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1.  The antimalarial amodiaquine causes autophagic-lysosomal and proliferative blockade sensitizing human melanoma cells to starvation- and chemotherapy-induced cell death 
Autophagy  2013;9(12):2087-2102.
Pharmacological inhibition of autophagic-lysosomal function has recently emerged as a promising strategy for chemotherapeutic intervention targeting cancer cells. Repurposing approved and abandoned non-oncological drugs is an alternative approach to the identification and development of anticancer therapeutics, and antimalarials that target autophagic-lysosomal functions have recently attracted considerable attention as candidates for oncological repurposing. Since cumulative research suggests that dependence on autophagy represents a specific vulnerability of malignant melanoma cells, we screened a focused compound library of antimalarials for antimelanoma activity. Here we report for the first time that amodiaquine (AQ), a clinical 4-aminoquinoline antimalarial with unexplored cancer-directed chemotherapeutic potential, causes autophagic-lysosomal and proliferative blockade in melanoma cells that surpasses that of its parent compound chloroquine. Monitoring an established set of protein markers (LAMP1, LC3-II, SQSTM1) and cell ultrastructural changes detected by electron microscopy, we observed that AQ treatment caused autophagic-lysosomal blockade in malignant A375 melanoma cells, a finding substantiated by detection of rapid inactivation of lysosomal cathepsins (CTSB, CTSL, CTSD). AQ-treatment was associated with early induction of energy crisis (ATP depletion) and sensitized melanoma cells to either starvation- or chemotherapeutic agent-induced cell death. AQ displayed potent antiproliferative effects, and gene expression array analysis revealed changes at the mRNA (CDKN1A, E2F1) and protein level (TP53, CDKN1A, CCND1, phospho-RB1 [Ser 780]/[Ser 807/811], E2F1) consistent with the observed proliferative blockade in S-phase. Taken together, our data suggest that the clinical antimalarial AQ is a promising candidate for repurposing efforts that aim at targeting autophagic-lysosomal function and proliferative control in malignant melanoma cells.
PMCID: PMC3981748  PMID: 24113242
malignant melanoma; amodiaquine; chloroquine; autophagy; lysosome; cathepsin; E2F1; CDKN1A
2.  Tanshinone I Activates the Nrf2-Dependent Antioxidant Response and Protects Against As(III)-Induced Lung Inflammation In Vitro and In Vivo 
Antioxidants & Redox Signaling  2013;19(14):1647-1661.
Aims: The NF-E2 p45-related factor 2 (Nrf2) signaling pathway regulates the cellular antioxidant response and activation of Nrf2 has recently been shown to limit tissue damage from exposure to environmental toxicants, including As(III). In an attempt to identify improved molecular agents for systemic protection against environmental insults, we have focused on the identification of novel medicinal plant-derived Nrf2 activators. Results: Tanshinones [tanshinone I (T-I), tanshinone IIA, dihydrotanshinone, cryptotanshinone], phenanthrenequinone-based redox therapeutics derived from the medicinal herb Salvia miltiorrhiza, have been tested as experimental therapeutics for Nrf2-dependent cytoprotection. Using a dual luciferase reporter assay overexpressing wild-type or mutant Kelch-like ECH-associated protein-1 (Keap1), we demonstrate that T-I is a potent Keap1-C151-dependent Nrf2 activator that stabilizes Nrf2 by hindering its ubiquitination. In human bronchial epithelial cells exposed to As(III), T-I displays pronounced cytoprotective activity with upregulation of Nrf2-orchestrated gene expression. In Nrf2 wild-type mice, systemic administration of T-I attenuates As(III) induced inflammatory lung damage, a protective effect not observed in Nrf2 knockout mice. Innovation: Tanshinones have been identified as a novel class of Nrf2-inducers for antioxidant tissue protection in an in vivo As(III) inhalation model, that is relevant to low doses of environmental exposure. Conclusion: T-I represents a prototype Nrf2-activator that displays cytoprotective activity upon systemic administration targeting lung damage originating from environmental insults. T-I based Nrf2-directed systemic intervention may provide therapeutic benefit in protecting other organs against environmental insults. Antioxid. Redox Signal. 19, 1647–1661.
PMCID: PMC3809600  PMID: 23394605
3.  Nrf2 Modulates Contractile and Metabolic Properties of Skeletal Muscle in Streptozotocin-Induced Diabetic Atrophy 
Experimental cell research  2013;319(17):10.1016/j.yexcr.2013.07.015.
The role of Nrf2 in disease prevention and treatment is well documented, however the specific role of Nrf2 in skeletal muscle is not well described. The current study investigated whether Nrf2 plays a protective role in an STZ-induced model of skeletal muscle atrophy.
Modulation of Nrf2 through siRNA resulted in a more robust differentiation of C2C12s, whereas increasing Nrf2 with sulforaphane treatment inhibited differentiation. Diabetic muscle atrophy was not dramatically influenced by Nrf2 genotype, since no differences were observed in total atrophy (all fiber types combined) between WT+STZ and KO+STZ animals. Nrf2-KO animals however, illustrated alterations in muscle size of Fast, Type II myosin expressing fibers. KO+STZ animals show significant alterations in myosin isoform expression in the GAST. Similarly, KO controls mimic both WT+STZ and KO+STZ muscle alterations in mitochondrial subunit expression. PGC-1α, a well-established player in mitochondrial biogenesis and myosin isoform expression, was decreased in KO control, WT+STZ and KO+STZ SOL muscle. Similarly, PGC-1α protein levels are correlated with Nrf2 levels in C2C12s after modulation by Nrf2 siRNA or sulforaphane treatment.
We provide experimental evidence indicating Nrf2 plays a role in myocyte differentiation and governs molecular alterations in contractile and metabolic properties in an STZ-induced model of muscle atrophy.
PMCID: PMC3809009  PMID: 23896025
Nrf2; skeletal muscle; atrophy; myosin; metabolism
4.  Zinc pyrithione impairs zinc homeostasis and upregulates stress response gene expression in reconstructed human epidermis 
Zinc ion homeostasis plays an important role in human cutaneous biology where it is involved in epidermal differentiation and barrier function, inflammatory and antimicrobial regulation, and wound healing. Zinc-based compounds designed for topical delivery therefore represent an important class of cutaneous therapeutics. Zinc pyrithione (ZnPT) is an FDA-approved microbicidal agent used worldwide in over-the-counter topical antimicrobials, and has also been examined as an investigational therapeutic targeting psoriasis and UVB-induced epidermal hyperplasia. Recently, we have demonstrated that cultured primary human skin keratinocytes display an exquisite sensitivity to nanomolar ZnPT concentrations causing induction of heat shock response gene expression and poly(ADP-ribose) polymerase (PARP)-dependent cell death (Cell Stress Chaperones 15:309–322, 2010). Here we demonstrate that ZnPT causes rapid accumulation of intracellular zinc in primary keratinocytes as observed by quantitative fluorescence microscopy and inductively coupled plasma mass spectrometry (ICP-MS), and that PARP activation, energy crisis, and genomic impairment are all antagonized by zinc chelation. In epidermal reconstructs (EpiDerm™) exposed to topical ZnPT (0.1–2% in Vanicream™), ICP-MS demonstrated rapid zinc accumulation, and expression array analysis demonstrated upregulation of stress response genes encoding metallothionein-2A (MT2A), heat shock proteins (HSPA6, HSPA1A, HSPB5, HSPA1L, DNAJA1, HSPH1, HSPD1, HSPE1), antioxidants (SOD2, GSTM3, HMOX1), and the cell cycle inhibitor p21 (CDKN1A). IHC analysis of ZnPT-treated EpiDerm™ confirmed upregulation of Hsp70 and TUNEL-positivity. Taken together our data demonstrate that ZnPT impairs zinc ion homeostasis and upregulates stress response gene expression in primary keratinocytes and reconstructed human epidermis, activities that may underlie therapeutic and toxicological effects of this topical drug.
PMCID: PMC4169675  PMID: 21424779
Zinc pyrithione; Keratinocyte; Reconstructed epidermis; Heat shock response; HSPA1A; ICP-MS
5.  DCPIP (2,6-dichlorophenolindophenol) as a genotype-directed redox chemotherapeutic targeting NQO1*2 breast carcinoma 
Free radical research  2010;45(3):276-292.
Accumulative experimental evidence suggests feasibility of chemotherapeutic intervention targeting human cancer cells by pharmacological modulation of cellular oxidative stress. Current efforts aim at personalization of redox chemotherapy through identification of predictive tumour genotypes and redox biomarkers. Based on earlier research demonstrating that anti-melanoma activity of the pro-oxidant 2,6-dichlorophenolindophenol (DCPIP) is antagonized by cellular NAD(P) H:quinone oxidoreductase (NQO1) expression, this study tested DCPIP as a genotype-directed redox chemotherapeutic targeting homozygous NQO1*2 breast carcinoma, a common missense genotype [rs1800566 polymorphism; NP_000894.1:p. Pro187Ser] encoding a functionally impaired NQO1 protein. In a panel of cultured breast carcinoma cell lines and NQO1-transfectants with differential NQO1 expression levels, homozygous NQO1*2 MDA-MB231 cells were hypersensitive to DCPIP-induced caspase-independent cell death that occurred after early onset of oxidative stress with glutathione depletion and loss of genomic integrity. Array analysis revealed upregulated expression of oxidative (GSTM3, HMOX1, EGR1), heat shock (HSPA6, HSPA1A, CRYAB) and genotoxic stress response (GADD45A, CDKN1A) genes confirmed by immunoblot detection of HO-1, Hsp70, Hsp70B′, p21 and phospho-p53 (Ser15). In a murine xenograft model of human homozygous NQO1*2-breast carcinoma, systemic administration of DCPIP displayed significant anti-tumour activity, suggesting feasibility of redox chemotherapeutic intervention targeting the NQO1*2 genotype.
PMCID: PMC4101082  PMID: 21034357
Redox chemotherapy; 2,6-dichlorophenolindophenol; xenograft; breast carcinoma; MDA-MB231; NQO1*2 genotype
6.  Autophagic-lysosomal dysregulation downstream of cathepsin B inactivation in human skin fibroblasts exposed to UVA 
Recently, using 2D-DIGE proteomics we have identified cathepsin B as a novel target of UVA in human Hs27 skin fibroblasts. In response to chronic exposure to noncytotoxic doses of UVA (9.9 J/cm2, twice a week, 3 weeks), photooxidative impairment of cathepsin B enzymatic activity occurred with accumulation of autofluorescent aggregates colocalizing with lysosomes, an effect mimicked by pharmacological antagonism of cathepsin B using the selective inhibitor CA074Me. Here, we have further explored the mechanistic involvement of cathepsin B inactivation in UVA-induced autophagic-lysosomal alterations using autophagy-directed PCR expression array analysis as a discovery tool. Consistent with lysosomal expansion, UVA upregulated cellular protein levels of the lysosomal marker glycoprotein Lamp-1, and increased levels of the lipidated autophagosomal membrane constituent LC3-II were detected. UVA did not alter expression of beclin 1 (BECN1), an essential factor for initiation of autophagy, but upregulation of p62 (sequestosome 1, SQSTM1), a selective autophagy substrate, and α-synuclein (SNCA), an autophagic protein substrate and aggresome component, was observed at the mRNA and protein level. Moreover, UVA downregulated transglutaminase-2 (TGM2), an essential enzyme involved in autophagolysosome maturation. Strikingly, UVA effects on Lamp-1, LC3-II, beclin 1, p62, α-synuclein, and transglutaminase-2 were mimicked by CA074Me treatment. Taken together, our data suggest that UVA-induced autophagic-lysosomal alterations occur as a consequence of impaired autophagic flux downstream of cathepsin B inactivation, a novel molecular mechanism potentially involved in UVA-induced skin photodamage.
PMCID: PMC4089038  PMID: 21773629
UVA; skin photodamage; cathepsin B; p62; oxidative stress; autophagy
7.  UVA Causes Dual Inactivation of Cathepsin B and L Underlying Lysosomal Dysfunction in Human Dermal Fibroblasts 
Cutaneous exposure to chronic solar UVA-radiation is a causative factor in photocarcinogenesis and photoaging. Recently, we have identified the thiol-dependent cysteine-protease cathepsin B as a novel UVA-target undergoing photo-oxidative inactivation upstream of autophagic-lysosomal dysfunction in fibroblasts. In this study, we examined UVA effects on a wider range of cathepsins and explored the occurrence of UVA-induced cathepsin inactivation in other cultured skin cell types. In dermal fibroblasts, chronic exposure to non-cytotoxic doses of UVA caused pronounced inactivation of the lysosomal cysteine-proteases cathepsin B and L, effects not observed in primary keratinocytes and occurring only to a minor extent in primary melanocytes. In order to determine if UVA-induced lysosomal impairment requires single or dual inactivation of cathepsin B and/or L, we used a genetic approach (siRNA) to selectively downregulate enzymatic activity of these target cathepsins. Monitoring an established set of protein markers (including LAMP1, LC3-II, and p62) and cell ultrastructural changes detected by electron microscopy, we observed that only dual genetic antagonism (targeting both CTSB and CTSL expression) could mimic UVA-induced autophagic-lysosomal alterations, whereas single knockdown (targeting CTSB or CTSL only) did not display ‘UVA-mimetic’ effects failing to reproduce the UVA-induced phenotype. Taken together, our data demonstrate that chronic UVA inhibits both cathepsin B and L enzymatic activity and that dual inactivation of both enzymes is a causative factor underlying UVA-induced impairment of lysosomal function in dermal fibroblasts.
PMCID: PMC3710731  PMID: 23603447
UVA; skin photodamage; cathepsin B; cathepsin L; fibroblast
8.  The Nrf2-inducers tanshinone I and dihydrotanshinone protect human skin cells and reconstructed human skin against solar simulated UV☆ 
Redox Biology  2013;1(1):532-541.
Exposure to solar ultraviolet (UV) radiation is a causative factor in skin photocarcinogenesis and photoaging, and an urgent need exists for improved strategies for skin photoprotection. The redox-sensitive transcription factor Nrf2 (nuclear factor-E2-related factor 2), a master regulator of the cellular antioxidant defense against environmental electrophilic insult, has recently emerged as an important determinant of cutaneous damage from solar UV, and the concept of pharmacological activation of Nrf2 has attracted considerable attention as a novel approach to skin photoprotection. In this study, we examined feasibility of using tanshinones, a novel class of phenanthrenequinone-based cytoprotective Nrf2 inducers derived from the medicinal plant Salvia miltiorrhiza, for protection of cultured human skin cells and reconstructed human skin against solar simulated UV. Using a dual luciferase reporter assay in human Hs27 dermal fibroblasts pronounced transcriptional activation of Nrf2 by four major tanshinones [tanshinone I (T-I), dihydrotanshinone (DHT), tanshinone IIA (T-II-A) and cryptotanshinone (CT)] was detected. In fibroblasts, the more potent tanshinones T-I and DHT caused a significant increase in Nrf2 protein half-life via blockage of ubiquitination, ultimately resulting in upregulated expression of cytoprotective Nrf2 target genes (GCLC, NQO1) with the elevation of cellular glutathione levels. Similar tanshinone-induced changes were also observed in HaCaT keratinocytes. T-I and DHT pretreatment caused significant suppression of skin cell death induced by solar simulated UV and riboflavin-sensitized UVA. Moreover, feasibility of tanshinone-based cutaneous photoprotection was tested employing a human skin reconstruct exposed to solar simulated UV (80 mJ/cm2 UVB; 1.53 J/cm2 UVA). The occurrence of markers of epidermal solar insult (cleaved procaspase 3, pycnotic nuclei, eosinophilic cytoplasm, acellular cavities) was significantly attenuated in DHT-treated reconstructs that displayed increased immunohistochemical staining for Nrf2 and γ-GCS together with the elevation of total glutathione levels. Taken together, our data suggest the feasibility of achieving tanshinone-based cutaneous Nrf2-activation and photoprotection.
Graphical abstract
•Tanshinones are phenanthrenequinone-based Nrf2 inducers active in human skin cells.•Tanshinones upregulate Nrf2 target gene expression with the elevation of glutathione.•Dihydrotanshinone protects cultured human skin cells against solar simulated UV.•Dihydrotanshinone protects reconstructed human skin against acute photodamage.
PMCID: PMC3836278  PMID: 24273736
CHX, cycloheximide; CT, cryptotanshinone; DHT, dihydrotanshinone; DMEM, Dulbecco's modified Eagle's medium; γ-GCS, gamma-glutamate-cysteine ligase; H&E, hematoxylin and eosin; HMOX1, heme oxygenase-1; IHC, immunohistochemistry; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide; NQO1, NAD(P)H quinone oxidoreductase 1; Nrf2, nuclear factor-E2-related factor 2; ROS, reactive oxygen species; SF, sulforaphane; SLL, solar simulated UV light; T-I, tanshinone I; T-II-A, tanshinone IIA; UVA, ultraviolet; UVB, ultraviolet B; Tanshinone I; Dihydrotanshinone; Nrf2; Solar simulated ultraviolet light; Skin photoprotection
9.  D-Penicillamine targets metastatic melanoma cells with induction of the unfolded protein response (UPR) and Noxa (PMAIP1)-dependent mitochondrial apoptosis 
D-penicillamine (3,3-Dimethyl-D-cysteine; DP) is an FDA-approved redox-active D-cysteine-derivative with antioxidant, disulfide-reducing, and metal chelating properties used therapeutically for the control of copper-related pathology in Wilson’s disease and reductive cystine-solubilization in cystinuria. Based on the established sensitivity of metastatic melanoma cells to pharmacological modulation of cellular oxidative stress, we tested feasibility of using DP for chemotherapeutic intervention targeting human A375 melanoma cells in vitro and in vivo. DP treatment induced caspase-dependent cell death in cultured human metastatic melanoma cells (A375, G361) without compromising viability of primary epidermal melanocytes, an effect not observed with the thiol-antioxidants N-acetyl-L-cysteine (NAC) and dithiothreitol. Focused gene expression array analysis followed by immunoblot detection revealed that DP rapidly activates the cytotoxic unfolded protein response (UPR; involving phospho-PERK, phospho-eIF2α, Grp78, CHOP, and Hsp70) and the mitochondrial pathway of apoptosis with p53 upregulation and modulation of Bcl-2 family members (involving Noxa, Mcl-1, and Bcl-2). DP (but not NAC) induced oxidative stress with early impairment of glutathione homeostasis and mitochondrial transmembrane potential. SiRNA-based antagonism of PMAIP1 expression blocked DP-induced upregulation of the proapoptotic BH3-only effector Noxa and prevented downregulation of the Noxa-antagonist Mcl-1, rescuing melanoma cells from DP-induced apoptosis. Intraperitoneal administration of DP displayed significant antimelanoma activity in a murine A375 xenograft model. It remains to be seen if melanoma cell-directed induction of UPR and apoptosis using DP or improved DP-derivatives can be harnessed for future chemotherapeutic intervention.
PMCID: PMC3779642  PMID: 22843330
Metastatic melanoma; D-penicillamine; Noxa (PMAIP1); Mcl-1; unfolded protein response (UPR); apoptosis
10.  The redox antimalarial dihydroartemisinin targets human metastatic melanoma cells but not primary melanocytes with induction of NOXA-dependent apoptosis 
Investigational New Drugs  2011;30(4):1289-1301.
Recent research suggests that altered redox control of melanoma cell survival, proliferation, and invasiveness represents a chemical vulnerability that can be targeted by pharmacological modulation of cellular oxidative stress. The endoperoxide artemisinin and semisynthetic artemisinin-derivatives including dihydroartemisinin (DHA) constitute a major class of antimalarials that kill plasmodium parasites through induction of iron-dependent oxidative stress. Here, we demonstrate that DHA may serve as a redox chemotherapeutic that selectively induces melanoma cell apoptosis without compromising viability of primary human melanocytes. Cultured human metastatic melanoma cells (A375, G361, LOX) were sensitive to DHA-induced apoptosis with upregulation of cellular oxidative stress, phosphatidylserine externalization, and activational cleavage of procaspase 3. Expression array analysis revealed DHA-induced upregulation of oxidative and genotoxic stress response genes (GADD45A, GADD153, CDKN1A, PMAIP1, HMOX1, EGR1) in A375 cells. DHA exposure caused early upregulation of the BH3-only protein NOXA, a proapototic member of the Bcl2 family encoded by PMAIP1, and genetic antagonism (siRNA targeting PMAIP1) rescued melanoma cells from apoptosis indicating a causative role of NOXA-upregulation in DHA-induced melanoma cell death. Comet analysis revealed early DHA-induction of genotoxic stress accompanied by p53 activational phosphorylation (Ser 15). In primary human epidermal melanocytes, viability was not compromised by DHA, and oxidative stress, comet tail moment, and PMAIP1 (NOXA) expression remained unaltered. Taken together, these data demonstrate that metastatic melanoma cells display a specific vulnerability to DHA-induced NOXA-dependent apoptosis and suggest feasibility of future antimelanoma intervention using artemisinin-derived clinical redox antimalarials.
PMCID: PMC3203350  PMID: 21547369
Malignant melanoma; Dihydroartemisinin; PMAIP1; Reactive oxygen species; Oxidative stress; Apoptosis
11.  Cinnamoyl-based Nrf2-Activators Targeting Human Skin Cell Photo-oxidative Stress 
Free radical biology & medicine  2008;45(4):385-395.
Strong experimental evidence suggests the involvement of photo-oxidative stress mediated by reactive oxygen species as a crucial mechanism of solar damage relevant to human skin photoaging and photocarcinogenesis. Based on the established role of antioxidant response element (ARE)-mediated gene expression in cancer chemoprevention, we tested the hypothesis that small molecule Nrf2-activators may serve a photo-chemopreventive role by targeting skin cell photo-oxidative stress. A luciferase-based reporter gene assay was used as a primary screen for the identification of novel agents that modulate the Nrf2-Keap1 signaling pathway. A series of cinnamoyl-based electrophilic Michael acceptors including cinnamic aldehyde and methyl-1-cinnamoyl-5-oxo-2-pyrrolidine-carboxylate was identified as potent Nrf2-activators. Hit confirmation was performed in a secondary screen, based on immunodetection of Nrf2 protein upregulation in human Hs27 skin fibroblasts, HaCaT keratinocytes, and primary skin keratinocytes. Bioefficacy profiling of positive test compounds in skin cells demonstrated compound-induced upregulation of hemeoxygenase I and NAD(P)H-quinone oxidoreductase, two Nrf2 target genes involved in the cellular antioxidant response. Pretreatment with cinnamoyl-based Nrf2-activators suppressed intracellular oxidative stress and protected against photo-oxidative induction of apoptosis in skin cells exposed to high doses of singlet oxygen. Our pilot studies suggest feasibility of developing cinnamoyl-based Nrf2-activators as novel photo-chemopreventive agents targeting skin cell photo-oxidative stress.
PMCID: PMC3710742  PMID: 18482591
Nrf2; skin cancer; photo-oxidative stress; photo-chemoprevention; Michael acceptor; cinnamic aldehyde; singlet oxygen
12.  Thiostrepton is an Inducer of Oxidative and Proteotoxic Stress that Impairs Viability of Human Melanoma Cells but not Primary Melanocytes 
Biochemical Pharmacology  2012;83(9):1229-1240.
Pharmacological induction of oxidative and proteotoxic stress has recently emerged as a promising strategy for chemotherapeutic intervention targeting cancer cells. Guided by a differential phenotypic drug screen for novel lead compounds that selectively induce melanoma cell apoptosis without compromising viability of primary human melanocytes, we have focused on the cyclic pyridinyl-polythiazolyl peptide-antimicrobial thiostrepton. Using comparative gene expression-array analysis, the early cellular stress response induced by thiostrepton was examined in human A375 metastatic melanoma cells and primary melanocytes. Thiostrepton displayed selective antimelanoma activity causing early induction of proteotoxic stress with massive upregulation of heat shock (HSPA6, HSPA1A, DNAJB4, HSPB1, HSPH1, HSPA1L, CRYAB, HSPA5, DNAJA1), oxidative stress (HMOX1, GSR, SOD1), and ER stress response (DDIT3) gene expression, confirmed by immunodetection (Hsp70, Hsp70B′, HO-1, phospho-eIF2α). Moreover, upregulation of p53, proapoptotic modulation of Bcl-2 family members (Bax, Noxa, Mcl-1, Bcl-2), and induction of apoptotic cell death were observed. Thiostrepton rapidly induced cellular oxidative stress followed by inactivation of chymotrypsin-like proteasomal activity and melanoma cell-directed accumulation of ubiquitinated proteins, not observed in melanocytes that were resistant to thiostrepton-induced apoptosis. Proteotoxic and apoptogenic effects were fully antagonized by antioxidant intervention. In RPMI 8226 multiple myeloma cells, known to be exquisitely sensitive to proteasome inhibition, early proteotoxic and apoptogenic effects of thiostrepton were confirmed by array analysis indicating pronounced upregulation of heat shock response gene expression. Our findings demonstrate that thiostrepton displays dual activity as a selective prooxidant and proteotoxic chemotherapeutic, suggesting feasibility of experimental intervention targeting metastatic melanoma and other malignancies including multiple myeloma.
PMCID: PMC3299892  PMID: 22321511
malignant melanoma; oxidative and proteotoxic stress; heat shock response; proteasome; thiostrepton
13.  Phenotypic Identification of the Redox Dye Methylene Blue as an Antagonist of Heat Shock Response Gene Expression in Metastatic Melanoma Cells 
Repurposing approved and abandoned non-oncological drugs is an alternative developmental strategy for the identification of anticancer therapeutics that has recently attracted considerable attention. Due to the essential role of the cellular heat shock response in cytoprotection through the maintenance of proteostasis and suppression of apoptosis, small molecule heat shock response antagonists can be harnessed for targeted induction of cytotoxic effects in cancer cells. Guided by gene expression array analysis and a phenotypic screen interrogating a collection of 3,7-diamino-phenothiazinium derivatives, we have identified the redox-drug methylene blue (MB), used clinically for the infusional treatment of methemoglobinemia, as a negative modulator of heat shock response gene expression in human metastatic melanoma cells. MB-treatment blocked thermal (43 °C) and pharmacological (celastrol, geldanamycin) induction of heat shock response gene expression, suppressing Hsp70 (HSPA1A) and Hsp27 (HSPB1) upregulation at the mRNA and protein level. MB sensitized melanoma cells to the apoptogenic activity of geldanamycin, an Hsp90 antagonist known to induce the counter-regulatory upregulation of Hsp70 expression underlying cancer cell resistance to geldanamycin chemotherapy. Similarly, MB-cotreatment sensitized melanoma cells to other chemotherapeutics (etoposide, doxorubicin). Taken together, these data suggest feasibility of repurposing the non-oncological redox drug MB as a therapeutic heat shock response antagonist for cancer cell chemosensitization.
PMCID: PMC3588094  PMID: 23429201
malignant melanoma; heat shock response; Hsp70; Hsp27; methylene blue; chemosensitization
14.  Therapeutic Potential of Nrf2 Activators in Streptozotocin-Induced Diabetic Nephropathy 
Diabetes  2011;60(11):3055-3066.
To determine whether dietary compounds targeting NFE2-related factor 2 (Nrf2) activation can be used to attenuate renal damage and preserve renal function during the course of streptozotocin (STZ)-induced diabetic nephropathy.
Diabetes was induced in Nrf2+/+ and Nrf2−/− mice by STZ injection. Sulforaphane (SF) or cinnamic aldehyde (CA) was administered 2 weeks after STZ injection and metabolic indices and renal structure and function were assessed (18 weeks). Markers of diabetes including blood glucose, insulin, polydipsia, polyuria, and weight loss were measured. Pathological alterations and oxidative damage in glomeruli were also determined. Changes in protein expression of the Nrf2 pathway, as well as transforming growth factor-β1 (TGF-β1), fibronectin (FN), collagen IV, and p21/WAF1Cip1 (p21) were analyzed. The molecular mechanisms of Nrf2-mediated protection were investigated in an in vitro model using human renal mesangial cells (HRMCs).
SF or CA significantly attenuated common metabolic disorder symptoms associated with diabetes in Nrf2+/+ but not in Nrf2−/− mice, indicating SF and CA function through specific activation of the Nrf2 pathway. Furthermore, SF or CA improved renal performance and minimized pathological alterations in the glomerulus of STZ-Nrf2+/+ mice. Nrf2 activation reduced oxidative damage and suppressed the expression of TGF-β1, extracellular matrix proteins and p21 both in vivo and in HRMCs. In addition, Nrf2 activation reverted p21-mediated growth inhibition and hypertrophy of HRMCs under hyperglycemic conditions.
We provide experimental evidence indicating that dietary compounds targeting Nrf2 activation can be used therapeutically to improve metabolic disorder and relieve renal damage induced by diabetes.
PMCID: PMC3198067  PMID: 22025779
15.  The Malondialdehyde-derived Fluorophore DHP-lysine is a Potent Sensitizer of UVA-induced Photooxidative Stress in Human Skin Cells 
Light-driven electron and energy transfer involving non-DNA skin chromophores as endogenous photosensitizers induces oxidative stress in UVA-exposed human skin, a process relevant to photoaging and photocarcinogenesis. Malondialdehyde is an electrophilic dicarbonyl-species derived from membrane lipid peroxidation. Here we present experimental evidence suggesting that the malondialdehyde-derived protein epitope dihydropyridine (DHP)-lysine is a potent endogenous UVA-photosensitizer of human skin cells. Immunohistochemical analysis revealed the abundant occurrence of malondialdehyde-derived and DHP-lysine epitopes in human skin. Using the chemically protected dihydropyridine-derivative (2S)-Boc-2-amino-6-(3,5-diformyl-4-methyl-4H-pyridin-1-yl)-hexanoic acid-t-butylester as a model of peptide-bound DHP-lysine, photodynamic inhibition of proliferation and induction of cell death were observed in human skin Hs27 fibroblasts as well as primary and HaCaT keratinocytes exposed to the combined action of UVA and DHP-lysine. DHP-lysine photosensitization induced intracellular oxidative stress, p38 MAP kinase activation, and upregulation of heme oxygenase-1 expression. Consistent with UVA-driven ROS formation from DHP-lysine, formation of superoxide, hydrogen peroxide, and singlet oxygen was detected in chemical assays, but little protection was achieved using SOD or catalase during cellular photosensitization. In contrast, inclusion of NaN3 completely abolished DHP-photosensitization. Taken together, these data demonstrate photodynamic activity of DHP-lysine and support the hypothesis that malondialdehyde-derived protein-epitopes may function as endogenous sensitizers of UVA-induced oxidative stress in human skin.
PMCID: PMC2963670  PMID: 20724175
photosensitization; UVA; lipid peroxidation; skin photooxidative stress; DHP-lysine
16.  Proteomic Identification of Cathepsin B and Nucleophosmin as Novel UVA-Targets in Human Skin Fibroblasts 
Photochemistry and photobiology  2010;86(6):1307-1317.
Solar UVA exposure plays a causative role in skin photoaging and photocarcinogenesis. Here we describe the proteomic identification of novel UVA-targets in human dermal fibroblasts following a 2D-DIGE (two-dimensional-difference-gel-electrophoresis) approach. Fibroblasts were exposed to non-cytotoxic doses of UVA or left untreated, and total protein extracts underwent CyDye-labeling followed by 2D-DIGE/mass spectrometric identification of differentially expressed proteins, confirmed independently by immunodetection. The protein displaying the most pronounced UVA-induced upregulation was identified as the nucleolar protein nucleophosmin. The protein undergoing the most pronounced UVA-induced downregulation was identified as cathepsin B, a lysosomal cysteine-protease displaying loss of enzymatic activity and altered maturation after cellular UVA exposure. Extensive lysosomal accumulation of lipofuscin-like autofluorescence and osmiophilic material occurred in UVA-exposed fibroblasts as detected by confocal fluorescence microscopy and transmission electron microscopy, respectively. Array analysis indicated UVA-induced upregulation of oxidative stress response gene expression, and UVA-induced loss of cathepsin B enzymatic activity in fibroblasts was suppressed by antioxidant intervention. Pharmacological cathepsin B-inhibition using CA074Me mimicked UVA-induced accumulation of lysosomal autofluorescence and deficient cathepsin B-maturation. Taken together, these data support the hypothesis that cathepsin B is a crucial target of UVA-induced photooxidative stress causatively involved in dermal photodamage through impairment of lysosomal removal of lipofuscin.
PMCID: PMC3001288  PMID: 20946361
17.  The Cinnamon-derived Dietary Factor Cinnamic Aldehyde Activates the Nrf2-dependent Antioxidant Response in Human Epithelial Colon Cells 
Molecules (Basel, Switzerland)  2010;15(5):3338-3355.
Colorectal cancer (CRC) is a major cause of tumor-related morbidity and mortality worldwide. Recent research suggests that pharmacological intervention using dietary factors that activate the redox sensitive Nrf2/Keap1-ARE signaling pathway may represent a promising strategy for chemoprevention of human cancer including CRC. In our search for dietary Nrf2 activators with potential chemopreventive activity targeting CRC, we have focused our studies on trans-cinnamic aldehyde (cinnamaldeyde, CA), the key flavor compound in cinnamon essential oil. Here we demonstrate that CA and an ethanolic extract (CE) prepared from Cinnamomum cassia bark, standardized for CA content by GC-MS analysis, display equipotent activity as inducers of Nrf2 transcriptional activity. In human colon cancer cells (HCT116, HT29) and non-immortalized primary fetal colon cells (FHC), CA- and CE-treatment upregulated cellular protein levels of Nrf2 and established Nrf2 targets involved in the antioxidant response including heme oxygenase 1 (HO-1) and γ-glutamylcysteine synthetase (γ-GCS, catalytic subunit). CA- and CE-pretreatment strongly upregulated cellular glutathione levels and protected HCT116 cells against hydrogen peroxide-induced genotoxicity and arsenic-induced oxidative insult. Taken together our data demonstrate that the cinnamon-derived food factor CA is a potent activator of the Nrf2-orchestrated antioxidant response in cultured human epithelial colon cells. CA may therefore represent an underappreciated chemopreventive dietary factor targeting colorectal carcinogenesis.
PMCID: PMC3101712  PMID: 20657484
colon cancer; Nrf2-activator; cinnamic aldehyde; antioxidant response
18.  GLO1 Overexpression in Human Malignant Melanoma 
Melanoma research  2010;20(2):85-96.
Glyoxalase I [lactoylglutathione lyase (EC encoded by GLO1] is a ubiquitous cellular defense enzyme involved in the detoxification of methylglyoxal, a cytotoxic byproduct of glycolysis. Accumulative evidence suggests an important role of GLO1 expression in protection against methylglyoxal-dependent protein adduction and cellular damage associated with diabetes, cancer, and chronological aging. Based on the hypothesis that GLO1 upregulation may play a functional role in glycolytic adaptations of cancer cells, we examined GLO1 expression status in human melanoma tissue. Quantitative RT-PCR analysis of a cDNA tissue array containing 40 human melanoma tissues (stages III and IV) and 13 healthy controls revealed pronounced upregulation of GLO1 expression at the mRNA level. Immunohistochemical analysis of a melanoma tissue microarray confirmed upregulation of glyoxalase 1 protein levels in malignant melanoma tissue versus healthy human skin. Consistent with an essential role of GLO1 in melanoma cell defense against methylglyoxal cytotoxicity, siRNA interference targeting GLO1-expression (siGLO1) sensitized A375 and G361 human metastatic melanoma cells towards the antiproliferative, apoptogenic, and oxidative stress-inducing activity of exogenous methylglyoxal. Protein adduction by methylglyoxal was increased in siGLO1-transfected cells as revealed by immunodetection using a monoclonal antibody directed against the major methylglyoxal-derived epitope argpyrimidine that detected a single band of methylglyoxal-adducted protein in human LOX, G361, and A375 total cell lysates. Using 2D-proteomics followed by mass spectrometry the methylglyoxal-adducted protein was identified as heat shock protein 27 (Hsp27; HSPB1). Taken together, our data suggest a function of GLO1 in the regulation of detoxification and target-adduction by the glycolytic byproduct methylglyoxal in malignant melanoma.
PMCID: PMC2891514  PMID: 20093988
Malignant melanoma; glyoxalase 1; methylglyoxal; heat shock protein 27; protein adduction
19.  Redox-Directed Cancer Therapeutics: Molecular Mechanisms and Opportunities 
Antioxidants & Redox Signaling  2009;11(12):3013-3069.
Redox dysregulation originating from metabolic alterations and dependence on mitogenic and survival signaling through reactive oxygen species represents a specific vulnerability of malignant cells that can be selectively targeted by redox chemotherapeutics. This review will present an update on drug discovery, target identification, and mechanisms of action of experimental redox chemotherapeutics with a focus on pro- and antioxidant redox modulators now in advanced phases of preclinal and clinical development. Recent research indicates that numerous oncogenes and tumor suppressor genes exert their functions in part through redox mechanisms amenable to pharmacological intervention by redox chemotherapeutics. The pleiotropic action of many redox chemotherapeutics that involves simultaneous modulation of multiple redox sensitive targets can overcome cancer cell drug resistance originating from redundancy of oncogenic signaling and rapid mutation. Moreover, some redox chemotherapeutics may function according to the concept of synthetic lethality (i.e., drug cytotoxicity is confined to cancer cells that display loss of function mutations in tumor suppressor genes or upregulation of oncogene expression). The impressive number of ongoing clinical trials that examine therapeutic performance of novel redox drugs in cancer patients demonstrates that redox chemotherapy has made the crucial transition from bench to bedside. Antioxid. Redox Signal. 11, 3013–3069.
The (redox) war on cancer
Developing anticancer redox chemotherapeutics
Redox chemotherapeutics: More than neocytotoxics?
Redox chemotherapeutics: Pleiotropic ‘dirty’ drugs?
Redox chemotherapeutics: Combinatorial or stand-alone drugs?
Redox chemotherapeutics and personalized medicine
Redox dysregulation as anticancer drug target
ROS in cancer chemotherapy: From toxicological liability to therapeutic asset
Reactive Pharmacophores for Anticancer Redox Chemotherapy
Organic endoperoxides: Artemisinins
Arsenicals: As2O3 and darinaparsin
Redox cyclers: Motexafin gadolinium
Motexafin gadolinium
Acetaminophen and O-acetylsalicylic acid
3,7-Diaminophenothiazinium redox dyes
Metal chelators: Disulfiram and triapine
Triapine and others
Di- and polysulfides: Varacin and diallyltrisulfide
Calicheamicin γ1I
Varacin and other polysulfides
Diallyldisulfide and diallyltrisulfide
Isothiocyanate organosulfur agents: β-Phenylethylisothiocyanate
Electrophilic Michael acceptors: Parthenolide and neratinib
Sacrificial antioxidants: L-Ascorbate
Molecular Targets for Anticancer Redox Chemotherapy
Targeting the SOD system
SOD inhibitors: ATN-224
SOD mimetics: Mangafodipir
MnTBAP and others
TEMPO and others
Targeting the glutathione redox system: Imexon and NOV002
Targeting the thioredoxin system: PX-12 and PMX464
Chaetocin and gliotoxin
Targeting the Nrf2/Keap1-ARE pathway
Targeting HO-1: Zinc protoporphyrin IX
Targeting NQO1: Dicoumarol and ES936
Targeting APE/Ref1: E3330 and PNRI-299
PNRI-299 and resveratrol
Lucanthone and CRT0044876
Targeting Cdc25 phosphatases: NSC 67121 and F-NSC 67121
NSC 67121 and F-NSC 67121
Targeting zinc finger transcription factors: DIBA
In search of a molecular target: elesclomol
Functional Targets for Anticancer Redox Chemotherapy
Prooxidant intervention targeting glucose metabolism: 2-DG and DCA
Prooxidant intervention targeting mitochondria
Targeting mitochondrial respiration: α-TOS, DIM, and Bz-423
Targeting VDACs: Erastin
Targeting tumor hypoxia
Hypoxia-activated redox chemotherapeutics: TPZ, AQ4N, and PR-104
AQ4N and PR-104
Targeting HIF-1α: PX-478
PMCID: PMC2824519  PMID: 19496700
20.  The topical antimicrobial zinc pyrithione is a heat shock response inducer that causes DNA damage and PARP-dependent energy crisis in human skin cells 
Cell Stress & Chaperones  2009;15(3):309-322.
The differentiated epidermis of human skin serves as an essential barrier against environmental insults from physical, chemical, and biological sources. Zinc pyrithione (ZnPT) is an FDA-approved microbicidal agent used worldwide in clinical antiseptic products, over-the-counter topical antimicrobials, and cosmetic consumer products including antidandruff shampoos. Here we demonstrate for the first time that cultured primary human skin keratinocytes and melanocytes display an exquisite vulnerability to nanomolar concentrations of ZnPT resulting in pronounced induction of heat shock response gene expression and impaired genomic integrity. In keratinocytes treated with nanomolar concentrations of ZnPT, expression array analysis revealed massive upregulation of genes encoding heat shock proteins (HSPA6, HSPA1A, HSPB5, HMOX1, HSPA1L, and DNAJA1) further confirmed by immunodetection. Moreover, ZnPT treatment induced rapid depletion of cellular ATP levels and formation of poly(ADP-ribose) polymers. Consistent with an involvement of poly(ADP-ribose) polymerase (PARP) in ZnPT-induced energy crisis, ATP depletion could be antagonized by pharmacological inhibition of PARP. This result was independently confirmed using PARP-1 knockout mouse embryonic fibroblasts that were resistant to ATP depletion and cytotoxicity resulting from ZnPT exposure. In keratinocytes and melanocytes, single-cell gel electrophoresis and flow cytometric detection of γ-H2A.X revealed rapid induction of DNA damage in response to ZnPT detectable before general loss of cell viability occurred through caspase-independent pathways. Combined with earlier experimental evidence that documents penetration of ZnPT through mammalian skin, our findings raise the possibility that this topical antimicrobial may target and compromise keratinocytes and melanocytes in intact human skin.
PMCID: PMC2866994  PMID: 19809895
Zinc pyrithione; Keratinocyte; Melanocyte; Comet assay; Heat shock response; PARP-dependent ATP depletion
21.  Antimelanoma Activity of the Redox Dye DCPIP (2,6-Dichlorophenolindophenol) is Antagonized by NQO1 
Biochemical pharmacology  2009;78(4):344-354.
Altered redox homeostasis involved in the control of cancer cell survival and proliferative signaling represents a chemical vulnerability that can be targeted by prooxidant redox intervention. Here, we demonstrate that the redox dye 2,6-dichlorophenolindophenol (DCPIP) may serve as a prooxidant chemotherapeutic targeting human melanoma cells in vitro and in vivo. DCPIP-apoptogenicity observed in the human melanoma cell lines A375 and G361 was inversely correlated with NAD(P)H:quinone oxidoreductase (NQO1) expression levels. In A375 cells displaying low NQO1 activity, DCPIP induced apoptosis with procaspase-3 and PARP cleavage, whereas G361 cells expressing high levels of enzymatically active NQO1 were resistant to DCPIP-cytotoxicity. Genetic (siRNA) or pharmacological (dicoumarol) antagonism of NQO1 strongly sensitized G361 cells to DCPIP apoptogenic activity. DCPIP-cytotoxicity was associated with the induction of oxidative stress and rapid depletion of glutathione in A375 and NQO1-modulated G361 cells. Expression array analysis revealed a DCPIP-induced stress response in A375 cells with massive up-regulation of genes encoding Hsp70B’ (HSPA6), Hsp70 (HSPA1A), heme oxygenase-1 (HMOX1), and early growth response protein 1 (EGR1) further confirmed by immunodetection. Systemic administration of DCPIP displayed significant antimelanoma activity in the A375 murine xenograft model. These findings suggest feasibility of targeting tumors that display low NQO1 enzymatic activity using DCPIP.
PMCID: PMC2742658  PMID: 19394313
melanoma; oxidative stress; NQO1; 2,6-dichlorophenolindophenol; Hsp70B’; xenograft
22.  The experimental chemotherapeutic N6-furfuryladenosine (kinetin-riboside) induces rapid ATP depletion, genotoxic stress, and CDKN1A (p21) upregulation in human cancer cell lines 
Biochemical pharmacology  2008;77(7):1125-1138.
Cytokinins and cytokinin nucleosides are purine derivatives with potential anticancer activity. N6-furfuryladenosine (FAdo, kinetin-riboside) displays antiproliferative and apoptogenic activity against various human cancer cell lines, and FAdo has recently been shown to suppress tumor growth in murine xenograft models of human leukemia and melanoma. In this study, FAdo-induced genotoxicity, stress response gene expression, and cellular ATP depletion were examined as early molecular consequences of FAdo-exposure in MiaPaCa-2 pancreas carcinoma, A375 melanoma, and other human cancer cell lines. FAdo, but not adenosine or N6-furfuryladenine, displayed potent antiproliferative activity that was also observed in human primary fibroblasts and keratinocytes. Remarkably, massive ATP depletion and induction of genotoxic stress as assessed by the alkaline comet assay occurred within 60 to 180 minutes of exposure to low micromolar concentrations of FAdo. This was followed by rapid upregulation of CDKN1A and other DNA damage/stress response genes (HMOX1, DDIT3, GADD45A) as revealed by expression array and Western analysis. Pharmacological and siRNA-based genetic inhibition of adenosine kinase suppressed FAdo cytotoxicity and also prevented ATP-depletion and p21-upregulation suggesting the importance of bioconversion of FAdo into the nucleotide form required for drug action. Taken together our data suggest that early induction of genotoxicity and energy crisis are important causative factors involved in FAdo cytotoxicity.
PMCID: PMC2656390  PMID: 19186174
kinetin; N6-furfuryladenosine; genotoxic stress; ATP depletion; CDKN1A; cancer
23.  The Cinnamon-derived Michael Acceptor Cinnamic Aldehyde Impairs Melanoma Cell Proliferation, Invasiveness, and Tumor Growth 
Free radical biology & medicine  2008;46(2):220-231.
Redox dysregulation in cancer cells represents a chemical vulnerability that can be targeted by prooxidant redox intervention. Dietary constituents that contain an electrophilic Michael acceptor pharmacophore may therefore display promising chemopreventive and chemotherapeutic anti-cancer activity. Here, we demonstrate that the cinnamon-derived dietary Michael acceptor trans-cinnamic aldehyde (CA) impairs melanoma cell proliferation and tumor growth. Feasibility of therapeutic intervention using high doses of CA (120 mg/kg, p.o., q.d., 10 days) was demonstrated in a human A375 melanoma SCID-mouse xenograft model. Low micromolar concentrations (IC50 < 10 μM) of CA, but not closely related CA-derivatives devoid of Michael acceptor activity, suppressed proliferation of human metastatic melanoma cell lines (A375, G361, LOX) with G1 cell cycle arrest, elevated intracellular ROS, and impaired invasiveness. Expression array analysis revealed that CA induced an oxidative stress response in A375 cells, up-regulating heme oxygenase-1 (HMOX1), sulfiredoxin 1 homolog (SRXN1), thioredoxin reductase 1 (TXNRD1), and other genes including the cell cycle regulator and stress-responsive tumor suppressor gene cyclin-dependent kinase inhibitor 1A (CDKN1A), a key mediator of G1 phase arrest. CA, but not Michael-inactive derivatives, inhibited NFκB transcriptional activity and TNFα-induced IL-8 production in A375 cells. These findings support a previously unrecognized role of CA as a dietary Michael acceptor with potential anticancer activity.
PMCID: PMC2650023  PMID: 19000754
melanoma; oxidative stress; Michael acceptor; cinnamic aldehyde; NFκB; p21 (CDKN1A); xenograft
24.  NQO1-activated Phenothiazinium Redox Cyclers for the Targeted Bioreductive Induction of Cancer Cell Apoptosis 
Free radical biology & medicine  2007;43(2):178-190.
Altered redox signaling and regulation in cancer cells represent a chemical vulnerability that can be targeted by selective chemotherapeutic intervention. Here, we demonstrate that 3,7-diaminophenothiazinium-based redoxcyclers (PRC) induce selective cancer cell apoptosis by NAD(P)H:quinone oxidoreductase (NQO1)-dependent bioreductive generation of cellular oxidative stress. Using PRC lead compounds including toluidine blue against human metastatic G361 melanoma cells, apoptosis occurred with phosphatidylserine-externalization, loss of mitochondrial transmembrane potential, cytochrome C release, caspase-3 activation, and massive ROS production. Consistent with reductive activation and subsequent redoxcycling as the mechanism of PRC cytotoxicity, co-incubation with catalase achieved cell protection, whereas reductive antioxidants enhanced PRC-cytotoxicity. Unexpectedly, human A375 melanoma cells were resistant to PRC-induced apoptosis, and PRC-sensitive G361 cells were protected by preincubation with the NQO1-inhibitor dicoumarol. Indeed, NQO1 specific enzymatic activity was nine fold higher in G361 than in A375 cells. The critical role of NQO1 in PRC-bioactivation and cytotoxicity was confirmed, when NQO1-transfected breast cancer cells (MCF7-DT15) stably overexpressing active NQO1 displayed strongly enhanced PRC-sensitivity as compared to vector-control transfected cells with base line NQO1 activity. Based on the known overexpression of NQO1 in various tumors these findings suggest the feasibility of developing PRC lead compounds into tumor-selective bioreductive chemotherapeutics.
PMCID: PMC2705808  PMID: 17603928
Apoptosis; Bioreductive Activation; Cancer; Melanoma; Methylene Blue; NQO1; Phenothiazinium Redox Cycler; Redox Chemotherapy; ROS; Toluidine Blue
25.  Nrf2 enhances resistance of cancer cells to chemotherapeutic drugs, the dark side of Nrf2 
Carcinogenesis  2008;29(6):1235-1243.
Drug resistance during chemotherapy is the major obstacle to the successful treatment of many cancers. Here, we report that inhibition of NF-E2-related factor 2 (Nrf2) may be a promising strategy to combat chemoresistance. Nrf2 is a critical transcription factor regulating a cellular protective response that defends cells against toxic insults from a broad spectrum of chemicals. Under normal conditions, the low constitutive amount of Nrf2 protein is maintained by the Kelch-like ECH-associated protein1 (Keap1)-mediated ubiquitination and proteasomal degradation system. Upon activation, this Keap1-dependent Nrf2 degradation mechanism is quickly inactivated, resulting in accumulation and activation of the antioxidant response element (ARE)-dependent cytoprotective genes. Since its discovery, Nrf2 has been viewed as a ‘good’ transcription factor that protects us from many diseases. In this study, we demonstrate the dark side of Nrf2: stable overexpression of Nrf2 resulted in enhanced resistance of cancer cells to chemotherapeutic agents including cisplatin, doxorubicin and etoposide. Inversely, downregulation of the Nrf2-dependent response by overexpression of Keap1 or transient transfection of Nrf2–small interfering RNA (siRNA) rendered cancer cells more susceptible to these drugs. Upregulation of Nrf2 by the small chemical tert-butylhydroquinone (tBHQ) also enhanced the resistance of cancer cells, indicating the feasibility of using small chemical inhibitors of Nrf2 as adjuvants to chemotherapy to increase the efficacy of chemotherapeutic agents. Furthermore, we provide evidence that the strategy of using Nrf2 inhibitors to increase efficacy of chemotherapeutic agents is not limited to certain cancer types or anticancer drugs and thus can be applied during the course of chemotherapy to treat many cancer types.
PMCID: PMC3312612  PMID: 18413364

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