Glutathione and thioredoxin antioxidant pathways synergize to drive cancer initiation and progression

Physical interaction between human ribonucleotide reductase large subunit and thioredoxin increases colorectal cancer malignancy

Ribonucleotide reductase (RR) is the rate-limiting enzyme in DNA synthesis, catalyzing the reduction of ribonucleotides to deoxyribonucleotides. During each enzymatic turnover, reduction of the active site disulfide in the catalytic large subunit is performed by a pair of shuttle cysteine residues in its C-terminal tail. Thioredoxin (Trx) and glutaredoxin (Grx) are ubiquitous redox proteins, catalyzing thiol-disulfide exchange reactions. Here, immunohistochemical examination of clinical colorectal cancer (CRC) specimens revealed that human thioredoxin1 (hTrx1), but not human glutaredoxin1 (hGrx1), was up-regulated along with human RR large subunit (RRM1) in cancer tissues, and the expression levels of both proteins were correlated with cancer malignancy stage. Ectopically expressed hTrx1 significantly increased RR activity, DNA synthesis, and cell proliferation and migration. Importantly, inhibition of both hTrx1 and RRM1 produced a synergistic anticancer effect in CRC cells and xenograft mice. Furthermore, hTrx1 rather than hGrx1 was the efficient reductase for RRM1 regeneration. We also observed a direct protein-protein interaction between RRM1 and hTrx1 in CRC cells. Interestingly, besides the known two conserved cysteines, a third cysteine (Cys⁷⁷⁹) in the RRM1 C terminus was essential for RRM1 regeneration and binding to hTrx1, whereas both Cys³² and Cys³⁵ in hTrx1 played a counterpart role. Our findings suggest that the up-regulated RRM1 and hTrx1 in CRC directly interact with each other and promote RR activity, resulting in enhanced DNA synthesis and cancer malignancy. We propose that the RRM1-hTrx1 interaction might be a novel potential therapeutic target for cancer treatment.

ROS signaling under metabolic stress: cross-talk between AMPK and AKT pathway

Cancer cells are frequently confronted with metabolic stress in tumor microenvironments due to their rapid growth and limited nutrient supply. Metabolic stress induces cell death through ROS-induced apoptosis. However, cancer cells can adapt to it by altering the metabolic pathways. AMPK and AKT are two primary effectors in response to metabolic stress: AMPK acts as an energy-sensing factor which rewires metabolism and maintains redox balance. AKT broadly promotes energy production in the nutrient abundance milieu, but the role of AKT under metabolic stress is in dispute. Recent studies show that AMPK and AKT display antagonistic roles under metabolic stress. Metabolic stress-induced ROS signaling lies in the hub between metabolic reprogramming and redox homeostasis. Here, we highlight the cross-talk between AMPK and AKT and their regulation on ROS production and elimination, which summarizes the mechanism of cancer cell adaptability under ROS stress and suggests potential options for cancer therapeutics.

ROS signaling under metabolic stress: cross-talk between AMPK and AKT pathway

Cancer cells are frequently confronted with metabolic stress in tumor microenvironments due to their rapid growth and limited nutrient supply. Metabolic stress induces cell death through ROS-induced apoptosis. However, cancer cells can adapt to it by altering the metabolic pathways. AMPK and AKT are two primary effectors in response to metabolic stress: AMPK acts as an energy-sensing factor which rewires metabolism and maintains redox balance. AKT broadly promotes energy production in the nutrient abundance milieu, but the role of AKT under metabolic stress is in dispute. Recent studies show that AMPK and AKT display antagonistic roles under metabolic stress. Metabolic stress-induced ROS signaling lies in the hub between metabolic reprogramming and redox homeostasis. Here, we highlight the cross-talk between AMPK and AKT and their regulation on ROS production and elimination, which summarizes the mechanism of cancer cell adaptability under ROS stress and suggests potential options for cancer therapeutics.

Redox-Related Epigenetic Mechanisms in Glioblastoma: Nuclear Factor (Erythroid-Derived 2)-Like 2, Cobalamin, and Dopamine Receptor Subtype 4

Glioblastoma is an exceptionally difficult cancer to treat. Cancer is universally marked by epigenetic changes, which play key roles in sustaining a malignant phenotype, in addition to disease progression and patient survival. Studies have shown strong links between the cellular redox state and epigenetics. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a redox-sensitive transcription factor that upregulates endogenous antioxidant production, and is aberrantly expressed in many cancers, including glioblastoma. Methylation of DNA and histones provides a mode of epigenetic regulation, and cobalamin-dependent reactions link the redox state to methylation. Antagonists of dopamine receptor subtype 4 (D₄ receptor) were recently shown to restrict glioblastoma stem cell growth by downregulating trophic signaling, resulting in inhibition of functional autophagy. In addition to stimulating glioblastoma stem cell growth, D₄ receptors have the unique ability to catalyze cobalamin-dependent phospholipid methylation. Therefore, D₄ receptors represent an important node in a molecular reflex pathway involving Nrf2 and cobalamin, operating in conjunction with redox status and methyl group donor availability. In this article, we describe the redox-related effects of Nrf2, cobalamin metabolism, and the D₄ receptor on the regulation of the epigenetic state in glioblastoma.

The Two Faces of Reactive Oxygen Species in Cancer

Reactive oxygen species (ROS), now appreciated for their cellular signaling capabilities, have a dual role in cancer. On the one hand, ROS can promote protumorigenic signaling, facilitating cancer cell proliferation, survival, and adaptation to hypoxia. On the other hand, ROS can promote antitumorigenic signaling and trigger oxidative stress-induced cancer cell death. To hyperactivate the cell signaling pathways necessary for cellular transformation and tumorigenesis, cancer cells increase their rate of ROS production compared with normal cells. Concomitantly, in order to maintain ROS homeostasis and evade cell death, cancer cells increase their antioxidant capacity. Compared with normal cells, this altered redox environment of cancer cells may increase their susceptibility to ROS-manipulation therapies. In this review, we discuss the two faces of ROS in cancer, the potential mechanisms underlying ROS signaling, and the opposing cancer therapeutic approaches to targeting ROS.

EF24 induces ROS-mediated apoptosis via targeting thioredoxin reductase 1 in gastric cancer cells

Gastric cancer (GC) is one of the leading causes of cancer mortality in the world, and finding novel agents for the treatment of advanced gastric cancer is of urgent need. Diphenyl difluoroketone (EF24), a molecule having structural similarity to curcumin, exhibits potent anti-tumor activities by arresting cell cycle and inducing apoptosis. Although EF24 demonstrates potent anticancer efficacy in numerous types of human cancer cells, the cellular targets of EF24 have not been fully defined. We report here that EF24 may interact with the thioredoxin reductase 1 (TrxR1), an important selenocysteine (Sec)-containing antioxidant enzyme, to induce reactive oxygen species (ROS)-mediated apoptosis in human gastric cancer cells. By inhibiting TrxR1 activity and increasing intracellular ROS levels, EF24 induces a lethal endoplasmic reticulum stress in human gastric cancer cells. Importantly, knockdown of TrxR1 sensitizes cells to EF24 treatment. In vivo, EF24 treatment markedly reduces the TrxR1 activity and tumor cell burden, and displays synergistic lethality with 5-FU against gastric cancer cells. Targeting TrxR1 with EF24 thus discloses a previously unrecognized mechanism underlying the biological activity of EF24, and reveals that TrxR1 is a good target for gastric cancer therapy.

Loss of Merlin induces metabolomic adaptation that engages dependence on Hedgehog signaling

The tumor suppressor protein Merlin is proteasomally degraded in breast cancer. We undertook an untargeted metabolomics approach to discern the global metabolomics profile impacted by Merlin in breast cancer cells. We discerned specific changes in glutathione metabolites that uncovered novel facets of Merlin in impacting the cancer cell metabolome. Concordantly, Merlin loss increased oxidative stress causing aberrant activation of Hedgehog signaling. Abrogation of GLI-mediated transcription activity compromised the aggressive phenotype of Merlin-deficient cells indicating a clear dependence of cells on Hedgehog signaling. In breast tumor tissues, GLI1 expression enhanced tissue identification and discriminatory power of Merlin, cumulatively presenting a powerful substantiation of the relationship between these two proteins. We have uncovered, for the first time, details of the tumor cell metabolomic portrait modulated by Merlin, leading to activation of Hedgehog signaling. Importantly, inhibition of Hedgehog signaling offers an avenue to target the vulnerability of tumor cells with loss of Merlin.

Crosstalk between calcium and reactive oxygen species signaling in cancer

The interplay between Ca²⁺ and reactive oxygen species (ROS) signaling pathways is well established, with reciprocal regulation occurring at a number of subcellular locations. Many Ca²⁺ channels at the cell surface and intracellular organelles, including the endoplasmic reticulum and mitochondria are regulated by redox modifications. In turn, Ca²⁺ signaling can influence the cellular generation of ROS, from sources such as NADPH oxidases and mitochondria. This relationship has been explored in great depth during the process of apoptosis, where surges of Ca²⁺ and ROS are important mediators of cell death. More recently, coordinated and localized Ca²⁺ and ROS transients appear to play a major role in a vast variety of pro-survival signaling pathways that may be crucial for both physiological and pathophysiological functions. While much work is required to firmly establish this Ca²⁺-ROS relationship in cancer, existing evidence from other disease models suggests this crosstalk is likely of significant importance in tumorigenesis. In this review, we describe the regulation of Ca²⁺ channels and transporters by oxidants and discuss the potential consequences of the ROS-Ca²⁺ interplay in tumor cells.

Cancer stem cells: Radioresistance, prediction of radiotherapy outcome and specific targets for combined treatments

Inactivation of cancer stem cells (CSCs) is of utmost importance for tumor cure after radiotherapy. An increasing body of evidence complies with a higher radioresistance of CSCs compared to the mass of tumor cells, supporting the use of CSC related biomarkers for prediction of radiotherapy outcome. Treatment individualization strategies for patient groups with vastly different risk of recurrence will most likely require application of more than one biomarker. Specifically, inclusion of established biomarkers like tumor size for primary radio(chemo)therapy or human papilloma virus (HPV) infection status in head and neck squamous cell carcinoma seems to be of very high relevance. The high heterogeneity of CSC subclones along with changes of the functional behavior of individual tumors under treatment underlines the importance of the selection of the optimal timepoint(s) of biomarker evaluation, but also provides a potential therapeutic target for combined treatment approaches with irradiation.

Cancer, Oxidative Stress, and Metastasis

Reactive oxygen species (ROS) are highly reactive molecules that arise from a number of cellular sources, including oxidative metabolism in mitochondria. At low levels they can be advantageous to cells, activating signaling pathways that promote proliferation or survival. At higher levels, ROS can damage or kill cells by oxidizing proteins, lipids, and nucleic acids. It was hypothesized that antioxidants might benefit high-risk patients by reducing the rate of ROS-induced mutations and delaying cancer initiation. However, dietary supplementation with antioxidants has generally proven ineffective or detrimental in clinical trials. High ROS levels limit cancer cell survival during certain windows of cancer initiation and progression. During these periods, dietary supplementation with antioxidants may promote cancer cell survival and cancer progression. This raises the possibility that rather than treating cancer patients with antioxidants, they should be treated with pro-oxidants that exacerbate oxidative stress or block metabolic adaptations that confer oxidative stress resistance.

GSH-dependent antioxidant defense contributes to the acclimation of colon cancer cells to acidic microenvironment

Due to increased glycolysis and poor local perfusion, solid tumors are usually immersed in an acidic microenvironment. While extracellular acidosis is cytotoxic, cancer cells eventually become acclimated to it. While previous studies have addressed the acute effect of acidosis on cancer cells, little is known about how cancer cells survive chronic acidosis. In this study we exposed colorectal cancer (CRC) cells (HCT15, HCT116 and LoVo) to acidic pH (pH 6.5) continuously for over three months and obtained CRC cells that become acclimated to acidic pH, designated as CRC-acidosis-acclimated or CRC-AA. We unexpectedly found that while acute exposure to low pH resulted in an increase in the level of intracellular reactive oxygen species (ROS), CRC-AA cells exhibited a significantly reduced level of ROS when compared to ancestor cells. CRC-AA cells were found to maintain a higher level of reduced glutathione, via the upregulation of CD44 and glutathione reductase (GSR), among others, than their ancestor cells. Importantly, CRC-AA cells were more sensitive to agents that deplete GSH. Moreover, downregulation of GSR by RNA interference was more deleterious to CRC-AA cells than to control cells. Together, our results demonstrate a critical role of glutathione-dependent antioxidant defense in acclimation of CRC cells to acidic extracellular pH.

Beyond the Oncogene Revolution: Four New Ways to Combat Cancer

It has become clear that tumorigenesis results from much more than just the activation of an oncogene and/or the inactivation of a tumor-suppressor gene, and that the cancer cell genome contains many more alterations than can be specifically targeted at once. This observation has led our group to a search for alternative ways to kill cancer cells (while sparing normal cells) by focusing on properties unique to the former. We have identified four approaches with the potential to generate new anticancer therapies: combatting the tactics by which cancers evade antitumor immune responses, targeting metabolic adaptations that tumor cells use to survive conditions that would kill normal cells, manipulating a cancer cell's response to excessive oxidative stress, and exploiting aneuploidy. This review describes our progress to date on these fronts.

Systemic depletion of L-cyst(e)ine with cyst(e)inase increases reactive oxygen species and suppresses tumor growth

Cancer cells experience higher oxidative stress from reactive oxygen species (ROS) than do non-malignant cells because of genetic alterations and abnormal growth; as a result, maintenance of the antioxidant glutathione (GSH) is essential for their survival and proliferation. Under conditions of elevated ROS, endogenous L-cysteine (L-Cys) production is insufficient for GSH synthesis. This necessitates uptake of L-Cys that is predominantly in its disulfide form, L-cystine (CSSC), via the xCT(-) transporter. We show that administration of an engineered and pharmacologically optimized human cyst(e)inase enzyme mediates sustained depletion of the extracellular L-Cys and CSSC pool in mice and non-human primates. Treatment with this enzyme selectively causes cell cycle arrest and death in cancer cells due to depletion of intracellular GSH and ensuing elevated ROS; yet this treatment results in no apparent toxicities in mice even after months of continuous treatment. Cyst(e)inase suppressed the growth of prostate carcinoma allografts, reduced tumor growth in both prostate and breast cancer xenografts and doubled the median survival time of TCL1-Tg:p53-/- mice, which develop disease resembling human chronic lymphocytic leukemia. It was observed that enzyme-mediated depletion of the serum L-Cys and CSSC pool suppresses the growth of multiple tumors, yet is very well tolerated for prolonged periods, suggesting that cyst(e)inase represents a safe and effective therapeutic modality for inactivating antioxidant cellular responses in a wide range of malignancies.

Mitochondrial control of apoptosis through modulation of cardiolipin oxidation in hepatocellular carcinoma: A novel link between oxidative stress and cancer

Altered redox status in cancer cells has been linked to lipid peroxidation induced by reactive oxygen species (ROS) and subsequent formation of reactive lipid electrophiles, especially 4-hydroxy-nonenal (4-HNE). Emerging evidence suggests that cancer cells manipulate redox status to acquire anti-apoptotic phenotype but the underlying mechanisms are poorly understood. Cardiolipin (CL), a mitochondria-specific inner membrane phospholipid, is critical for maintaining mitochondrial function. Paradoxically, liver tissues contain tetralinoleoyl cardiolipin (TLCL) as the major CL in mitochondria yet emerging evidence suggests that ROS generated in mitochondria may lead to CL peroxidation and activation of intrinsic apoptosis. It remains unclear how CL oxidation leads to apoptosis and its relevance to the pathogenesis of hepatocellular carcinoma (HCC). We employed a mass spectrometry-based lipidomic approach to profile lipids in human tissues of HCC and found that CL was gradually decreased in tumor comparing to peripheral non-cancerous tissues, accompanied by a concomitant decrease of oxidized CL and its oxidation product, 4-HNE. Incubation of liver cancer cells with TLCL significantly restored apoptotic sensitivity accompanied by an increase of CL and its oxidation products when treated with staurosporine (STS) or Sorafenib (the standard treatment for late stage HCC patients). Our studies uncovered a novel mechanism by which cancer cells adopt to evade apoptosis, highlighting the importance of mitochondrial control of apoptosis through modulation of CL oxidation and subsequent 4-HNE formation in HCC. Thus manipulation of mitochondrial CL oxidation and lipid electrophile formation may have potential therapeutic value for diseases linked to oxidative stress and mitochondrial dysfunctions.

Hypoxia-inducible factors: coupling glucose metabolism and redox regulation with induction of the breast cancer stem cell phenotype

Reduced oxygen availability (hypoxia) leads to increased production of reactive oxygen species (ROS) by the electron transport chain. Here, I review recent work delineating mechanisms by which hypoxia-inducible factor 1 (HIF-1) mediates adaptive metabolic responses to hypoxia, including increased flux through the glycolytic pathway and decreased flux through the tricarboxylic acid cycle, in order to decrease mitochondrial ROS production. HIF-1 also mediates increased flux through the serine synthesis pathway and mitochondrial one-carbon (folate cycle) metabolism to increase mitochondrial antioxidant production (NADPH and glutathione). Dynamic maintenance of ROS homeostasis is required for induction of the breast cancer stem cell phenotype in response to hypoxia or cytotoxic chemotherapy. Consistently, inhibition of phosphoglycerate dehydrogenase, the first enzyme of the serine synthesis pathway, in breast cancer cells impairs tumor initiation, metastasis, and response to cytotoxic chemotherapy. I discuss how these findings have important implications for understanding the logic of the tumor microenvironment and for improving therapeutic responses in women with breast cancer.

Mitochondria and Cancer

Mitochondria are bioenergetic, biosynthetic, and signaling organelles that are integral in stress sensing to allow for cellular adaptation to the environment. Therefore, it is not surprising that mitochondria are important mediators of tumorigenesis, as this process requires flexibility to adapt to cellular and environmental alterations in addition to cancer treatments. Multiple aspects of mitochondrial biology beyond bioenergetics support transformation, including mitochondrial biogenesis and turnover, fission and fusion dynamics, cell death susceptibility, oxidative stress regulation, metabolism, and signaling. Thus, understanding mechanisms of mitochondrial function during tumorigenesis will be critical for the next generation of cancer therapeutics.

Dimeric peroxiredoxins are druggable targets in human Burkitt lymphoma

Burkitt lymphoma is a fast-growing tumor derived from germinal center B cells. It is mainly treated with aggressive chemotherapy, therefore novel therapeutic approaches are needed due to treatment toxicity and developing resistance. Disturbance of red-ox homeostasis has recently emerged as an efficient antitumor strategy. Peroxiredoxins (PRDXs) are thioredoxin-family antioxidant enzymes that scavenge cellular peroxides and contribute to red-ox homeostasis. PRDXs are robustly expressed in various malignancies and critically involved in cell proliferation, differentiation and apoptosis. To elucidate potential role of PRDXs in lymphoma, we studied their expression level in B cell-derived primary lymphoma cells as well as in cell lines. We found that PRDX1 and PRDX2 are upregulated in tumor B cells as compared with normal counterparts. Concomitant knockdown of PRDX1 and PRDX2 significantly attenuated the growth rate of lymphoma cells. Furthermore, in human Burkitt lymphoma cell lines, we isolated dimeric 2-cysteine peroxiredoxins as targets for SK053, a novel thiol-specific small-molecule peptidomimetic with antitumor activity. We observed that treatment of lymphoma cells with SK053 triggers formation of covalent PRDX dimers, accumulation of intracellular reactive oxygen species, phosphorylation of ERK1/2 and AKT and leads to cell cycle arrest and apoptosis. Based on site-directed mutagenesis and modeling studies, we propose a mechanism of SK053-mediated PRDX crosslinking, involving double thioalkylation of active site cysteine residues. Altogether, our results suggest that peroxiredoxins are novel therapeutic targets in Burkitt lymphoma and provide the basis for new approaches to the treatment of this disease.

Targeting reactive oxygen species in development and progression of pancreatic cancer

INTRODUCTION

Pancreatic ductal adenocarcinoma (PDA) is characterized by expression of oncogenic KRas which drives all aspects of tumorigenesis. Oncogenic KRas induces the formation of reactive oxygen species (ROS) which have been implicated in initiation and progression of PDA. To facilitate tumor promoting levels and to avoid oncogene-induced senescence or cytotoxicity, ROS homeostasis in PDA cells is balanced by additional up-regulation of antioxidant systems. Areas covered: We examine the sources of ROS in PDA, the mechanisms by which ROS homeostasis is maintained, and the biological consequences of ROS in PDA. Additionally, we discuss the potential mechanisms for targeting ROS homoeostasis as a point of therapeutic intervention. An extensive review of the relevant literature as it relates to the topic was conducted using PubMed. Expert commentary: Even though oncogenic mutations in the KRAS gene have been detected in over 95% of human pancreatic adenocarcinoma, targeting its gene product, KRas, has been difficult. The dependency of PDA cells on balancing ROS homeostasis could be an angle for new prevention or treatment strategies. These include use of antioxidants to prevent formation or progression of precancerous lesions, or methods to increase ROS in tumor cells to toxic levels.

A redox state-dictated signalling pathway deciphers the malignant cell specificity of CD40-mediated apoptosis

CD40, a member of the tumour necrosis factor receptor (TNFR) superfamily, has the capacity to cause extensive apoptosis in carcinoma cells, while sparing normal epithelial cells. Yet, apoptosis is only achieved by membrane-presented CD40 ligand (mCD40L), as soluble receptor agonists are but weakly pro-apoptotic. Here, for the first time we have identified the precise signalling cascade underpinning mCD40L-mediated death as involving sequential TRAF3 stabilisation, ASK1 phosphorylation, MKK4 (but not MKK7) activation and JNK/AP-1 induction, leading to a Bak- and Bax-dependent mitochondrial apoptosis pathway. TRAF3 is central in the activation of the NADPH oxidase (Nox)-2 component p40phox and the elevation of reactive oxygen species (ROS) is essential in apoptosis. Strikingly, CD40 activation resulted in down-regulation of Thioredoxin (Trx)-1 to permit ASK1 activation and apoptosis. Although soluble receptor agonist alone could not induce death, combinatorial treatment incorporating soluble CD40 agonist and pharmacological inhibition of Trx-1 was functionally equivalent to the signal triggered by mCD40L. Finally, we demonstrate using normal, ‘para-malignant' and tumour-derived cells that progression to malignant transformation is associated with increase in oxidative stress in epithelial cells, which coincides with increased susceptibility to CD40 killing, while in normal cells CD40 signalling is cytoprotective. Our studies have revealed the molecular nature of the tumour specificity of CD40 signalling and explained the differences in pro-apoptotic potential between soluble and membrane-bound CD40 agonists. Equally importantly, by exploiting a unique epithelial culture system that allowed us to monitor alterations in the redox-state of epithelial cells at different stages of malignant transformation, our study reveals how pro-apoptotic signals can elevate ROS past a previously hypothesised ‘lethal pro-apoptotic threshold' to induce death; an observation that is both of fundamental importance and carries implications for cancer therapy.

Role of thioredoxin reductase 1 in dysplastic transformation of human breast epithelial cells triggered by chronic oxidative stress

Thioredoxin reductase 1 (TrxR1) is a pivotal intracellular redox sensor and antioxidant enzyme. On the other hand, overexpression of TrxR1 is closely correlated with the initiation of various tumors including breast cancer, though the detailed mechanism remains unclear. Here we investigated the role of TrxR1 in dysplastic transformation of human breast epithelial cell line MCF-10A induced by chronic oxidative stress. Not surprisingly, sustained exposure to H₂O₂ significantly augmented the expression and activity of TrxR1 in MCF-10A cells. The dysplastically transformed MCF-10A (MCF-10AT) cells undergoing 8-week H₂O₂ treatment exhibited a certain degree of malignancy in tumorigenicity evaluation. Moreover, TrxR1 inhibitor ethaselen (BBSKE) could partially reverse some malignant phenotypes including epithelial to mesenchymal transition (EMT) of MCF-10AT as well as MCF-7 cells. Collectively, our results supported the considerable involvement of TrxR1 in the onset of breast cancer and BBSKE may be a promising agent against breast cancer.

Mitochondrial dysfunction enhances cisplatin resistance in human gastric cancer cells via the ROS-activated GCN2-eIF2α-ATF4-xCT pathway

Mitochondrial DNA mutations and defects in mitochondrial enzymes have been identified in gastric cancers, and they might contribute to cancer progression. In previous studies, mitochondrial dysfunction was induced by oligomycin-enhanced chemoresistance to cisplatin. Herein, we dissected the regulatory mechanism for mitochondrial dysfunction-enhanced cisplatin resistance in human gastric cancer cells. Repeated cisplatin treatment-induced cisplatin-resistant cells exhibited high SLC7A11 (xCT) expression, and xCT inhibitors (sulfasalazine or erastin), xCT siRNA, or a GSH synthesis inhibitor (buthionine sulphoximine, BSO) could sensitize these cells to cisplatin. Clinically, the high expression of xCT was associated with a poorer prognosis for gastric cancer patients under adjuvant chemotherapy. Moreover, we found that mitochondrial dysfunction enhanced cisplatin resistance and up-regulated xCT expression, as well as intracellular glutathione (GSH). The xCT inhibitors, siRNA against xCT or BSO decreased mitochondrial dysfunction-enhanced cisplatin resistance. We further demonstrated that the upregulation of the eIF2α-ATF4 pathway contributed to mitochondrial dysfunction-induced xCT expression, and activated eIF2α kinase GCN2, but not PERK, stimulated the eIF2α-ATF4-xCT pathway in response to mitochondrial dysfunction-increased reactive oxygen species (ROS) levels. In conclusion, our results suggested that the ROS-activated GCN2-eIF2α-ATF4-xCT pathway might contribute to mitochondrial dysfunction-enhanced cisplatin resistance and could be a potential target for gastric cancer therapy.

Altered metabolite levels in cancer: implications for tumour biology and cancer therapy

Altered cell metabolism is a characteristic feature of many cancers. Aside from well-described changes in nutrient consumption and waste excretion, altered cancer cell metabolism also results in changes to intracellular metabolite concentrations. Increased levels of metabolites that result directly from genetic mutations and cancer-associated modifications in protein expression can promote cancer initiation and progression. Changes in the levels of specific metabolites, such as 2-hydroxyglutarate, fumarate, succinate, aspartate and reactive oxygen species, can result in altered cell signalling, enzyme activity and/or metabolic flux. In this Review, we discuss the mechanisms that lead to changes in metabolite concentrations in cancer cells, the consequences of these changes for the cells and how they might be exploited to improve cancer therapy.

Antioxidant Activity during Tumor Progression: A Necessity for the Survival of Cancer Cells?

Antioxidant defenses encompass a variety of distinct compounds and enzymes that are linked together through their capacity to neutralize and scavenge reactive oxygen species (ROS). While the relationship between ROS and tumorigenesis is clearly complex and context dependent, a number of recent studies have suggested that neutralizing ROS can facilitate tumor progression and metastasis in multiple cancer types through distinct mechanisms. These studies therefore infer that antioxidant activity may be necessary to support the viability and/or the invasive capacity of cancer cells during tumor progression and metastasis. Here, we discuss some of the accumulating evidence suggesting a role for antioxidant activity in facilitating tumor progression.

Gene regulatory effects of disease-associated variation in the NRF2 network

Reactive oxygen species (ROS), which are both a natural byproduct of oxidative metabolism and an undesirable byproduct of many environmental stressors, can damage all classes of cellular macromolecules and promote diseases from cancer to neurodegeneration. The actions of ROS are mitigated by the transcription factor NRF2, which regulates expression of antioxidant genes via its interaction with cis-regulatory antioxidant response elements (AREs). However, despite the seemingly straightforward relationship between the opposing forces of ROS and NRF2, regulatory precision in the NRF2 network is essential. Genetic variants that alter NRF2 stability or alter ARE sequences have been linked to a range of diseases. NRF2 hyperactivating mutations are associated with tumorigenesis. On the subtler end of the spectrum, single nucleotide variants (SNVs) that alter individual ARE sequences have been linked to neurodegenerative disorders including progressive supranuclear palsy and Parkinson’s disease, as well as other diseases. Although the human health implications of NRF2 dysregulation have been recognized for some time, a systems level view of this regulatory network is beginning to highlight key NRF2-targeted AREs consistently associated with disease.

Regulation of Reactive Oxygen Species and the Antioxidant Protein DJ-1 in Mastocytosis

Neoplastic accumulation of mast cells in systemic mastocytosis (SM) associates with activating mutations in the receptor tyrosine kinase KIT. Constitutive activation of tyrosine kinase oncogenes has been linked to imbalances in oxidant/antioxidant mechanisms in other myeloproliferative disorders. However, the impact of KIT mutations on the redox status in SM and the potential therapeutic implications are not well understood. Here, we examined the regulation of reactive oxygen species (ROS) and of the antioxidant protein DJ-1 (PARK-7), which increases with cancer progression and acts to lessen oxidative damage to malignant cells, in relationship with SM severity. ROS levels were increased in both indolent (ISM) and aggressive variants of the disease (ASM). However, while DJ-1 levels were reduced in ISM with lower mast cell burden, they rose in ISM with higher mast cell burden and were significantly elevated in patients with ASM. Studies on mast cell lines revealed that activating KIT mutations induced constant ROS production and consequent DJ-1 oxidation and degradation that could explain the reduced levels of DJ-1 in the ISM population, while IL-6, a cytokine that increases with disease severity, caused a counteracting transcriptional induction of DJ-1 which would protect malignant mast cells from oxidative damage. A mouse model of mastocytosis recapitulated the biphasic changes in DJ-1 and the escalating IL-6, ROS and DJ-1 levels as mast cells accumulate, findings which were reversed with anti-IL-6 receptor blocking antibody. Our findings provide evidence of increased ROS and a biphasic regulation of the antioxidant DJ-1 in variants of SM and implicate IL-6 in DJ-1 induction and expansion of mast cells with KIT mutations. We propose consideration of IL-6 blockade as a potential adjunctive therapy in the treatment of patients with advanced mastocytosis, as it would reduce DJ-1 levels making mutation-positive mast cells vulnerable to oxidative damage.

Dual pharmacological inhibition of glutathione and thioredoxin systems synergizes to kill colorectal carcinoma stem cells

NRF2 stabilizes redox potential through genes for glutathione and thioredoxin antioxidant systems. Whether blockade of glutathione and thioredoxin is useful in eliminating cancer stem cells remain unknown. We used xenografts derived from colorectal carcinoma patients to investigate the pharmacological inhibition of glutathione and thioredoxin systems. Higher expression of five glutathione S‐transferase isoforms (GSTA1, A2, M4, O2, and P1) was observed in xenograft‐derived spheroids than in fibroblasts. Piperlongumine (2.5–10 μmol/L) and auranofin (0.25–4 μmol/L) were used to inhibit glutathione S‐transferase π and thioredoxin reductase, respectively. Piperlongumine or auranofin alone up‐regulated the expression of NRF2 target genes, but not TP53 targets. While piperlongumine showed modest cancer‐specific cell killing (IC₅₀ difference between cancer spheroids and fibroblasts: P = 0.052), auranofin appeared more toxic to fibroblasts (IC₅₀ difference between cancer spheroids and fibroblasts: P = 0.002). The synergism of dual inhibition was evaluated by determining the Combination Index, based on the number of surviving cells with combination treatments. Molar ratios indicated synergism in cancer spheroids, but not in fibroblasts: (auranofin:piperlongumine) = 2:5, 1:5, 1:10, and 1:20. Cancer‐specific cell killing was achieved at the following drug concentrations (auranofin:piperlongumine): 0.25:2.5 μmol/L, 0.5:2.5 μmol/L, or 0.25:5 μmol/L. The dual inhibition successfully decreased CD44v9 surface presentation and delayed tumor emergence in nude mouse. However, a small subpopulation persistently survived and accumulated phosphorylated histone H2A. Such “persisters” still retained lesser but significant tumorigenicity. Thus, dual inhibition of glutathione S‐transferase π and thioredoxin reductase could be a feasible option for decreasing the tumor mass and CD44v9‐positive fraction by disrupting redox regulation.

RETRACTED: PHGDH Expression Is Required for Mitochondrial Redox Homeostasis, Breast Cancer Stem Cell Maintenance, and Lung Metastasis

Intratumoral hypoxia stimulates enrichment of breast cancer stem cells (BCSC), which are critical for metastasis and patient mortality. Here we report a metabolic adaptation that is required for hypoxia-induced BCSC enrichment and metastasis. Hypoxia-inducible factors coordinately regulate expression of genes encoding phosphoglycerate dehydrogenase (PHGDH) and five downstream enzymes in the serine synthesis pathway and mitochondrial one-carbon (folate) cycle. RNAi-mediated silencing of PHGDH expression in both estrogen receptor-positive and negative breast cancer cells led to decreased NADPH levels, disturbed mitochondrial redox homeostasis, and increased apoptosis, which abrogated BCSC enrichment under hypoxic conditions. PHGDH-deficient cells exhibited increased oxidant levels and apoptosis, as well as loss of BCSC enrichment, in response to treatment with carboplatin or doxorubicin. PHGDH-deficient cells were relatively weakly tumorigenic and tumors that did form were deficient in BCSCs, abolishing metastatic capacity. Our findings highlight a role for PHGDH in the formation of secondary (recurrent or metastatic) tumors, with potential implications for therapeutic targeting of advanced cancers. Cancer Res; 76(15); 4430-42. ©2016 AACR.

Inhibitory nitrosylation of mammalian thioredoxin reductase 1: Molecular characterization and evidence for its functional role in cellular nitroso-redox imbalance

Mammalian thioredoxin 1 (Trx1) and the selenoprotein Trx reductase 1 (TrxR1) are key cellular enzymes that function coordinately in thiol-based redox regulation and signaling. Recent studies have revealed that the Trx1/TrxR1 system has an S-nitrosothiol reductase (denitrosylase) activity through which it can regulate nitric oxide-related cellular processes. In this study we revealed that TrxR1 is itself susceptible to nitrosylation, characterized the underlying mechanism, and explored its functional significance. We found that nitrosothiol or nitric oxide donating agents rapidly and effectively inhibited the activity of recombinant or endogenous TrxR1. In particular, the NADPH-reduced TrxR1 was partially and reversibly inhibited upon exposure to low concentrations (<10μM) of S-nitrosocysteine (CysNO) and markedly and continuously inhibited at higher doses. Concurrently, TrxR1 very efficiently reduced low, but not high, levels of CysNO. Biochemical and mass spectrometric analyses indicated that its active site selenocysteine residue renders TrxR1 highly susceptible to nitrosylation-mediated inhibition, and revealed both thiol and selenol modifications at the two redox active centers of the enzyme. Studies in HeLa cancer cells demonstrated that endogenous TrxR1 is sensitive to nitrosylation-dependent inactivation and pointed to an important role for glutathione in reversing or preventing this process. Notably, depletion of cellular glutathione with l-buthionine-sulfoximine synergized with nitrosating agents in promoting sustained nitrosylation and inactivation of TrxR1, events that were accompanied by significant oxidation of Trx1 and extensive cell death. Collectively, these findings expand our knowledge of the role and regulation of the mammalian Trx system in relation to cellular nitroso-redox imbalance. The observations raise the possibility of exploiting the nitrosylation susceptibility of TrxR1 for killing tumor cells.

Differentiation-inducing and anti-proliferative activities of isoliquiritigenin and all-trans-retinoic acid on B16F0 melanoma cells: Mechanisms profiling by RNA-seq

Melanoma is a cancer that arises from melanocytes, specialized pigmented cells that are found predominantly in the skin. The incidence of malignant melanoma has significantly increased over the last decade. With the development of therapy, the survival rate of some kind of cancer has been improved greatly. But the treatment of melanoma remains unsatisfactory. Much of melanoma's resistance to traditional chemotherapy is believed to arise intrinsically, by virtue of potent growth and cell survival-promoting genetic alteration. Therefore, significant attention has recently been focused on differentiation therapy, as well as differentiation inducer compounds. In previous study, we found isoliquiritigenin (ISL), a natural product extracted from licorice, could induce B16F0 melanoma cell differentiation. Here we investigated the transcriptional response of melanoma differentiation process induced by ISL and all-trans-retinoic acid (RA). Results showed that 390 genes involves in 201 biochemical pathways were differentially expressed in ISL treatment and 304 genes in 193 pathways in RA treatment. Differential expressed genes (DGEs, fold-change (FC)≥10) with the function of anti-proliferative and differentiation inducing indicated a loss of grade malignancy characteristic. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated glutathione metabolism, glycolysis/gluconeogenesis and pentose phosphate pathway were the top three relative pathway perturbed by ISL, and mitogen-activated protein kinase (MAPK) signaling pathway was the most important pathway in RA treatment. In the analysis of hierarchical clustering of DEGs, we discovered 72 DEGs involved in the process of drug action. We thought Cited1, Tgm2, Xaf1, Cd59a, Fbxo2, Adh7 may have critical role in the differentiation of melanoma. The evidence displayed herein confirms the critical role of reactive oxygen species (ROS) in melanoma pathobiology and provides evidence for future targets in the development of next-generation biomarkers and therapeutics.

Intracellular glutathione determines bortezomib cytotoxicity in multiple myeloma cells

Multiple myeloma (myeloma in short) is an incurable cancer of antibody-producing plasma cells that comprise 13% of all hematological malignancies. The proteasome inhibitor bortezomib has improved treatment significantly, but inherent and acquired resistance to the drug remains a problem. We here show that bortezomib-induced cytotoxicity was completely dampened when cells were supplemented with cysteine or its derivative, glutathione (GSH) in ANBL-6 and INA-6 myeloma cell lines. GSH is a major component of the antioxidative defense in eukaryotic cells. Increasing intracellular GSH levels fully abolished bortezomib-induced cytotoxicity and transcriptional changes. Elevated intracellular GSH levels blocked bortezomib-induced nuclear factor erythroid 2-related factor 2 (NFE2L2, NRF2)-associated stress responses, including upregulation of the xCT subunit of the Xc- cystine-glutamate antiporter. INA-6 cells conditioned to increasing bortezomib doses displayed reduced bortezomib sensitivity and elevated xCT levels. Inhibiting Xc- activity potentiated bortezomib-induced cytotoxicity in myeloma cell lines and primary cells, and re-established sensitivity to bortezomib in bortezomib-conditioned cells. We propose that intracellular GSH level is the main determinant of bortezomib-induced cytotoxicity in a subset of myeloma cells, and that combined targeting of the proteasome and the Xc- cystine-glutamate antiporter can circumvent bortezomib resistance.

Glutaminolysis as a target for cancer therapy

Cancer cells display an altered metabolic circuitry that is directly regulated by oncogenic mutations and loss of tumor suppressors. Mounting evidence indicates that altered glutamine metabolism in cancer cells has critical roles in supporting macromolecule biosynthesis, regulating signaling pathways, and maintaining redox homeostasis, all of which contribute to cancer cell proliferation and survival. Thus, intervention in these metabolic processes could provide novel approaches to improve cancer treatment. This review summarizes current findings on the role of glutaminolytic enzymes in human cancers and provides an update on the development of small molecule inhibitors to target glutaminolysis for cancer therapy.

Sickle Cells Abolish Melanoma Tumorigenesis in Hemoglobin SS Knockin Mice and Augment the Tumoricidal Effect of Oncolytic Virus In Vivo

Insights from the study of cancer resistance in animals have led to the discovery of novel anticancer pathways and opened new venues for cancer prevention and treatment. Sickle cells (SSRBCs) from subjects with homozygous sickle cell anemia (SCA) have been shown to target hypoxic tumor niches, induce diffuse vaso-occlusion, and potentiate a tumoricidal response in a heme- and oxidant-dependent manner. These findings spawned the hypothesis that SSRBCs and the vasculopathic microenvironment of subjects with SCA might be inimical to tumor outgrowth and thereby constitute a natural antitumor defense. We therefore implanted the B16F10 melanoma into humanized hemoglobin SS knockin mice which exhibit the hematologic and vasculopathic sequelae of human SCA. Over the 31-day observation period, hemoglobin SS mice showed no significant melanoma outgrowth. By contrast, 68-100% of melanomas implanted in background and hemoglobin AA knockin control mice reached the tumor growth end point (p < 0.0001). SS knockin mice also exhibited established markers of underlying vasculopathy, e.g., chronic hemolysis (anemia, reticulocytosis) and vascular inflammation (leukocytosis) that differed significantly from all control groups. Genetic differences or normal AA gene knockin do not explain the impaired tumor outgrowth in SS knockin mice. These data point instead to the chronic pro-oxidative vasculopathic network in these mice as the predominant cause. In related studies, we demonstrate the ability of the sickle cell component of this system to function as a therapeutic vehicle in potentiating the oncolytic/vasculopathic effect of RNA reovirus. Sickle cells were shown to efficiently adsorb and transfer the virus to melanoma cells where it induced apoptosis even in the presence of anti-reovirus neutralizing antibodies. In vivo, SSRBCs along with their viral cargo rapidly targeted the tumor and initiated a tumoricidal response exceeding that of free virus and similarly loaded normal RBCs without toxicity. Collectively, these data unveil two hitherto unrecognized findings: hemoglobin SS knockin mice appear to present a natural barrier to melanoma tumorigenesis while SSRBCs demonstrate therapeutic function as a vehicle for enhancing the oncolytic effect of free reovirus against established melanoma.

Redox Homeostasis and Cellular Antioxidant Systems: Crucial Players in Cancer Growth and Therapy

Reactive oxygen species (ROS) and their products are components of cell signaling pathways and play important roles in cellular physiology and pathophysiology. Under physiological conditions, cells control ROS levels by the use of scavenging systems such as superoxide dismutases, peroxiredoxins, and glutathione that balance ROS generation and elimination. Under oxidative stress conditions, excessive ROS can damage cellular proteins, lipids, and DNA, leading to cell damage that may contribute to carcinogenesis. Several studies have shown that cancer cells display an adaptive response to oxidative stress by increasing expression of antioxidant enzymes and molecules. As a double-edged sword, ROS influence signaling pathways determining beneficial or detrimental outcomes in cancer therapy. In this review, we address the role of redox homeostasis in cancer growth and therapy and examine the current literature regarding the redox regulatory systems that become upregulated in cancer and their role in promoting tumor progression and resistance to chemotherapy.

Stress Regulates Aquaporin-8 Permeability to Impact Cell Growth and Survival

Aquaporin-8 (AQP8) allows the bidirectional transport of water and hydrogen peroxide across biological membranes. Depending on its concentration, H2O2 exerts opposite roles, amplifying growth factor signaling in physiological conditions, but causing severe cell damage when in excess. Thus, H2O2 permeability is likely to be tightly controlled in living cells.

AIMS

In this study, we investigated whether and how the transport of H2O2 through plasma membrane AQP8 is regulated, particularly during cell stress.

RESULTS

We show that diverse cellular stress conditions, including heat, hypoxia, and ER stress, reversibly inhibit the permeability of AQP8 to H2O2 and water. Preventing the accumulation of intracellular reactive oxygen species (ROS) during stress counteracts AQP8 blockade. Once inhibition is established, AQP8-dependent transport can be rescued by reducing agents. Neither H2O2 nor water transport is impaired in stressed cells expressing a mutant AQP8, in which cysteine 53 had been replaced by serine. Cells expressing this mutant are more resistant to stress-, drug-, and radiation-induced growth arrest and death.

INNOVATION AND CONCLUSION

The control of AQP8-mediated H2O2 transport provides a novel mechanism to regulate cell signaling and survival during stress. Antioxid. Redox Signal. 24, 1031-1044.

ROS homeostasis and metabolism: a dangerous liason in cancer cells

Tumor cells harbor genetic alterations that promote a continuous and elevated production of reactive oxygen species. Whereas such oxidative stress conditions would be harmful to normal cells, they facilitate tumor growth in multiple ways by causing DNA damage and genomic instability, and ultimately, by reprogramming cancer cell metabolism. This review outlines the metabolic-dependent mechanisms that tumors engage in when faced with oxidative stress conditions that are critical for cancer progression by producing redox cofactors. In particular, we describe how the mitochondria has a key role in regulating the interplay between redox homeostasis and metabolism within tumor cells. Last, we will discuss the potential therapeutic use of agents that directly or indirectly block metabolism.

Metabolic, autophagic, and mitophagic activities in cancer initiation and progression

Cancer is a complex disease marked by uncontrolled cell growth and invasion. These processes are driven by the accumulation of genetic and epigenetic alterations that promote cancer initiation and progression. Contributing to genome changes are the regulation of oxidative stress and reactive species-induced damage to molecules and organelles. Redox regulation, metabolic plasticity, autophagy, and mitophagy play important and interactive roles in cancer hallmarks including sustained proliferation, activated invasion, and replicative immortality. However, the impact of these processes can differ depending on the signaling pathways altered in cancer, tumor type, tumor stage, and/or the differentiation state. Here, we highlight some of the representative studies on the impact of oxidative and nitrosative activities, mitochondrial bioenergetics, metabolism, and autophagy and mitophagy in the context of tumorigenesis. We discuss the implications of these processes for cellular activities in cancer for anti-cancer-based therapeutics.

Metabolic rewiring in melanoma

Oncogene-driven metabolic rewiring is an adaptation to low nutrient and oxygen conditions in the tumor microenvironment that enables cancer cells of diverse origin to hyperproliferate. Aerobic glycolysis and enhanced reliance on glutamine utilization are prime examples of such rewiring. However, tissue of origin as well as specific genetic and epigenetic changes determines gene expression profiles underlying these metabolic alterations in specific cancers. In melanoma, activation of the mitogen-activated protein kinase (MAPK) pathway driven by mutant BRAF or NRAS is a primary cause of malignant transformation. Activity of the MAPK pathway, as well as other factors, such as HIF1α, Myc and MITF, are among those that control the balance between non-oxidative and oxidative branches of central carbon metabolism. Here, we discuss the nature of metabolic alterations that underlie melanoma development and affect its response to therapy.

Mechanisms of ferroptosis

Ferroptosis is a non-apoptotic form of cell death that can be triggered by small molecules or conditions that inhibit glutathione biosynthesis or the glutathione-dependent antioxidant enzyme glutathione peroxidase 4 (GPX4). This lethal process is defined by the iron-dependent accumulation of lipid reactive oxygen species and depletion of plasma membrane polyunsaturated fatty acids. Cancer cells with high level RAS-RAF-MEK pathway activity or p53 expression may be sensitized to this process. Conversely, a number of small molecule inhibitors of ferroptosis have been identified, including ferrostatin-1 and liproxstatin-1, which can block pathological cell death events in brain, kidney and other tissues. Recent work has identified a number of genes required for ferroptosis, including those involved in lipid and amino acid metabolism. Outstanding questions include the relationship between ferroptosis and other forms of cell death, and whether activation or inhibition of ferroptosis can be exploited to achieve desirable therapeutic ends.

Generation of small molecules to interfere with regulated necrosis

Interference with regulated necrosis for clinical purposes carries broad therapeutic relevance and, if successfully achieved, has a potential to revolutionize everyday clinical routine. Necrosis was interpreted as something that no clinician might ever be able to prevent due to the unregulated nature of this form of cell death. However, given our growing understanding of the existence of regulated forms of necrosis and the roles of key enzymes of these pathways, e.g., kinases, peroxidases, etc., the possibility emerges to identify efficient and selective small molecule inhibitors of pathologic necrosis. Here, we review the published literature on small molecule inhibition of regulated necrosis and provide an outlook on how combination therapy may be most effective in treatment of necrosis-associated clinical situations like stroke, myocardial infarction, sepsis, cancer and solid organ transplantation.

Role of mitochondrial dysfunction in cancer progression

Deregulated cellular energetics was one of the cancer hallmarks. Several underlying mechanisms of deregulated cellular energetics are associated with mitochondrial dysfunction caused by mitochondrial DNA mutations, mitochondrial enzyme defects, or altered oncogenes/tumor suppressors. In this review, we summarize the current understanding about the role of mitochondrial dysfunction in cancer progression. Point mutations and copy number changes are the two most common mitochondrial DNA alterations in cancers, and mitochondrial dysfunction induced by chemical depletion of mitochondrial DNA or impairment of mitochondrial respiratory chain in cancer cells promotes cancer progression to a chemoresistance or invasive phenotype. Moreover, defects in mitochondrial enzymes, such as succinate dehydrogenase, fumarate hydratase, and isocitrate dehydrogenase, are associated with both familial and sporadic forms of cancer. Deregulated mitochondrial deacetylase sirtuin 3 might modulate cancer progression by regulating cellular metabolism and oxidative stress. These mitochondrial defects during oncogenesis and tumor progression activate cytosolic signaling pathways that ultimately alter nuclear gene expression, a process called retrograde signaling. Changes in the intracellular level of reactive oxygen species, Ca(2+), or oncometabolites are important in the mitochondrial retrograde signaling for neoplastic transformation and cancer progression. In addition, altered oncogenes/tumor suppressors including hypoxia-inducible factor 1 and tumor suppressor p53 regulate mitochondrial respiration and cellular metabolism by modulating the expression of their target genes. We thus suggest that mitochondrial dysfunction plays a critical role in cancer progression and that targeting mitochondrial alterations and mitochondrial retrograde signaling might be a promising strategy for the development of selective anticancer therapy.

So You Can Teach Old Fibroblasts New Tricks

New data show that as dermal fibroblasts grow old, they increase their secretion of the WNT antagonist sFRP2 to drive melanoma cell metastasis. sFRP2 suppresses β-catenin and MITF signaling in melanoma cells, downregulating the redox regulator APE1, making melanoma cells more sensitive to oxidative stress and driving resistance to BRAF inhibitors. Thus, the aging microenvironment in elderly patient skin activates a signaling pathway that drives more aggressive melanoma cell behavior. Cancer Discov; 6(6); 581-3. ©2016 AACR.

Piperlongumine as a direct TrxR1 inhibitor with suppressive activity against gastric cancer

Piperlongumine (PL), a natural alkaloid isolated from the fruit of long pepper, is known to selectively kill tumor cells while sparing their normal counterparts. However, the cellular target and potent anticancer efficacy of PL in numerous types of human cancer cells have not been fully defined. We report here that PL may interact with the thioredoxin reductase 1 (TrxR1), an important selenocysteine (Sec)-containing antioxidant enzyme, to induce reactive oxygen species (ROS)-mediated apoptosis in human gastric cancer cells. By inhibiting TrxR1 activity and increasing intracellular ROS levels, PL induces a lethal endoplasmic reticulum stress and mitochondrial dysfunction in human gastric cancer cells. Importantly, knockdown of TrxR1 sensitizes cells to PL treatment, and PL displays synergistic lethality with GSH inhibitors (BSO and Erastin) against gastric cancer cells. In vivo, PL treatment markedly reduces the TrxR1 activity and tumor cell burden. Remarkably, TrxR1 was significantly overexpressed in gastric cancer cell lines and human gastric cancer tissues. Targeting TrxR1 with PL thus discloses a previously unrecognized mechanism underlying the biological activity of PL and provides an in-depth insight into the action of PL in the treatment of gastric cancer.

Imbalance in Protein Thiol Redox Regulation and Cancer-Preventive Efficacy of Selenium

Although several experimental studies showed cancer-preventive efficacy of supplemental dietary selenium, human clinical trials questioned this efficacy. Identifying its molecular targets and mechanism is important in understanding this discrepancy. Methylselenol, the active metabolite of selenium, reacts with lipid hydroperoxides bound to protein kinase C (PKC) and is oxidized to methylseleninic acid (MSA). This locally generated MSA selectively inactivates PKC by oxidizing its critical cysteine sulfhydryls. The peroxidatic redox cycle occurring in this process may explain how extremely low concentrations of selenium catalytically modify specific membrane-bound proteins compartmentally separated from glutathione and selectively induce cytotoxicity in promoting cells. Mammalian thioredoxin reductase (TR) is itself a selenoenzyme with a catalytic selenocysteine residue. Together with thioredoxin (Trx), it catalyzes reduction of selenite and selenocystine by NADPH generating selenide which in the presence of oxygen redox cycles producing reactive oxygen species. Trx binds with high affinity to PKC and reverses PKC inactivation. Therefore, established tumor cells overexpressing TR and Trx may escape the cancer-preventive actions of selenium. This suggests that in some cases, certain selenoproteins may counteract selenometabolite actions. Lower concentrations of selenium readily inactivate antiapoptotic PKC isoenzymes e and a which have a cluster of vicinal thiols, thereby inducing apoptosis. Higher concentrations of selenium also inactivate proapoptotic enzymes such as proteolytically activated PKCd fragment, holo-PKCz, caspase-3, and c-Jun N-terminal kinase, which all have a limited number of critical cysteine residues and make tumor cells resistant to selenium-induced apoptosis. This may explain the intriguing U-shaped curve that is seen with dietary selenium intake and the extent of cancer prevention.

Coping with the metabolic stress of leaving home

Detachment from extracellular matrix causes metabolic defects that transformed cells must overcome in order to survive and proliferate outside of their normal niche. A recent report from Jiang et al. published in Nature describes how cancer cells grown in suspension utilize reductive carboxylation of glutamine to transfer reducing power from the cytosol to mitochondria to detoxify reactive oxygen species and promote anchorage-independent growth and survival.

Discovery and Characterization of Carotenoid-Oxygen Copolymers in Fruits and Vegetables with Potential Health Benefits

We reported previously that the spontaneous oxidation of β-carotene and other carotenoids proceeds predominantly by formation of carotenoid-oxygen copolymers and that β-carotene copolymers exhibit immunological activity, including priming innate immune function and limiting inflammatory processes. Oxidative loss of carotenoids in fruits and vegetables occurs during processing. Here we report evidence for the occurrence of associated analogous copolymer compounds. Geronic acid, an indirect, low molecular weight marker of β-carotene oxidation at ∼2% of β-carotene copolymers, is found to occur in common fresh or dried foods, including carrots, tomatoes, sweet potatoes, paprika, rosehips, seaweeds, and alfalfa, at levels encompassing an approximately thousand-fold range, from low ng/g in fresh foods to μg/g in dried foods. Copolymers isolated from several dried foods reach mg/g levels: comparable to initial carotenoid levels. In vivo biological activity of supplemental β-carotene copolymers has been previously documented at μg/g levels, suggesting that some foods could have related activity.