Inhibition of mammalian thioredoxin reductase by black tea and its constituents: implications for anticancer actions

Significance of the mitochondrial thioredoxin reductase in cancer cells: An update on role, targets and inhibitors

Thioredoxin reductase 2 (TrxR2) is a key component of the mitochondrial thioredoxin system able to transfer electrons to peroxiredoxin 3 (Prx3) in a reaction mediated by thioredoxin 2 (Trx2). In this way, both the level of hydrogen peroxide and thiol redox state are modulated. TrxR2 is often overexpressed in cancer cells conferring apoptosis resistance. Due to their exposed flexible arm containing selenocysteine, both cytosolic and mitochondrial TrxRs are inhibited by a large number of molecules. The various classes of inhibitors are listed and the molecules acting specifically on TrxR2 are extensively described. Particular emphasis is given to gold(I/III) complexes with phosphine, carbene or other ligands and to tamoxifen-like metallocifens. Also chemically unrelated organic molecules, including natural compounds and their derivatives, are taken into account. An important feature of many TrxR2 inhibitors is provided by their nature of delocalized lipophilic cations that allows their accumulation in mitochondria exploiting the organelle membrane potential. The consequences of TrxR2 inhibition are presented focusing especially on the impact on mitochondrial pathophysiology. Inhibition of TrxR2, by hindering the activity of Trx2 and Prx3, increases the mitochondrial concentration of reactive oxygen species and shifts the thiol redox state toward a more oxidized condition. This is reflected by alterations of specific targets involved in the release of pro-apoptotic factors such as cyclophilin D which acts as a regulator of the mitochondrial permeability transition pore. Therefore, the selective inhibition of TrxR2 could be utilized to induce cancer cell apoptosis.

Antioxidants from black and green tea: from dietary modulation of oxidative stress to pharmacological mechanisms

The consumption of tea (Camellia sinensis) has been correlated with a low incidence of chronic pathologies, such as cardiovascular disease and cancer, in which oxidative stress plays a critical role. Tea catechins and theaflavins are, respectively, the bioactive phytochemicals responsible for the antioxidant activity of green tea (GT) and black tea (BT). In addition to their redox properties, tea catechins and theaflavins could have also pharmacological activities, such as the ability to lower glucose, lipid and uric acid (UA) levels. These activities are mediated by pharmacological mechanisms such as enzymatic inhibition and interaction with transporters. Epigallocatechin gallate is the most active compound at inhibiting the enzymes involved in cholesterol and UA metabolism (hydroxy-3-methyl-glutaryl-CoA reductase and xanthine oxidase respectively) and affecting glucose transporters. The structural features of catechins that significantly contribute to their pharmacological effect are the presence/absence of the galloyl moiety and the number and positions of the hydroxyl groups on the rings. Although the inhibitory effects on α-glucosidase, maltase, amylase and lipase, multidrug resistance 1, organic anion transporters and proton-coupled folate transport occur at higher concentrations than those apparent in the circulation, these effects could be relevant in the gut. In conclusion, despite the urgent need for further research in humans, the regular consumption of moderate quantities of GT and BT can effectively modulate their antioxidant capacity, mainly in people subjected to oxidative stress, and could improve the metabolism of glucose, lipid and UA.

LINKED ARTICLES

This article is part of a themed section on Principles of Pharmacological Research of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.11/issuetoc.

Eucommia bark (Du-Zhong) improves diabetic nephropathy without altering blood glucose in type 1-like diabetic rats

Background

Eucommia bark, Eucommia ulmoides Oliver barks (Du-Zhong in Mandarin), is an herb used for renal dysfunction in Chinese traditional medicine. In an attempt to develop this herb as a treatment for diabetic nephropathy (DN), we investigated the effects of Du-Zhong on renal dysfunction in type 1-like diabetic rats.

Methods

Streptozotocin (STZ) was used to induce type 1-like diabetes in rats (STZ-diabetic rats). In addition to hyperglycemia, STZ-diabetic rats showed significant nephropathy, including higher plasma levels of blood urea nitrogen, creatinine, and renal fibrosis. Western blot analysis of renal cortical tissue was applied to characterize the changes in potential signals related to nephropathy.

Results

Oral administration of Du-Zhong (1 g/kg/day) to STZ-diabetic rats for 20 days not only decreased the plasma levels of blood urea nitrogen and creatinine but also improved renal fibrosis, whereas the plasma glucose level was not changed. The higher expressions of protein levels of transforming growth factor-beta (TGF-β) and connective tissue growth factor in diabetic rats were markedly attenuated by Du-Zhong. The increased phosphorylation of Smad2/3 in STZ-diabetic rats was also reduced by Du-Zhong. However, Du-Zhong cannot reverse the hyperglycemia-induced overproduction of signal transducers and activators of transcription 3 in the diabetic kidney.

Conclusion

Oral administration of Du-Zhong improves STZ-induced DN in rats by inhibiting TGF-β/Smad signaling and suppressing TGF-β/connective tissue growth factor expression. Therefore, active principle from Du-Zhong is suitable to develop as new agent for DN in the future.

Polyphenols as mitochondria-targeted anticancer drugs

Mitochondria are the respiratory and energetic centers of the cell where multiple intra- and extracellular signal transduction pathways converge leading to dysfunction of those organelles and, consequently, apoptotic or/and necrotic cell death. Mitochondria-targeted anticancer drugs are referred to as mitocans; they have recently been classified by Neuzil et al. (2013) according to their molecular mode of action into: hexokinase inhibitors; mimickers of the Bcl-2 homology-3 (BH3) domains; thiol redox inhibitors; deregulators of voltage-dependent anionic channel (VDAC)/adenine nucleotide translocase (ANT) complex; electron redox chain-targeting agents; lipophilic cations targeting the mitochondrial inner membrane; tricarboxylic acid cycle-targeting agents; and mitochondrial DNA-targeting agents. Polyphenols of plant origin and their synthetic or semisynthetic derivatives exhibit pleiotropic biological activities, including the above-mentioned modes of action characteristic of mitocans. Some of them have already been tested in clinical trials. Gossypol has served as a lead compound for developing more efficient BH3 mimetics such as ABT-737 and its orally available structural analog ABT-263 (Navitoclax). Furthermore, mitochondriotropic derivatives of phenolic compounds such as quercetin and resveratrol have been synthesized and reported to efficiently induce cancer cell death in vitro.

Induction of Thioredoxin Reductase 1 by Korean Red Ginseng Water Extract Regulates Cytoprotective Effects on Human Endothelial Cells

Korean Red Ginseng is a popular herbal medicine and is widely used in many food products. KRG has biological benefits related to vascular diseases including diabetes, hypertension, atherosclerosis, and other cardiac diseases and KRG has antioxidant and anti-hyperlipidemic actions. KRG decreases the level of oxidative stress and suppresses proinflammatory cytokines and cell adhesion molecules, thus protecting endothelial dysfunction. Mammalian Thioredoxin reductase 1 is an NADPH-dependent selenoprotein, essential for antioxidant defense and DNA synthesis and repair, that regulates the redox system by modulating redox-sensitive transcription factors and thiol-containing proteins. Here, we show that KRG water extract increases the expression of TrxR1 in human umbilical vein endothelial cells via the p38 and PKC-δ signaling pathways. The induction of TrxR1 expression by KRG was confirmed by Western blot analysis and reverse transcription polymerase chain reaction. However, the increase in TrxR1 expression was abolished by specific silencing of the p38 and PKC-δ genes. In addition, we demonstrated that auranofin, a TrxR1 inhibitor, weakens the protective effect of KRG against H₂O₂-induced cell death as measured by the terminal transferase dUTP nick end labeling assay. These results suggest that KRG may have protective effects in vascular diseases by upregulating TrxR1 in endothelial cells, thereby inhibiting the generation of reactive oxygen species and cell death.

UPLC-MS/MS determination and gender-related pharmacokinetic study of five active ingredients in rat plasma after oral administration of Eucommia cortex extract

ETHNOPHARMACOLOGICAL RELEVANCE

Eucommiae cortex (EC), the bark of Eucommia ulmoides Oliv., has been traditionally used to treat many diseases in China for more than 2000 years. The pharmacological effects are primarily attributed to the presence of lignans, iridoids and phenolics, which are main active ingredients in EC.

AIM OF THE STUDY

First, to investigate the active ingredients that can be absorbed into the rat plasma according to which ingredients exhibit significant correlation of drug concentration-time curve. Second, to establish an efficient ultra-performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) method for simultaneous determination of ingredients absorbed in rat plasma. Finally, to investigate gender effect on the pharmacokinetics of the ingredients absorbed in male and female rats plasma after oral administration with EC extract.

MATERIALS AND METHODS

18 ingredients from EC were detected by UPLC-MS/MS, 9 out of 18 ingredients were absorbed into rat plasma. And 5 ingredients exhibit significant correlation of drug concentration-time curve. They were pinoresinol di-O-β-d-glucopyranoside (PDG), geniposide (GE), geniposidic acid (GA), aucubin (AN) and chlorogenic acid (CA). The analytes were extracted from rat plasma via a simple protein precipitation procedure and osalmid was used as the internal standard. Chromatographic separation was achieved on a Waters ACQUITY HSS T3 column (2.1mm×100mm, 1.8μm) using a gradient elution program with acetonitrile and 0.1% formic acid water as the mobile phase, with a flow rate of 0.3mLmin(-1). The detection was performed on a triple-quadrupole tandem mass spectrometer by multiple reactions monitoring (MRM) mode in a positive ion mode via electrospray ionization (ESI). The transition monitored were /z 683.00[M+H](+)→235.10 for PDG, / z 389.00[M+H](+)→208.80 for GE, m/z 375.00[M+H](+)→194.79 for GA, m/z 364.00[M+NH4](+)→148.81 for AN, m/z 355.10[M+H](+)→162.84 for CA and m/z 230.03[M+H](+)→120.77 for internal standard.

RESULTS

The developed method showed good linearity over a wide concentration range, the lower limits of quantification and higher accuracy and precision for determination of the 5 analytes. Then the method was applied to study the pharmacokinetics in rats, and the results indicated that there were significant differences in pharmacokinetic parameters of the analytes between the male and female rats, and absorptions of these analytes in male group were all significantly higher than those in female group.

CONCLUSION

This study established an efficient, sensitive and selective UPLC-MS/MS method for simultaneous determination of the five ingredients in rat plasma, and it could be successfully applied to the comparative pharmacokinetic studies in male and female rats after oral administration with EC extract.

Antimutagenic and anticancer activity of Darjeeling tea in multiple test systems

Background

Darjeeling tea, a most popular variety of black tea, though consumed by the people in different parts of world but its beneficial health effects have not been investigated in details. In this study, the antimutagenic and anticancer effect of Darjeeling tea extract (DTE) has been evaluated.

Methods

Antimutagenic activity of the DTE was carried out in two different strains of Salmonella typhimurium by AMES test against a known mutagen benzo[a]pyrene (B[a]P) with S9 activation. Moreover, anticlastogenic property of DTE was also measured by micronuclei formation (MN) against B[a]P with S9 activation in human lymphocytes. The anticancer activity of the same was studied on U937 cell line. Here, Human PBMCs were used as the normal cell control to identify selective anticancer activity of the extract against U937 cells.

Results

The results showed significant antimutagenic activity on bacterial strains. A significant decrease in MN was also observed in the DTE treated human lymphocyte cultures pretreated with B[a]P when compared with B[a]P treated cultures alone. The study clearly exhibited anticancer activity of the extract on U937 cell line. Further studies also revealed that apoptosis induction is an important mechanism behind the anticancer effect of DTE.

Conclusion

Overall, this study indicates that DTE has significant antimutagenic and anticancer activities on bacterial and mammalian cells respectively.

5'-AMP-activated protein kinase attenuates adriamycin-induced oxidative podocyte injury through thioredoxin-mediated suppression of the apoptosis signal-regulating kinase 1-P38 signaling pathway

Oxidative stress-induced podocyte injury is one of the major mechanisms underlying the initiation and progression of glomerulosclerosis. 5'-AMP-activated protein kinase (AMPK), a serine/threonine kinase that senses intracellular energy status and maintains energy homeostasis, is reported to have antioxidative effects. However, little is known about its application and mechanism. In this study, we investigated whether and how AMPK affected oxidative podocyte injury induced by Adriamycin (ADR; Wako Pure Chemical, Osaka, Japan). Exposure of podocytes to ADR resulted in cell injury, which was preceded by increased reactive oxygen species (ROS) generation and P38 activation. Prevention of oxidative stress with the antioxidant N-acetyl-cysteine and glutathione or inhibition of P38 with SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole] attenuated cell injury. Activation of AMPK with three structurally different AMPK activators also protected podocytes from ADR-elicited cell injury. This effect was associated with strong suppression of oxidative stress-sensitive kinase apoptosis signal-regulating kinase 1 (ASK1) and P38 without obvious influence on ROS level. Further analyses revealed that AMPK promoted thioredoxin (Trx) binding to ASK1. Consistently, AMPK potently suppressed the expression of thioredoxin-interacting protein (TXNIP), a negative regulator of Trx, whereas it significantly enhanced the activity of Trx reductases that convert oxidized Trx to reduced form. In further support of a key role of Trx, downregulation or inhibition of Trx exaggerated but downregulation of TXNIP attenuated the cell injury. These results indicate that AMPK prevents oxidative cell injury through Trx-mediated suppression of ASK1-P38 signaling pathway. Our findings thus provide novel mechanistic insights into the antioxidative actions of AMPK. AMPK could be developed as a novel therapeutic target for treatment of oxidative cell injury.

Mechanism of the Inhibitory Effects of Eucommia ulmoides Oliv. Cortex Extracts (EUCE) in the CCl 4 -Induced Acute Liver Lipid Accumulation in Rats

Eucommia ulmoides Oliv. (EU) has been used for treatment of liver diseases. The protective effects of Eucommia Ulmoides Oliv. cortex extracts (EUCE) on the carbon tetrachloride- (CCl4-) induced hepatic lipid accumulation were examined in this study. Rats were orally treated with EUCE in different doses prior to an intraperitoneal injection of 1 mg/kg CCl4. Acute injection of CCl4 decreased plasma triglyceride but increased hepatic triglyceride and cholesterol as compared to control rats. On the other hand, the pretreatment with EUCE diminished these effects at a dose-dependent manner. CCl4 treatment decreased glutathione (GSH) and increased malondialdehyde (MDA) accompanied by activated P450 2E1. The pretreatment with EUCE significantly improved these deleterious effects of CCl4. CCl4 treatment increased P450 2E1 activation and ApoB accumulation. Pretreatment with EUCE reversed these effects. ER stress response was significantly increased by CCl4, which was inhibited by EUCE. One of the possible ER stress regulatory mechanisms, lysosomal activity, was examined. CCl4 reduced lysosomal enzymes that were reversed with the EUCE. The results indicate that oral pretreatment with EUCE may protect liver against CCl4-induced hepatic lipid accumulation. ER stress and its related ROS regulation are suggested as a possible mechanism in the antidyslipidemic effect of EUCE.

Small molecule inhibitors of mammalian thioredoxin reductase

Mammalian thioredoxin reductases (TrxRs) are a family of NADPH-dependent flavoproteins with a penultimate selenocysteine residue at the carboxy-terminus. Besides their native substrate thioredoxins (Trx), the enzymes show a broad substrate specificity, at least partially, because of the C-terminal redox-active site that is easily accessible in the reduced form. TrxRs are ubiquitous in all kinds of cells and have a critical role in regulating intracellular redox signaling. In recent years, a wealth of evidence has revealed that overactivation/dysfunction of TrxRs is closely related to various diseases, especially in tumor development, and thus the past decades have witnessed an expanding interest in finding TrxRs inhibitors, which might be promising agents for cancer chemotherapy. Herein we reviewed the small molecule inhibitors of mammalian TrxRs, with an emphasis on those that have potential anticancer activity. This review includes the nonpatent references up to 2010 that deal with mammalian TrxR inhibitors.

Epigallocatechin-3-gallate potently inhibits the in vitro activity of hydroxy-3-methyl-glutaryl-CoA reductase

Hydroxy-3-methyl-glutaryl-CoA reductase (HMGR) is the rate-controlling enzyme of cholesterol synthesis, and owing to its biological and pharmacological relevance, researchers have investigated several compounds capable of modulating its activity with the hope of developing new hypocholesterolemic drugs. In particular, polyphenol-rich extracts were extensively tested for their cholesterol-lowering effect as alternatives, or adjuvants, to the conventional statin therapies, but a full understanding of the mechanism of their action has yet to be reached. Our work reports on a detailed kinetic and equilibrium study on the modulation of HMGR by the most-abundant catechin in green tea, epigallocatechin-3-gallate (EGCG). Using a concerted approach involving spectrophotometric, optical biosensor, and chromatographic analyses, molecular docking, and site-directed mutagenesis on the cofactor site of HMGR, we have demonstrated that EGCG potently inhibits the in vitro activity of HMGR (K(i) in the nanomolar range) by competitively binding to the cofactor site of the reductase. Finally, we evaluated the effect of combined EGCG-statin administration.

Hepatocytes lacking thioredoxin reductase 1 have normal replicative potential during development and regeneration

Cells require ribonucleotide reductase (RNR) activity for DNA replication. In bacteria, electrons can flow from NADPH to RNR by either a thioredoxin-reductase- or a glutathione-reductase-dependent route. Yeast and plants artificially lacking thioredoxin reductases exhibit a slow-growth phenotype, suggesting glutathione-reductase-dependent routes are poor at supporting DNA replication in these organisms. We have studied proliferation of thioredoxin-reductase-1 (Txnrd1)-deficient hepatocytes in mice. During development and regeneration, normal mice and mice having Txnrd1-deficient hepatocytes exhibited similar liver growth rates. Proportions of hepatocytes that immunostained for PCNA, phosphohistone H3 or incorporated BrdU were also similar, indicating livers of either genotype had similar levels of proliferative, S and M phase hepatocytes, respectively. Replication was blocked by hydroxyurea, confirming that RNR activity was required by Txnrd1-deficient hepatocytes. Regenerative thymidine incorporation was similar in normal and Txnrd1-deficient livers, further indicating that DNA synthesis was unaffected. Using genetic chimeras in which a fluorescently marked subset of hepatocytes was Txnrd1-deficient while others were not, we found that the multigenerational contributions of both hepatocyte types to development and to liver regeneration were indistinguishable. We conclude that, in mouse hepatocytes, a Txnrd1-independent route for the supply of electrons to RNR can fully support DNA replication and normal proliferative growth.

Thioredoxin and Cancer: A Role for Thioredoxin in all States of Tumor Oxygenation

Thioredoxin is a small redox-regulating protein, which plays crucial roles in maintaining cellular redox homeostasis and cell survival and is highly expressed in many cancers. The tumor environment is usually under either oxidative or hypoxic stress and both stresses are known up-regulators of thioredoxin expression. These environments exist in tumors because their abnormal vascular networks result in an unstable oxygen delivery. Therefore, the oxygenation patterns in human tumors are complex, leading to hypoxia/re-oxygenation cycling. During carcinogenesis, tumor cells often become more resistant to hypoxia or oxidative stress-induced cell death and most studies on tumor oxygenation have focused on these two tumor environments. However, recent investigations suggest that the hypoxic cycling occurring within tumors plays a larger role in the contribution to tumor cell survival than either oxidative stress or hypoxia alone. Thioredoxin is known to have important roles in both these cellular responses and several studies implicate thioredoxin as a contributor to cancer progression. However, only a few studies exist that investigate the regulation of thioredoxin in the hypoxic and cycling hypoxic response in cancers. This review focuses on the role of thioredoxin in the various states of tumor oxygenation.