor relapse after therapy. A number of molecular mechanisms underlie the chemoresistance of CSCs, including high expression levels of drug efflux transporters. We investigated the role of the antioxidant transcription factor NF-E2-related factor 2 (NRF2) in chemoresistance development, using a CSC-enriched colonosphere system. HCT116 colonospheres were more resistant to doxorubicin-induced cell death and expressed higher levels of drug efflux transporters such as P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) compared to HCT116 monolayers. Notably, levels of NRF2 and expression of its target genes were substantially elevated in colonospheres, and these increases were linked to doxorubicin resistance. When NRF2 expression was silenced in colonospheres, Pgp and BCRP expression was downregulated, and doxorubicin resistance was diminished. Collectively, these results indicate that NRF2 activation contributes to chemoresistance acquisition in CSC-enriched colonospheres through the upregulation of drug efflux transporters.

Activation of NRF2 by p62 and proteasome reduction in sphere-forming breast carcinoma cells

Cancer stem cells (CSCs) express high levels of drug efflux transporters and antioxidant genes, and are therefore believed to be responsible for cancer recurrence following chemo/radiotherapy intervention. In this study, we investigated the role of NF-E2-related factor 2 (NRF2), a master regulator of antioxidant gene expression, in the growth and stress resistance of CSC-enriched mammosphere. The MCF7 mammospheres expressed significantly higher levels of the NRF2 protein and target gene expression compared to the monolayer. As underlying mechanisms, we observed that proteolytic activity and expression of the proteasome catalytic subunits were decreased in the mammospheres. Additionally, mammospheres retained a high level of p62 and the silencing of p62 was observed to attenuate NRF2 activation. NRF2 increase was confirmed in sphere-cultures of the colon and ovarian cancer cells. The functional implication of NRF2 was demonstrated in NRF2-knockdown mammospheres. NRF2-silenced mammospheres demonstrated increased cell death and retarded sphere growth as a result of target gene repression. Moreover, unlike the control mammospheres, NRF2-knockdown mammospheres did not develop anticancer drug resistance. Collectively, these results indicated that altered proteasome function and p62 expression caused NRF2 activation in CSC-enriched mammospheres. In addition, NRF2 appeared to play a role in CSC survival and anticancer drug resistance.

Beneficial Effects of Sarpogrelate and Rosuvastatin in High Fat Diet/Streptozotocin-Induced Nephropathy in Mice

Chronic kidney disease (CKD) is a major complication of metabolic disorders such as diabetes mellitus, obesity, and hypertension. Comorbidity of these diseases is the factor exacerbating CKD progression. Statins are commonly used in patients with metabolic disorders to decrease the risk of cardiovascular complications. Sarpogrelate, a selective antagonist of 5-hydroxytryptamine (5-HT) 2A receptor, inhibits platelet aggregation and is used to improve peripheral circulation in diabetic patients. Here, we investigated the effects of sarpogrelate and rosuvastatin on CKD in mice that were subjected to a high fat diet (HFD) for 22 weeks and a single low dose of streptozotocin (STZ, 40 mg/kg). When mice were administrated sarpogrelate (50 mg/kg, p.o.) for 13 weeks, albuminuria and urinary cystatin C excretion were normalized and histopathological changes such as glomerular mesangial expansion, tubular damage, and accumulations in lipid droplets and collagen were significantly improved. Sarpogrelate treatment repressed the HFD/STZ-induced CD31 and vascular endothelial growth factor receptor-2 expressions, indicating the attenuation of glomerular endothelial proliferation. Additionally, sarpogrelate inhibited interstitial fibrosis by suppressing the increases in transforming growth factor-β1 (TGF-β1) and plasminogen activator inhibitor-1 (PAI-1). All of these functional and histological improvements were also seen in rosuvastatin (20 mg/kg) group and, notably, the combinatorial treatment with sarpogrelate and rosuvastatin showed additive beneficial effects on histopathological changes by HFD/STZ. Moreover, sarpogrelate reduced circulating levels of PAI-1 that were elevated in the HFD/STZ group. As supportive in vitro evidence, sarpogrelate incubation blocked TGF-β1/5-HT-inducible PAI-1 expression in murine glomerular mesangial cells. Taken together, sarpogrelate and rosuvastatin may be advantageous to control the progression of CKD in patients with comorbid metabolic disorders, and particularly, the use of sarpogrelate as adjunctive therapy with statins may provide additional benefits on CKD.

Abstract B27: Involvement of c-MET signaling in BCRP/ABCG2 activation and resistance to doxorubicin and photodynamic therapy

Overexpression of BCRP/ABCG2, a xenobiotic efflux transporter, is related to anticancer drug resistance in tumors. Proto-oncogene c-MET induces cancer cell proliferation, motility, and survival, and its aberrant activation was found to be a prognostic factor in advanced ovarian cancers. In this study, we demonstrate the potential cross-resistance of doxorubicin-resistant ovarian cancer cells to the pheophorbide a (Pba)-based photodynamic therapy (PDT) and suggest c-MET and BCRP/ABCG2 overexpression as an underlying molecular mechanism. The doxorubicin resistant A2780 cell line (A2780DR) showed enhanced resistance to PDT cytotoxicity with decreased level of reactive oxygen species generation and Pba accumulation than A2780. In microarray analysis, BCRP/ABCG2 is the sole drug transporter which was upregulated in A2780DR. The incubation with the BCRP/ABCG2 specific inhibitor reversed the both Pba-PDT and doxorubicin resistance in A2780DR. Importantly, we identified that the expression of c-Met, which is an oncogene activating PI3K signaling, was increased in A2780DR and the inhibition of PI3K/AKT or c-MET repressed resistance to doxorubicin and PDT. Finally, we showed that the pharmacological and genetic inhibition of c-MET diminished levels of BCRP/ABCG2 in A2780DR. Collectively, our results provide evidence that c-MET overexpression is linked to BCRP/ABCG2 activation and this mechanism leads to crossresistance to both chemotherapy and PDT.

Citation Format: Bo-hyun Choi, Kyeong-Ah Jung, Mi-Kyoung Kwak. Involvement of c-MET signaling in BCRP/ABCG2 activation and resistance to doxorubicin and photodynamic therapy. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr B27.

Abstract 2224: Functional role of NRF2 signaling in sphere cultured carcinoma cells

Cancer stem cells (CSCs) are known to express high levels of antioxidant proteins and ABC transporters. In this study, we investigated the potential involvement of NF-E2-related factor 2 (NRF2) in CSC survival and stress response by using the MCF7 mammosphere culture, which is accepted as a CSCs-enriched system. The levels of NRF2 protein, reporter transcription activity, and target gene expression such as efflux transporters were higher in mammospheres than in the monolayer. We observed that proteasome activity as well as catalytic subunits expression was decreased in the mammospheres. Additionally, mammospheres showed an elevated p62 protein level and p62-knockdown suppressed NRF2 activation. The role of NRF2 in mammospheres could be demonstrated in NRF2-knockdown mammospheres. Sphere growth was retarded in knockdown group and anticancer drugs resistance was not obtained in NRF2-knockdown mammospheres in contrast to the control mammospheres. NRF2 activation could be confirmed in spheres-culture of colon cancer and ovarian cancer cells. These results show that reduced proteasome function and elevated p62 level contributed to NRF2 activation in CSCs-enriched mammospheres. The study further suggests a potential role of NRF2 in CSCs survival and stress resistance.

Citation Format: In-geun Ryoo, Mi-Kyoung Kwak. Functional role of NRF2 signaling in sphere cultured carcinoma cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2224. doi:10.1158/1538-7445.AM2015-2224

NRF2 Signaling Negatively Regulates Phorbol-12-Myristate-13-Acetate (PMA)-Induced Differentiation of Human Monocytic U937 Cells into Pro-Inflammatory Macrophages

Blood monocytes are recruited to injured tissue sites and differentiate into macrophages, which protect against pathogens and repair damaged tissues. Reactive oxygen species (ROS) are known to be an important contributor to monocytes’ differentiation and macrophages’ function. NF-E2-related factor 2 (NRF2), a transcription factor regulating cellular redox homeostasis, is known to be a critical modulator of inflammatory responses. We herein investigated the role of NRF2 in macrophage differentiation using the human monocytic U937 cell line and phorbol-12-myristate-13-acetate (PMA). In U937 cells with NRF2 silencing, PMA-stimulated cell adherence was significantly facilitated when compared to control U937 cells. Both transcript and protein levels for pro-inflammatory cytokines, including interleukine-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNFα) were highly elevated in PMA-stimulated NRF2-silenced U937 compared to the control. In addition, PMA-inducible secretion of monocyte chemotactic protein 1 (MCP-1) was significantly high in NRF2-silenced U937. As an underlying mechanism, we showed that NRF2-knockdown U937 retained high levels of cellular ROS and endoplasmic reticulum (ER) stress markers expression; and subsequently, PMA-stimulated levels of Ca²⁺ and PKCα were greater in NRF2-knockdown U937 cells, which caused enhanced nuclear accumulation of nuclear factor-ҡB (NFҡB) p50 and extracellular signal-regulated kinase (ERK)-1/2 phosphorylation. Whereas the treatment of NRF2-silenced U937 cells with pharmacological inhibitors of NFҡB or ERK1/2 largely blocked PMA-induced IL-1β and IL-6 expression, indicating that these pathways are associated with cell differentiation. Taken together, our results suggest that the NRF2 system functions to suppress PMA-stimulated U937 cell differentiation into pro-inflammatory macrophages and provide evidence that the ROS-PKCα-ERK-NFҡB axis is involved in PMA-facilitated differentiation of NRF2-silenced U937 cells.

The c-MET/PI3K signaling is associated with cancer resistance to doxorubicin and photodynamic therapy by elevating BCRP/ABCG2 expression

Overexpression of BCRP/ABCG2, a xenobiotic efflux transporter, is associated with anticancer drug resistance in tumors. Proto-oncogene c-MET induces cancer cell proliferation, motility, and survival, and its aberrant activation was found to be a prognostic factor in advanced ovarian cancers. In the present study, we investigated the potential crossresistance of doxorubicin-resistant ovarian cancer cells to the pheophorbide a (Pba)-based photodynamic therapy (PDT), and suggest c-MET and BCRP/ABCG2 overexpression as an underlying molecular mechanism. The doxorubicin-resistant A2780 cell line (A2780DR), which was established by incubating A2780 with stepwise increasing concentrations of doxorubicin, showed low levels of cellular Pba accumulation and reactive oxygen species generation, and was more resistant to PDT cytotoxicity than A2780. In a microarray analysis, BCRP/ABCG2 was found to be the only drug transporter whose expression was upregulated in A2780DR; this increase was confirmed by Western blot and immunocytochemical analyses. As functional evidence, the treatment with a BCRP/ABCG2-specific inhibitor reversed A2780DR resistance to both doxorubicin and PDT. We identified that c-MET increase is related to BCRP/ABCG2 activation. The c-MET downstream phosphoinositide 3-kinase (PI3K)/AKT signaling was activated in A2780DR and the inhibition of PI3K/AKT or c-MET repressed resistance to doxorubicin and PDT. Finally, we showed that the pharmacological and genetic inhibition of c-MET diminished levels of BCRP/ABCG2 in A2780DR. Moreover, c-MET inhibition could repress BCRP/ABCG2 expression in breast carcinoma MDA-MB-231 and colon carcinoma HT29, resulting in sensitization to doxorubicin. Collectively, our results provide a novel link of c-MET overexpression to BCRP/ABCG2 activation, suggesting that this mechanism leads to crossresistance to both chemotherapy and PDT.

The sensitivity of cancer cells to pheophorbide a-based photodynamic therapy is enhanced by Nrf2 silencing

Photodynamic therapy (PDT) has emerged as an effective treatment for various solid tumors. The transcription factor NRF2 is known to protect against oxidative and electrophilic stress; however, its constitutive activity in cancer confers resistance to anti-cancer drugs. In the present study, we investigated NRF2 signaling as a potential molecular determinant of pheophorbide a (Pba)-based PDT by using NRF2-knockdown breast carcinoma MDA-MB-231 cells. Cells with stable NRF2 knockdown showed enhanced cytotoxicity and apoptotic/necrotic cell death following PDT along with increased levels of singlet oxygen and reactive oxygen species (ROS). A confocal microscopic visualization of fluorogenic Pba demonstrated that NRF2-knockdown cells accumulate more Pba than control cells. A subsequent analysis of the expression of membrane drug transporters showed that the basal expression of BCRP is NRF2-dependent. Among measured drug transporters, the basal expression of breast cancer resistance protein (BCRP; ABCG2) was only diminished by NRF2-knockdown. Furthermore, after incubation with the BCRP specific inhibitor, differential cellular Pba accumulation and ROS in two cell lines were abolished. In addition, NRF2-knockdown cells express low level of peroxiredoxin 3 compared to the control, which implies that diminished mitochondrial ROS defense system can be contributing to PDT sensitization. The role of the NRF2-BCRP pathway in Pba-PDT response was further confirmed in colon carcinoma HT29 cells. Specifically, NRF2 knockdown resulted in enhanced cell death and increased singlet oxygen and ROS levels following PDT through the diminished expression of BCRP. Similarly, PDT-induced ROS generation was substantially increased by treatment with NRF2 shRNA in breast carcinoma MCF-7 cells, colon carcinoma HCT116 cells, renal carcinoma A498 cells, and glioblastoma A172 cells. Taken together, these results indicate that the manipulation of NRF2 can enhance Pba-PDT sensitivity in multiple cancer cells.

Effect of redox modulating NRF2 activators on chronic kidney disease

Chronic kidney disease (CKD) is featured by a progressive decline of kidney function and is mainly caused by chronic diseases such as diabetes mellitus and hypertension. CKD is a complex disease due to cardiovascular complications and high morbidity; however, there is no single treatment to improve kidney function in CKD patients. Since biological markers representing oxidative stress are significantly elevated in CKD patients, oxidative stress is receiving attention as a contributing factor to CKD pathology. Nuclear factor erythroid-2 related factor 2 (NRF2) is a predominant transcription factor that regulates the expression of a wide array of genes encoding antioxidant proteins, thiol molecules and their generating enzymes, detoxifying enzymes, and stress response proteins, all of which can counteract inflammatory and oxidative damages. There is considerable experimental evidence suggesting that NRF2 signaling plays a protective role in renal injuries that are caused by various pathologic conditions. In addition, impaired NRF2 activity and consequent target gene repression have been observed in CKD animals. Therefore, a pharmacological intervention activating NRF2 signaling can be beneficial in protecting against kidney dysfunction in CKD. This review article provides an overview of the role of NRF2 in experimental CKD models and describes current findings on the renoprotective effects of naturally occurring NRF2 activators, including sulforaphane, resveratrol, curcumin, and cinnamic aldehyde. These experimental results, coupled with recent clinical experiences with a synthetic triterpenoid, bardoxolone methyl, have brought a light of hope for ameliorating CKD progression by preventing oxidative stress and maintaining cellular redox homeostasis.

Inhibitory role of the KEAP1-NRF2 pathway in TGFβ1-stimulated renal epithelial transition to fibroblastic cells: a modulatory effect on SMAD signaling

Transforming growth factor β1 (TGFβ1) is a potent stimulator of epithelial-to-mesenchymal transition (EMT) and has been associated with chronic kidney diseases by activating profibrotic gene expression. In this study, we investigated the role of the KEAP1-NRF2 pathway, which is a master regulator of the cellular antioxidant system, in TGFβ1-stimulated EMT gene changes using human renal tubular epithelial HK2. Treatment with TGFβ1 enhanced the levels of reactive oxygen species (ROS) and TGFβ1-stimulated EMT gene changes, including an increase in profibrotic fibronectin-1 and collagen 1A1, were diminished by the antioxidant N-acetylcysteine. In HK2, TGFβ1 suppressed NRF2 activity and thereby reduced the expression of GSH synthesizing enzyme through the elevation of ATF3 level. Therefore, the activation of NRF2 signaling with sulforaphane effectively attenuated the TGFβ1-stimulated increase in fibronectin-1 and collagen 1A1. Conversely, the TGFβ1-EMT gene changes were further enhanced by NRF2 knockdown compared to the control cells. The relationship of NRF2 signaling and TGFβ1-EMT changes was further confirmed in a stable KEAP1-knockdown HK2, which is a model of pure activation of NRF2. The TGFβ1-mediated increase of collagen 1A1 and fibronectin-1 in KEAP1 knockdown HK2 was suppressed. In particular, TGFβ1-SMAD signaling was modulated in KEAP1 knockdown HK2: the TGFβ1-stimulated SMAD2/3 phosphorylation and SMAD transcriptional activity were repressed. Additionally, the protein level of SMAD7, an inhibitor of SMAD signaling, was elevated and the level of SMURF1, an E3 ubiquitin ligase for SMAD7 protein, was diminished in KEAP1 knockdown HK2. Finally, the inhibition of SMAD7 expression in KEAP1 knockdown HK2 restored TGFβ1 response, indicating that SMURF1-SMAD7 may be a molecular signaling linking the NRF2-GSH pathway to TGFβ1-EMT changes. Collectively, these results indicate that the KEAP1-NRF2 antioxidant system can be an effective modulator of TGFβ1-stimulated renal epithelial transition to fibroblastic cells through the SMUR1-SMAD7 signaling, and further implies the beneficial role of NRF2 in chronic renal diseases.

PPARδ deficiency disrupts hypoxia-mediated tumorigenic potential of colon cancer cells

Peroxisome proliferator-activated receptor (PPAR) δ is highly expressed in colon epithelial cells and closely linked to colon carcinogenesis. However, the role of PPARδ in colon cancer cells in a hypoxic tumor microenvironment is not fully understood. We found that expression of the tumor-promoting cytokines, IL-8 and VEGF, induced by hypoxia (<1% O2) and deferoxamine (a hypoxia mimetic) was significantly attenuated in PPARδ-deficient HCT116 colon cancer cells. Consequently, PPARδ-knockout colon cancer cells exposed to hypoxia and deferoxamine failed to stimulate endothelial cell vascularization and macrophage migration/proliferation, whereas wild-type cells were able to induce angiogenesis and macrophage activation in response to hypoxic stress. Hypoxic stress induced transcriptional activation of PPARδ, but not its protein expression, in HCT116 cells. Exogenous expression of p300 potentiated deferoxamine-induced PPARδ transactivation, while siRNA knockdown of p300 abolished hypoxia- and deferoxamine-induced PPARδ transactivation. PPARδ associated with p300 upon hypoxic stress as demonstrated by coimmunoprecipitation studies. PI3K inhibitors or siRNA knockdown of Akt suppressed the PPARδ transactivation induced by hypoxia and deferoxamine in HCT116 cells, leading to decreased expression of IL-8 and VEGF. Collectively, these results reveal that PPARδ is required for hypoxic stress-mediated cytokine expression in colon cancer cells, resulting in promotion of angiogenesis, macrophage recruitment, and macrophage proliferation in the tumor microenvironment. p300 and the PI3K/Akt pathway play a role in the regulation of PPARδ transactivation induced by hypoxic stress. Our results demonstrate the positive crosstalk between PPARδ in tumor cells and the hypoxic tumor microenvironment and provide potential therapeutic targets for colon cancer.

Piper betle induces phase I & II genes through Nrf2/ARE signaling pathway in mouse embryonic fibroblasts derived from wild type and Nrf2 knockout cells

Background

Nuclear factor-erythroid 2 p45 related factor 2 (Nrf2) is a primary transcription factor, protecting cells from oxidative stress by regulating a number of antioxidants and phase II detoxifying enzymes. Dietary components such as sulforaphane in broccoli and quercetin in onions have been shown to be inducers of Nrf2. Piper betle (PB) grows well in tropical climate and the leaves are used in a number of traditional remedies for the treatment of stomach ailments and infections among Asians. The aim of this study was to elucidate the effect of Piper betle (PB) leaves extract in Nrf2 signaling pathway by using 2 types of cells; mouse embryonic fibroblasts (MEFs) derived from wild-type (WT) and Nrf2 knockout (N0) mice.

Methods

WT and N0 cells were treated with 5 and 10 μg/ml of PB for 10 and 12-h for the determination of nuclear translocation of Nrf2 protein. Luciferase reporter gene activity was performed to evaluate the antioxidant response element (ARE)-induction by PB. Real-time PCR and Western blot were conducted on both WT and N0 cells after PB treatment for the determination of antioxidant enzymes [superoxide dismutase (SOD1) and heme-oxygenase (HO-1)], phase I oxidoreductase enzymes [NAD(P)H: quinone oxidoreductase (NQO1)] and phase II detoxifying enzyme [glutathione S-transferase (GST)].

Results

Nuclear translocation of Nrf2 by PB in WT cells was better after 10 h incubation compared to 12 h. Real time PCR and Western blot analysis showed increased expressions of Nrf2, NQO1 and GSTA1 genes with corresponding increases in glutathione, NQO1 and HO-1 proteins in WT cells. Reporter gene ARE was stimulated by PB as shown by ARE/luciferase assay. Interestingly, PB induced SOD1 gene and protein expressions in N0 cells but not in WT cells.

Conclusion

The results of this study confirmed that PB activated Nrf2-ARE signaling pathway which subsequently induced some phase I oxidoreductase, phase II detoxifying and antioxidant genes expression via ARE reporter gene involved in the Nrf2 pathway with the exception of SOD1 which may not be dependent on this pathway.

Caffeic acid phenethyl ester activation of Nrf2 pathway is enhanced under oxidative state: structural analysis and potential as a pathologically targeted therapeutic agent in treatment of colonic inflammation

Caffeic acid phenethyl ester (CAPE) is a polyphenolic natural product that possesses numerous biological activities including anti-inflammatory effects. CAPE-mediated nuclear factor-erythroid 2 p45 (NF-E2)-related factor 2 (Nrf2) activation is likely responsible for some of its biological effects. CAPE was chemically modified to yield CAPE analogues that were subjected to experiments examining cellular Nrf2 activity. CAPE and the CAPE analogue with a catechol moiety, but not the other analogues, activated the Nrf2 pathway. In addition, only biotin-labeled CAPE analogues with the catechol moiety precipitated Kelch-like ECH associated protein 1 (Keap1) when incubated with cell lysates and streptavidin agarose beads. Sodium hypochlorite (NaOCl) oxidation of the catechol moiety in CAPE produced an oxidized, electrophilic form of CAPE (Oxi-CAPE) and greatly enhanced the ability of CAPE to activate Nrf2 and to bind to Keap1. Rectal administration of CAPE ameliorated 2,4,6-trinitrobenzene sulfonic acid-induced rat colitis and activated the Nrf2 pathway in the inflamed colon, and incubation of CAPE in the lumen of the inflamed distal colon generated Oxi-CAPE. However, these biological effects and chemical change of CAPE were not observed in the normal colon. Our data suggest that CAPE requires the catechol moiety for the oxidation-enhanced activation of the Nrf2 pathway and has potential as a pathologically targeted Nrf2-activating agent that is exclusively activated in pathological states with oxidative stress such as colonic inflammation.

The impact of saddle embolism on the major adverse event rate of patients with non-high-risk pulmonary embolism

OBJECTIVE

Wider application of CT angiography (CTA) improves the diagnosis of acute pulmonary embolism (PE). It also permits the visualisation of saddle embolism (SE), namely thrombi, which are located at the bifurcation of the main pulmonary artery. The aim of this study was to assess the prevalence of SE and whether SE predicts a complicated clinical course in patients with non-high-risk PE.

METHODS

In total, 297 consecutive patients with non-high-risk PE confirmed using CTA in the emergency department were studied. The presence of SE and its ability to predict the occurrence of major adverse events (MAEs) within 1 month were determined.

RESULTS

Of the 297 patients, 27 (9.1%) had an SE. The overall mortality at 1 month was 12.5%; no significant difference was observed between the SE and non-SE groups (18.5% vs 11.9%, p=0.32). However, patients with SE were more likely to receive thrombolytic therapy (29.6% vs 8.1%, p<0.01) and had significantly more MAEs (59.3% vs 25.6%, p<0.01).

CONCLUSION

At the time of diagnosis, SE, as determined using CTA, is associated with the development of MAE within 1 month. It may be a simple method for risk stratification of patients with non-high-risk PE.

ADVANCES IN KNOWLEDGE

The prognosis of patients with SE, especially those who are haemodynamically stable, is unclear. This study shows that patients with SE, determined with CTA, is associated with the development of MAE.

Intracardiac thrombus formation induced by carbon monoxide poisoning

INTRODUCTION

Carbon monoxide (CO) is one of the leading causes of poisoning; it inhibits oxygen delivery, subsequently causing ischemic changes and ultimately death by multiorgan failure. Furthermore, thromboembolic episodes due to CO poisoning have been reported. However, intracardiac thrombus formation following exposure to CO has been very rarely described. Here, a case of right atrial large thrombus formation after CO poisoning is presented.

CASE PRESENTATION

A previously healthy 24-year-old woman was referred for CO poisoning. She has attempted suicide, and her initial mental status was drowsy with focal memory loss. Her initial CO fraction was 16%, and initial laboratory data showed creatinine kinase-myocardial bound of 90.6 ng/mL (upper limit 5 ng/mL) and troponin I of 1.899 ng/mL (upper limit 1.5 ng/mL). A transthoracic echocardiography was performed 24 h after the accident, revealing a 30 15 mm nodular echogenic mass in the right atrium. Anticoagulation with low-molecular-weight heparin was started along with hyperbaric oxygen therapy. After 7 days of heparinization, the large thrombus in right atrium had resolved.

CONCLUSION

This report describes an intracardiac thrombus formation induced by CO poisoning. Because intracardiac thrombus can result in pulmonary embolism and cerebral embolic infarction, its consideration following CO poisoning is important.

Involvement of the Nrf2-proteasome pathway in the endoplasmic reticulum stress response in pancreatic β-cells

The ubiquitin-proteasome system plays a central role in protein quality control through endoplasmic reticulum (ER)-associated degradation (ERAD) of unfolded and misfolded proteins. NF-E2-related factor 2 (Nrf2) is a transcription factor that controls the expression of an array of phase II detoxification and antioxidant genes. Nrf2 signaling has additionally been shown to upregulate the expression of the proteasome catalytic subunits in several cell types. Here, we investigated the role of Nrf2 in tunicamycin-induced ER stress using a murine insulinoma β-cell line, βTC-6. shRNA-mediated silencing of Nrf2 expression in βTC-6 cells significantly increased tunicamycin-induced cytotoxicity, elevated the expression of the pro-apoptotic ER stress marker Chop10, and inhibited tunicamycin-inducible expression of the proteasomal catalytic subunits Psmb5 and Psmb6. The effects of 3H-1,2-dithiole-3-thione (D3T), a small molecule Nrf2 activator, on ER stress were also examined in βTC-6 cells. D3T pretreatment reduced tunicamycin cytotoxicity and attenuated the tunicamycin-inducible Chop10 and protein kinase RNA-activated-like ER kinase (Perk). The protective effect of D3T was shown to be associated with increased ERAD. D3T increased the expression of Psmb5 and Psmb6 and elevated chymotrypsin-like peptidase activity; proteasome inhibitor treatment blocked D3T effects on tunicamycin cytotoxicity and ER stress marker changes. Similarly, silencing of Nrf2 abolished the protective effect of D3T against ER stress. These results indicate that the Nrf2 pathway contributes to the ER stress response in pancreatic β-cells by enhancing proteasome-mediated ERAD.

NRF2 inhibition represses ErbB2 signaling in ovarian carcinoma cells: implications for tumor growth retardation and docetaxel sensitivity

NF-E2-related factor 2 (NRF2) is a transcription factor that regulates the expression of various antioxidant and detoxifying enzymes. Although the benefit of NRF2 in cancer prevention is well established, its role in cancer pathobiology was recently discovered. In this study, the role of NRF2 in tumor growth and docetaxel sensitivity was investigated in ErbB2-overexpressing ovarian carcinoma SKOV3 cells. Interfering RNA-mediated stable inhibition of NRF2 in SKOV3 cells repressed NRF2 signaling, resulting in cell growth arrest at G(0)/G(1) phase and tumor growth retardation in mouse xenografts. Microarray analysis revealed that ErbB2 expression is substantially reduced in NRF2-inhibited SKOV3 and this was further confirmed by RT-PCR and immunoblot analysis. Repression of ErbB2 led to a decrease in phospho-AKT and enhanced p27 protein, reinforcing the effect of NRF2 knockdown on SKOV3 growth. Furthermore, NRF2 inhibition-mediated ErbB2 repression increases the sensitivity of these cells to docetaxel cytotoxicity and apoptosis. The linkage between NRF2 and ErbB2 was confirmed in the ErbB2-positive breast cancer cell line BT-474: NRF2 knockdown suppressed ErbB2 expression and enhanced docetaxel sensitivity. Our results provide insight into the coordinated regulation of signaling molecules responding to environmental stress and suggest that NRF2 modulation might be a therapeutic strategy to limit tumor growth and enhance sensitivity to taxane-based chemotherapy.

Abstract 2066: Identification of NRF2 target genes in human cells: Aldoketo reductases as a potential biomarker for NRF2 activity

The transcription factor NF-E2-related factor 2 (NRF2) plays a critical role in cellular defense system by up-regulating multiple antioxidant genes. Therefore, the identification of biomarker genes reflecting NRF2 activity would be important for the prediction of high-risk populations to environmental stresses. In the current study, we have investigated human genes of which expression is highly dependent on NRF2 to establish biomarker genes for NRF2 activity. For this purpose, NRF2-specific interfering RNA was stably introduced in normal human renal epithelial cells (HK-2) and human bronchial epithelial cells (NL-20), and changes in inducible genes were determined following treatment with 4 types of NRF2 activators: thiol-reacting sulforaphane, free-radical generating tert-butylhydroquinone, Michael acceptor cinnamic aldehyde and pro-oxidant hydrogen peroxide. These treatments showed relatively common alterations in gene expression, and among these, the expression of aldoketo reductase (AKR) 1C1 was highly inducible by all of these treatments in an NRF2-depedent manner. The levels for AKRs were found to be constitutively high in renal carcinoma A498 cells, of which NRF2 activity is elevated compared to normal renal epithelial HK-2 cells. In addition, the expression of AKRs was greatly enhanced by NRF2 activator treatments in human monocytes (U937), implying the potential utility of AKRs as a peripheral NRF2 marker. In conclusion, our results indicate that the expression of AKRs is highly dependent on NRF2 in human cells; therefore AKRs can be an effective biomarker for predicting NRF2 activity in human cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2066. doi:1538-7445.AM2012-2066

Role of the Nrf2-heme oxygenase-1 pathway in silver nanoparticle-mediated cytotoxicity

Silver nanoparticles (nano-Ag) have been widely used in various commercial products including textiles, electronic appliances and biomedical products. However, there remains insufficient information on the potential risk of nano-Ag to human health and environment. In the current study, we have investigated the role of NF-E2-related factor 2 (Nrf2) transcription factor in nano-Ag-induced cytotoxicity. When Nrf2 expression was blocked using interring RNA expression in ovarian carcinoma cell line, nano-Ag treatment showed a substantial decrease in cell viability with concomitant increases in apoptosis and DNA damage compared to the control cells. Target gene analysis revealed that the expression of heme oxygenase-1 (HO-1) was highly elevated by nano-Ag in nonspecific shRNA expressing cells, while Nrf2 knockdown cells (NRF2i) did not increase HO-1 expression. The role of HO-1 in cytoprotection against nano-Ag was reinforced by results using pharmacological inducer of HO-1: cobalt protoporphyrin-mediated HO-1 activation in the NRF2i cells prevented nano-Ag-mediated cell death. Similarly, pharmacological or genetic inhibition of HO-1 in nonspecific control cells exacerbated nano-Ag toxicity. As the upstream signaling mechanism, nano-Ag required the phosphoinositide 3-kinase (PI3K) and p38MAPK signaling cascades for HO-1 induction. The treatment with either PI3K inhibitor or p38MAPK inhibitor suppressed HO-1 induction and intensified nano-Ag-induced cell death. Taken together, these results suggest that Nrf2-dependent HO-1 up-regulation plays a protective role in nano-Ag-induced DNA damage and consequent cell death. In addition, nano-Ag-mediated HO-1 induction is associated with the PI3K and p38MAPK signaling pathways.

Cell growth inhibition and apoptosis by SDS-solubilized single-walled carbon nanotubes in normal rat kidney epithelial cells

Carbon nanotubes (CNTs), promising novel nanomaterials, have been applied to drug delivery and bio-imaging; however, their potential harmful effects on human health and environment have gained much attention recently. In the present study, we investigated cytotoxic effect of solubilized single-walled CNTs (SWCNTs), which were dispersed in water by sodium dodesyl sulfate (SDS), in normal rat kidney epithelial cells (NRK-52E). SDS-SWCNT (0.125-10 μg/mL)-treated NRK-52E cells showed decreased cell viability and enhanced cytotoxicity marker levels following 24-48 h incubation. In addition, SDS-SWCNT treatment evoked the cell growth inhibition: 8 μg/mL SDS-SWCNT induced the growth arrest at G(0)/G(1) phase and levels of cell cycle-related proteins such as CDK2, CDK6 and phosphorylated-retinoblastoma (pRB) were significantly reduced by CNT. Whereas, at higher concentration of SDS-SWCNT, the percentage of cell numbers in apoptotic sub-G(1) phase was substantially increased. Along with these changes, SDS-SWCNT treatment elevated protein levels for p53 and p21 with a concomitant increase in the single strand DNA breakage. Taken together, these results suggest that SDS-solubilized SWCNTs exert genotoxic effect in renal epithelial cells, and p53-dependent signaling can be associated with the growth arrest and apoptosis events upon CNT-induced DNA damage.

NRF2 blockade suppresses colon tumor angiogenesis by inhibiting hypoxia-induced activation of HIF-1α

Transcription factor NRF2 is an important modifier of cellular responses to oxidative stress. Although its cytoprotective effects are firmly established, recent evidence suggesting important roles in cancer pathobiology has yet to be mechanistically developed. In the current study, we investigated the role of NRF2 in colon tumor angiogenesis. Stable RNAi-mediated knockdown of NRF2 in human colon cancer cells suppressed tumor growth in mouse xenograft settings with a concomitant reduction in blood vessel formation and VEGF expression. Similar antiangiogenic effects of NRF2 knockdown were documented in chick chorioallantoic membrane assays and endothelial tube formation assays. Notably, NRF2-inhibited cancer cells failed to accumulate HIF-1α protein under hypoxic conditions, limiting expression of VEGF and other HIF-1α target genes. In these cells, HIF-1α was hydroxylated but pharmacological inhibition of PHD domain-containing prolyl hydroxylases was sufficient to restore hypoxia-induced accumulation of HIF-1α. Mechanistic investigations demonstrated that reduced mitochondrial O(2) consumption in NRF2-inhibited cells was probably responsible for HIF-1α degradation during hypoxia; cellular O(2) consumption and ATP production were lower in NRF2 knockdown cells than in control cells. Our findings offer novel insights into how cellular responses to O(2) and oxidative stress are integrated in cancer cells, and they highlight NRF2 as a candidate molecular target to control tumor angiogenesis by imposing a blockade to HIF-1α signaling.

Transcription factor Nrf2 maintains the basal expression of Mdm2: An implication of the regulation of p53 signaling by Nrf2

Co-operated regulation of oxidative stress-response transcription factors would be an important issue for animals to determine the cell fate under environmental stress. This notion raises a possibility that NF-E2-related factor 2 (Nrf2), which confers cytoprotection against oxidative stress, and p53 can have a direct co-regulation network. In the current study, we have indentified that the expression of murine double minute 2 (Mdm2) is repressed in nrf2-deleted murine embryonic fibroblasts (MEFs). This was confirmed by microarray, RT-PCR, and immunoblot analyses, and further promoter analysis showed that Nrf2 is directly involved in the basal expression of Mdm2 through the antioxidant response element, which is located in the first intron of this gene. This linkage between Nrf2 and Mdm2 appears to cause the accumulation of p53 protein in nrf2-deficent MEFs. In addition, we show that ovarian carcinoma A2780 cells with Nrf2 shRNA expression displayed higher levels of p53 activation in response to hydrogen peroxide treatment, leading to increased cell death. Collectively, our results suggest novel evidence that the inhibition of Nrf2 can suppress Mdm2 expression, which may result in p53 signaling modulation. In addition, this observation supports the concept that Nrf2 inhibition in cancer cells can facilitate apoptotic response upon environmental stress.

Regulation of notch1 signaling by nrf2: implications for tissue regeneration

The Keap1-Nrf2-ARE signaling pathway elicits an adaptive response for cell survival after endogenous and exogenous stresses, such as inflammation and carcinogens, respectively. Keap1 inhibits the transcriptional activation activity of Nrf2 (p45 nuclear factor erythroid-derived 2-related factor 2) in unstressed cells by facilitating its degradation. Through transcriptional analyses in Keap1- or Nrf2-disrupted mice, we identified interactions between the Keap1-Nrf2-ARE and the Notch1 signaling pathways. We found that Nrf2 recognized a functional antioxidant response element (ARE) in the promoter of Notch1. Notch1 regulates processes such as proliferation and cell fate decisions. We report a functional role for this cross talk between the two pathways and show that disruption of Nrf2 impeded liver regeneration after partial hepatectomy and was rescued by reestablishment of Notch1 signaling.