Targeting redox vulnerability of cancer cells by prooxidative intervention of a glutathione-activated Cu(II) pro-ionophore: Hitting three birds with one stone

Identification of tanshinone I as a natural Cu(II) ionophore

The development of Cu(II) ionophores for targeted disruption of aberrant redox homeostasis in cancer cells has been considered an appealing strategy in the field of anticancer research. This study presents the first identification of tanshinone I (Ts1), a natural o-quinone, as a Cu(II) ionophore. Structure-activity relationship studies on tanshinones and mechanistic investigations reveal that the presence of Cu(II) effectively promotes the tautomerization of Ts1 from its diketo to keto-enol forms, thereby facilitating its sequential proton-loss Cu(II) chelation, and enabling it to function as a Cu(II) ionophore due to its structural features including the presence of an o-quinone moiety, a benzyl hydrogen, and a large conjugated system. The unique property allows Ts1 to preferentially induce copper accumulation in human hepatoma HepG2 cells over human umbilical vein endothelial cells, by releasing copper driven by reduced glutathione (GSH). This copper accumulation leads to a reduction in the GSH-to-oxidized glutathione ratio and the generation of reactive oxygen species, ultimately triggering apoptosis of HepG2 cells. The findings not only provide support for o-quinones as innovative types of anticancer Cu(II) ionophores, but also shed light on the previously unrecognized role of Ts1 as a potent Cu(II) ionophore for eradicating cancer cells by selectively disrupting their redox regulation programs, resembling a "Trojan horse".

A high-contrast NIR excitation probe for monitoring Cu2+ in the endoplasmic reticulum for synergistic cuproptosis and ferroptosis anticancer therapy

Currently, the study of cuproptosis focuses on the Cu2+-induced morphology changes in mitochondria (Mito), and the observation of the effect of endoplasmic reticulum (ER)-related Cu2+ content on cuproptosis is relatively lacking. Herein, we have developed a hydroxyflavone (HF)-based NIR excited two-photon fluorescent probe, BHCO, that exhibits specific recognition of Cu2+ with high resolution. BHCO-Cu2+ (Cu2BC) can lead to DLAT protein aggregation, triggering cuproptosis. Furthermore, Cu2BC can upregulate reactive oxygen species (ROS) under 720 nm excitation, which facilitates ferroptosis. The synergistic effect of ferroptosis and cuproptosis leads to the damage of cellular mitochondria and endoplasmic reticulum, resulting in the severe death of cancer cells. We are firmly convinced that our nonlinear optical (NLO) small molecule probe for monitoring Cu2+ could provide a valid tool for rapid tumor elimination through synergistic ferroptosis and cuproptosis.

Neurotoxic Effect of Myricitrin in Copper-Induced Oxidative Stress Is Mediated by Increased Intracellular Ca2+ Levels and ROS/p53/p38 Axis

Although commonly appreciated for their anti-oxidative and neuroprotective properties, flavonoids can also exhibit pro-oxidative activity, potentially reducing cell survival, particularly in the presence of metal ions. Disrupted copper homeostasis is a known contributor to neuronal dysfunction through oxidative stress induction. This study investigated the effects of myricitrin (1-20 μg/mL) on copper-induced toxicity (0.5 mM CuSO4) in the neuroblastoma SH-SY5Y cell line. At non-toxic concentrations, myricitrin exacerbated copper's toxic effects. The myricitrin-induced decrease in survival was accompanied with increased reactive oxygen species (ROS) production, reduced superoxide dismutase activity, and a lower GSH/GSSG ratio. In combination with copper, myricitrin also activated caspase-3/7, promoted nuclear chromatin changes, and compromised membrane integrity. At the protein level, myricitrin upregulated p53 and PUMA expression. The toxic effects of myricitrin were alleviated by the p38 inhibitor SB203580, the intracellular calcium chelator BAPTA-AM, and the NMDA receptor blocker MK-801, highlighting the significant role of the ROS/p53/p38 axis in cell death and the critical involvement of calcium ions in apoptosis induction. The atomic force microscopy was used to assess the surface morphology and nanomechanical properties of SH-SY5Y cells, revealing changes following myricitrin treatment. This research highlights the toxic potential of myricitrin and emphasizes the need for caution when considering flavonoid supplementation in conditions with elevated copper levels.

Mechanisms of cuproptosis and its relevance to distinct diseases

Copper is a trace element required by the organism, but once the level of copper exceeds the threshold, it becomes toxic and even causes death. The underlying mechanisms of copper-induced death are inconclusive, with different studies showing different opinions on the mechanism of copper-induced death. Multiple investigations have shown that copper induces oxidative stress, endoplasmic reticulum stress, nucleolar stress, and proteasome inhibition, all of which can result in cell death. The latest research elucidates a copper-dependent death and denominates it as cuproptosis. Cuproptosis takes place through the combination of copper and lipoylated proteins of the tricarboxylic acid cycle, triggering agglomeration of lipoylated proteins and loss of iron-sulfur cluster proteins, leading to proteotoxic stress and ultimately death. Given the toxicity and necessity of copper, abnormal levels of copper lead to diseases such as neurological diseases and cancer. The development of cancer has a high demand for copper, neurological diseases involve the change of copper contents and the binding of copper to proteins. There is a close relationship between these two kinds of diseases and copper. Here, we summarize the mechanisms of copper-related death, and the association between copper and diseases, to better figure out the influence of copper in cell death and diseases, thus advancing the clinical remedy of these diseases.

Glutathione-triggered prodrugs: Design strategies, potential applications, and perspectives

The burgeoning prodrug strategy offers a promising avenue toward improving the efficacy and specificity of cytotoxic drugs. Elevated intracellular levels of glutathione (GSH) have been regarded as a hallmark of tumor cells and characteristic feature of the tumor microenvironment. Considering the pivotal involvement of elevated GSH in the tumorigenic process, a diverse repertoire of GSH-triggered prodrugs has been developed for cancer therapy, facilitating the attenuation of deleterious side effects associated with conventional chemotherapeutic agents and/or the attainment of more efficacious therapeutic outcomes. These prodrug formulations encompass a spectrum of architectures, spanning from small molecules to polymer-based and organic-inorganic nanomaterial constructs. Although the GSH-triggered prodrugs have been gaining increasing interests, a comprehensive review of the advancements made in the field is still lacking. To fill the existing lacuna, this review undertakes a retrospective analysis of noteworthy research endeavors, based on a categorization of these molecules by their diverse recognition units (i.e., disulfides, diselenides, Michael acceptors, and sulfonamides/sulfonates). This review also focuses on explaining the distinct benefits of employing various chemical architecture strategies in the design of these prodrug agents. Furthermore, we highlight the potential for synergistic functionality by incorporating multiple-targeting conjugates, theranostic entities, and combinational treatment modalities, all of which rely on the GSH-triggering. Overall, an extensive overview of the emerging field is presented in this review, highlighting the obstacles and opportunities that lie ahead. Our overarching goal is to furnish methodological guidance for the development of more efficacious GSH-triggered prodrugs in the future. By assessing the pros and cons of current GSH-triggered prodrugs, we expect that this review will be a handful reference for prodrug design, and would provide a guidance for improving the properties of prodrugs and discovering novel trigger scaffolds for constructing GSH-triggered prodrugs.

Cuproptosis and its application in different cancers: an overview

Heavy metal ions are essential micronutrients for human health. They are also indispensable to maintaining health and regular operation of organs. Increasing or decreasing these metal ions will lead to cell death, such as ferroptosis. Tsvetkov et al. have recently proposed a novel cell death method called "Cuproptosis". Many researchers have linked this form of death to the diagnosis, prognosis, microenvironment infiltration, and prediction of immunotherapeutic efficacy of various tumors to better understand these tumors. Similarly, with the proposal of this mechanism, the killing effect of copper ionophores on cancer cells has come to our attention again. We introduced the mechanism of cuproptosis in detail and described the establishment of the corresponding prognostic model and risk score for uveal melanoma through cuproptosis. In addition, we describe the current progress in the study of cancer in other organs through cuproptosis and summarize the treatment of tumours by copper ionophore and its future research direction. With further research, the concept of cuproptosis may help us understand cancer and guide its clinical treatment.

Copper metabolism in cell death and autophagy

Copper is an essential trace element in biological systems, maintaining the activity of enzymes and the function of transcription factors. However, at high concentrations, copper ions show increased toxicity by inducing regulated cell death, such as apoptosis, paraptosis, pyroptosis, ferroptosis, and cuproptosis. Furthermore, copper ions can trigger macroautophagy/autophagy, a lysosome-dependent degradation pathway that plays a dual role in regulating the survival or death fate of cells under various stress conditions. Pathologically, impaired copper metabolism due to environmental or genetic causes is implicated in a variety of human diseases, such as rare Wilson disease and common cancers. Therapeutically, copper-based compounds are potential chemotherapeutic agents that can be used alone or in combination with other drugs or approaches to treat cancer. Here, we review the progress made in understanding copper metabolic processes and their impact on the regulation of cell death and autophagy. This knowledge may help in the design of future clinical tools to improve cancer diagnosis and treatment.Abbreviations: ACSL4, acyl-CoA synthetase long chain family member 4; AIFM1/AIF, apoptosis inducing factor mitochondria associated 1; AIFM2, apoptosis inducing factor mitochondria associated 2; ALDH, aldehyde dehydrogenase; ALOX, arachidonate lipoxygenase; AMPK, AMP-activated protein kinase; APAF1, apoptotic peptidase activating factor 1; ATF4, activating transcription factor 4; ATG, autophagy related; ATG13, autophagy related 13; ATG5, autophagy related 5; ATOX1, antioxidant 1 copper chaperone; ATP, adenosine triphosphate; ATP7A, ATPase copper transporting alpha; ATP7B, ATPase copper transporting beta; BAK1, BCL2 antagonist/killer 1; BAX, BCL2 associated X apoptosis regulator; BBC3/PUMA, BCL2 binding component 3; BCS, bathocuproinedisulfonic acid; BECN1, beclin 1; BID, BH3 interacting domain death agonist; BRCA1, BRCA1 DNA repair associated; BSO, buthionine sulphoximine; CASP1, caspase 1; CASP3, caspase 3; CASP4/CASP11, caspase 4; CASP5, caspase 5; CASP8, caspase 8; CASP9, caspase 9; CCS, copper chaperone for superoxide dismutase; CD274/PD-L1, CD274 molecule; CDH2, cadherin 2; CDKN1A/p21, cyclin dependent kinase inhibitor 1A; CDKN1B/p27, cyclin-dependent kinase inhibitor 1B; COMMD10, COMM domain containing 10; CoQ10, coenzyme Q 10; CoQ10H2, reduced coenzyme Q 10; COX11, cytochrome c oxidase copper chaperone COX11; COX17, cytochrome c oxidase copper chaperone COX17; CP, ceruloplasmin; CYCS, cytochrome c, somatic; DBH, dopamine beta-hydroxylase; DDIT3/CHOP, DNA damage inducible transcript 3; DLAT, dihydrolipoamide S-acetyltransferase; DTC, diethyldithiocarbamate; EIF2A, eukaryotic translation initiation factor 2A; EIF2AK3/PERK, eukaryotic translation initiation factor 2 alpha kinase 3; ER, endoplasmic reticulum; ESCRT-III, endosomal sorting complex required for transport-III; ETC, electron transport chain; FABP3, fatty acid binding protein 3; FABP7, fatty acid binding protein 7; FADD, Fas associated via death domain; FAS, Fas cell surface death receptor; FASL, Fas ligand; FDX1, ferredoxin 1; GNAQ/11, G protein subunit alpha q/11; GPX4, glutathione peroxidase 4; GSDMD, gasdermin D; GSH, glutathione; HDAC, histone deacetylase; HIF1, hypoxia inducible factor 1; HIF1A, hypoxia inducible factor 1 subunit alpha; HMGB1, high mobility group box 1; IL1B, interleukin 1 beta; IL17, interleukin 17; KRAS, KRAS proto-oncogene, GTPase; LOX, lysyl oxidase; LPCAT3, lysophosphatidylcholine acyltransferase 3; MAP1LC3, microtubule associated protein 1 light chain 3; MAP2K1, mitogen-activated protein kinase kinase 1; MAP2K2, mitogen-activated protein kinase kinase 2; MAPK, mitogen-activated protein kinases; MAPK14/p38, mitogen-activated protein kinase 14; MEMO1, mediator of cell motility 1; MT-CO1/COX1, mitochondrially encoded cytochrome c oxidase I; MT-CO2/COX2, mitochondrially encoded cytochrome c oxidase II; MTOR, mechanistic target of rapamycin kinase; MTs, metallothioneins; NAC, N-acetylcysteine; NFKB/NF-Κb, nuclear factor kappa B; NLRP3, NLR family pyrin domain containing 3; NPLOC4/NPL4, NPL4 homolog ubiquitin recognition factor; PDE3B, phosphodiesterase 3B; PDK1, phosphoinositide dependent protein kinase 1; PHD, prolyl-4-hydroxylase domain; PIK3C3/VPS34, phosphatidylinositol 3-kinase catalytic subunit type 3; PMAIP1/NOXA, phorbol-12-myristate-13-acetate-induced protein 1; POR, cytochrome P450 oxidoreductase; PUFA-PL, PUFA of phospholipids; PUFAs, polyunsaturated fatty acids; ROS, reactive oxygen species; SCO1, synthesis of cytochrome C oxidase 1; SCO2, synthesis of cytochrome C oxidase 2; SLC7A11, solute carrier family 7 member 11; SLC11A2/DMT1, solute carrier family 11 member 2; SLC31A1/CTR1, solute carrier family 31 member 1; SLC47A1, solute carrier family 47 member 1; SOD1, superoxide dismutase; SP1, Sp1 transcription factor; SQSTM1/p62, sequestosome 1; STEAP4, STEAP4 metalloreductase; TAX1BP1, Tax1 binding protein 1; TEPA, tetraethylenepentamine; TFEB, transcription factor EB; TM, tetrathiomolybdate; TP53/p53, tumor protein p53; TXNRD1, thioredoxin reductase 1; UCHL5, ubiquitin C-terminal hydrolase L5; ULK1, Unc-51 like autophagy activating kinase 1; ULK1, unc-51 like autophagy activating kinase 1; ULK2, unc-51 like autophagy activating kinase 2; USP14, ubiquitin specific peptidase 14; VEGF, vascular endothelial gro wth factor; XIAP, X-linked inhibitor of apoptosis.

New anti-cancer explorations based on metal ions

Due to the urgent demand for more anti-cancer methods, the new applications of metal ions in cancer have attracted increasing attention. Especially the three kinds of the new mode of cell death, including ferroptosis, calcicoptosis, and cuproptosis, are of great concern. Meanwhile, many metal ions have been found to induce cell death through different approaches, such as interfering with osmotic pressure, triggering biocatalysis, activating immune pathways, and generating the prooxidant effect. Therefore, varieties of new strategies based on the above approaches have been studied and applied for anti-cancer applications. Moreover, many contrast agents based on metal ions have gradually become the core components of the bioimaging technologies, such as MRI, CT, and fluorescence imaging, which exhibit guiding significance for cancer diagnosis. Besides, the new nano-theranostic platforms based on metal ions have experimentally shown efficient response to endogenous and exogenous stimuli, which realizes simultaneous cancer therapy and diagnosis through a more controlled nano-system. However, most metal-based agents have still been in the early stages, and controlled clinical trials are necessary to confirm or not the current expectations. This article will focus on these new explorations based on metal ions, hoping to provide some theoretical support for more anti-cancer ideas.

Cuproptosis-Related Risk Score Predicts Prognosis and Characterizes the Tumor Microenvironment in Hepatocellular Carcinoma

Aims

Cuproptosis is a recently identified form of programmed cell death; however, its role in hepatocellular carcinoma (HCC) remains unclear.

Methods

A set of bioinformatic tools was integrated to analyze the expression and prognostic significance of ferredoxin 1 (FDX1), the key regulator of cuproptosis. A cuproptosis-related risk score (CRRS) was developed via correlation analyses, least absolute shrinkage and selection operator (LASSO) Cox regression, and multivariate Cox regression. The metabolic features, mutation signatures, and immune profile of CRRS-classified HCC patients were investigated, and the role of CRRS in therapy guidance was analyzed.

Results

FDX1 was significantly downregulated in HCC, and its high expression was associated with longer survival time. HCC patients in the high-CRRS group showed a significantly lower overall survival (OS) and enriched in cancer-related pathways. Mutation analyses revealed that the high-CRRS HCC patients had a high mutational frequency of some tumor suppressors such as tumor protein P53 (TP53) and Breast-cancer susceptibility gene 1 (BRCA1)-associated protein 1 (BAP1) and a low frequency of catenin beta 1 (CTNNB1). Besides, HCC patients with high CRRS showed an increase of protumor immune infiltrates and a high expression of immune checkpoints. Moreover, the area under the curve (AUC) values of CRRS in predicting the efficiency of sorafenib and the non-responsiveness to transcatheter arterial chemoembolization (TACE) in HCC patients reached 0.877 and 0.764, respectively.

Significance

The cuproptosis-related signature is helpful in prognostic prediction and in guiding treatment for HCC patients.

Redox-Based Strategy for Selectively Inducing Energy Crisis Inside Cancer Cells: An Example of Modifying Dietary Curcumin to Target Mitochondria

Reprograming of energy metabolism is a major hallmark of cancer, but its effective intervention is still a challenging task due to metabolic heterogeneity and plasticity of cancer cells. Herein, we report a general redox-based strategy for meeting the challenge. The strategy was exemplified by a dietary curcumin analogue (MitoCur-1) that was designed to target mitochondria (MitoCur-1). By virtue of its electrophilic and mitochondrial-targeting properties, MitoCur-1 generated reactive oxygen species (ROS) more effectively and selectively in HepG2 cells than in L02 cells via the inhibition of mitochondrial antioxidative thioredoxin reductase 2 (TrxR2). The ROS generation preferentially mediated the energy crisis of HepG2 cells in a dual-inhibition fashion against both mitochondrial and glycolytic metabolisms, which could hit the metabolic plasticity of HepG2 cells. The ROS-dependent energy crisis also allowed its preferential killing of HepG2 cells (IC₅₀ = 1.4 μM) over L02 cells (IC₅₀ = 9.1 μM), via induction of cell-cycle arrest, apoptosis and autophagic death, and its high antitumor efficacy in vivo, in nude mice bearing HepG2 tumors (15 mg/kg). These results highlight that inhibiting mitochondrial TrxR2 to produce ROS by electrophiles is a promising redox-based strategy for the effective intervention of cancer cell energy metabolic reprograming.

Selective Targeting of Cancer Cells by Copper Ionophores: An Overview

Conventional cancer therapies suffer from severe off-target effects because most of them target critical facets of cells that are generally shared by all rapidly proliferating cells. The development of new therapeutic agents should aim to increase selectivity and therefore reduce side effects. In addition, these agents should overcome cancer cell resistance and target cancer stem cells. Some copper ionophores have shown promise in this direction thanks to an intrinsic selectivity in preferentially inducing cuproptosis of cancer cells compared to normal cells. Here, Cu ionophores are discussed with a focus on selectivity towards cancer cells and on the mechanisms responsible for this selectivity. The proposed strategies, to further improve the targeting of cancer cells by copper ionophores, are also reported.

Copper-induced tumor cell death mechanisms and antitumor theragnostic applications of copper complexes

Recent studies found that unbalanced copper homeostasis affect tumor growth, causing irreversible damage. Copper can induce multiple forms of cell death, including apoptosis and autophagy, through various mechanisms, including reactive oxygen species accumulation, proteasome inhibition, and antiangiogenesis. Hence, copper in vivo has attracted tremendous attention and is in the research spotlight in the field of tumor treatment. This review first highlights three typical forms of copper's antitumor mechanisms. Then, the development of diverse biomaterials and nanotechnology allowing copper to be fabricated into diverse structures to realize its theragnostic action is discussed. Novel copper complexes and their clinical applications are subsequently described.

Application of glutathione depletion in cancer therapy: Enhanced ROS-based therapy, ferroptosis, and chemotherapy

Glutathione (GSH) is an important member of cellular antioxidative system. In cancer cells, a high level of GSH is indispensable to scavenge excessive reactive oxygen species (ROS) and detoxify xenobiotics, which make it a potential target for cancer therapy. Plenty of studies have shown that loss of intracellular GSH makes cancer cells more susceptible to oxidative stress and chemotherapeutic agents. GSH depletion has been proved to improve the therapeutic efficacy of ROS-based therapy (photodynamic therapy, sonodynamic therapy, and chemodynamic therapy), ferroptosis, and chemotherapy. In this review, various strategies for GSH depletion used in cancer therapy are comprehensively summarized and discussed. First, the functions of GSH in cancer cells are analyzed to elucidate the necessity of GSH depletion in cancer therapy. Then, the synthesis and metabolism of GSH are briefly introduced to bring up some crucial targets for GSH modulation. Finally, different approaches to GSH depletion in the literature are classified and discussed in detail according to their mechanisms. Particularly, functional materials with GSH-consuming ability based on nanotechnology are elaborated due to their unique advantages and potentials. This review presents the ingenious application of GSH-depleting strategy in cancer therapy for improving the outcomes of various therapeutic regimens, which may provide useful guidance for designing intelligent drug delivery system.

Histone Deacetylase Inhibitor-Induced CDKN2B and CDKN2D Contribute to G2/M Cell Cycle Arrest Incurred by Oxidative Stress in Hepatocellular Carcinoma Cells via Forkhead Box M1 Suppression

Forkhead box protein M1 (FOXM1) is a pivotal regulator of G2/M cell cycle progression in many types of cancer. Previously, our study demonstrated that histone deacetylase inhibition (HDACi) sensitizes hepatocellular carcinoma cells (HCC) to oxidative stress through FOXM1 suppression. However, the mechanism underlying its suppression by HDACi still requires elucidation. We hypothesized that HDACi induce genes responsible for destabilizing and inactivating FOXM1. The transcriptome in the HepG2 was revealed by massive analysis of cDNA end (MACE). Expression of mRNA and proteins were analyzed by quantitative real-time PCR (qPCR) and western blot, respectively. Cell cycle was analyzed by fluorescence-activated cell sorting (FACS). Oxidative stress and HDACi suppressed CDK4/6 levels while enhancing CDK inhibitor 2B and 2D (CDKN2B and CDKN2D) expressions in HCC. Palbociclib, a specific inhibitor of CDK4/6, induced G2/M cell cycle arrest in HCC by down-regulating phosphorylation level of FOXM1, and its downstream target genes such as aurora kinase A (AURKA) and polo-like kinase 1 (PLK1). HDACi treatment increased the ubiquitination level of FOXM1 by suppressing ubiquitin-specific peptidase 21 (USP21), which deubiquitinates FOXM1. Inhibiting FOXM1 degradation with MG132 treatment affected neither palbociclib-induced G2/M cell cycle arrest nor expression of its target genes. Double knockdown of CDKN2B and CDKN2D reduced the oxidative stress and HDACi-induced G/2M cell cycle arrest. In conclusion, oxidative stress and HDACi synergistically cause G2/M cell cycle arrest via CDKN2 induction, which sequentially inhibits CDK4/6, FOXM1, and its downstream target genes AURKA, PLK1, and CCNB1 phosphorylation in HCC.

Glutathione-Depleting Nanomedicines for Synergistic Cancer Therapy

Cancer cells frequently exhibit resistance to various molecular and nanoscale drugs, which inevitably affects the drugs' therapeutic outcomes. Overexpression of glutathione (GSH) has been observed in many cancer cells, and solid evidence has corroborated the resulting tumor resistance to a variety of anticancer therapies, suggesting that this biochemical characteristic of cancer cells can be developed as a potential target for cancer treatments. The single treatment of GSH-depleting agents can potentiate the responses of the cancer cells to different cell death stimuli; therefore, as an adjunctive strategy, GSH depletion is usually combined with mainstream cancer therapies for enhancing the therapeutic outcomes. Propelled by the rapid development of nanotechnology, GSH-depleting agents can be readily constructed into anticancer nanomedicines, which have shown a steep rise over the past decade. Here, we review the common GSH-depleting nanomedicines which have been widely applied in synergistic cancer treatments in recent years. Some current challenges and future perspectives for GSH depletion-based cancer therapies are also presented. With the understanding of the structure-property relationship and action mechanisms of these biomaterials, we hope that the GSH-depleting nanotechnology will be further developed to realize more effective disease treatments and even achieve successful clinical translations.

Small molecules regulating reactive oxygen species homeostasis for cancer therapy

Elevated intracellular reactive oxygen species (ROS) and antioxidant defense systems have been recognized as one of the hallmarks of cancer cells. Compared with normal cells, cancer cells exhibit increased ROS to maintain their malignant phenotypes and are more dependent on the "redox adaptation" mechanism. Thus, there are two apparently contradictory but virtually complementary therapeutic strategies for the regulation of ROS to prevent or treat cancer. The first strategy, that is, chemoprevention, is to prevent or reduce intracellular ROS either by suppressing ROS production pathways or by employing antioxidants to enhance ROS clearance, which protects normal cells from malignant transformation and inhibits the early stage of tumorigenesis. The second strategy is the ROS-mediated anticancer therapy, which stimulates intracellular ROS to a toxicity threshold to activate ROS-induced cell death pathways. Therefore, targeting the regulation of intracellular ROS-related pathways by small-molecule candidates is considered to be a promising treatment for tumors. We herein first briefly introduce the source and regulation of ROS, and then focus on small molecules that regulate ROS-related pathways and show efficacy in cancer therapy from the perspective of pharmacophores. Finally, we discuss several challenges in developing cancer therapeutic agents based on ROS regulation and propose the direction of future development.

Redox-Active Polymers Connecting Living Microbial Cells to an Extracellular Electrical Circuit

Microbial electrochemical systems in which metabolic electrons in living microbes have been extracted to or injected from an extracellular electrical circuit have attracted considerable attention as environmentally-friendly energy conversion systems. Since general microbes cannot exchange electrons with extracellular solids, electron mediators are needed to connect living cells to an extracellular electrode. Although hydrophobic small molecules that can penetrate cell membranes are commonly used as electron mediators, they cannot be dissolved at high concentrations in aqueous media. The use of hydrophobic mediators in combination with small hydrophilic redox molecules can substantially increase the efficiency of the extracellular electron transfer process, but this method has side effects, in some cases, such as cytotoxicity and environmental pollution. In this Review, recently-developed redox-active polymers are highlighted as a new type of electron mediator that has less cytotoxicity than many conventional electron mediators. Owing to the design flexibility of polymer structures, important parameters that affect electron transport properties, such as redox potential, the balance of hydrophobicity and hydrophilicity, and electron conductivity, can be systematically regulated.

A hydrogen peroxide-activated Cu(II) pro-ionophore strategy for modifying naphthazarin as a promising anticancer agent with high selectivity for generating ROS in HepG2 cells over in L02 cells

Targeting redox vulnerability of cancer cells by pro-oxidants capable of generating reactive oxygen species (ROS) has surfaced as an important anticancer strategy. Due to the intrinsic narrow therapeutic window and other dangerous side effects of ROS generation, it is highly needed and challenging to develop pro-oxidative anticancer agents (PAAs) with high selectivity for generating ROS in cancer cells. Herein we report a hydrogen peroxide (H₂O₂)-activated Cu(II) pro-ionophore strategy to develop naphthazarin (Nap) as such type of PAAs based on the H₂O₂-mediated conversion of boronate to free phenol. The boronate-protected Nap (PNap) can exploit increased levels of H₂O₂ in HepG2 cells to in situ release Nap followed by its efflux via conjugation with reduced glutathione (GSH), allowing that the Nap-GSH adduct works as a Cu(II) ionophore to induce continuously GSH depletion via a reduction-dependent releasing of Cu(I) by GSH. This strategy endows PNap with the unprecedented ability to hit multi-redox characteristics (increased levels of H₂O₂, GSH and copper) of HepG2 cells, leading to ROS generation preferentially in HepG2 cells along with their selective death.

Sensitization to oxidative stress and G2/M cell cycle arrest by histone deacetylase inhibition in hepatocellular carcinoma cells

Oxidative stress resistance in cancer cells has contributed to multi-drug resistance, which poses a serious challenge to cancer therapy. To surmount this, combinatorial treatment involving anticancer drugs and histone deacetylase inhibitors (HDACi) have emerged as a chemotherapeutic option. Yet, HDACi's role in redox states of cancer cells still requires elucidation. In the present study, we hypothesized that HDACi sensitizes cancer cells to oxidative stress and results in G2/M cell cycle arrest. Cell viability and cell cycle were analyzed using Cell Counting Kit 8 (CCK8) and fluorescent activated cell sorting (FACS), respectively. The transcriptomes of cells were investigated by massive analysis of cDNA end (MACE). Expression of mRNA and proteins were analyzed by quantitative real-time PCR (qPCR) and Western blot, respectively. Intracellular oxidative stress induced by tert-Butyl hydroperoxide (tBHP) reduced cell viability and resulted in G2/M cell cycle arrest in a dose-dependent manner in hepatocellular carcinoma (HCC) cells. The effects of sorafenib on cell cycle arrest and HCC viability were enhanced through HDACi treatment. MACE revealed that genes related to progression of G2/M cell cycle including Foxm1, Aurka, Plk1, and Ccnb1 were significantly down-regulated in tBHP and HDACi-treated HepG2 cells. Inhibition of FOXM1 with thiostrepton also resulted in reduced cell viability and expression of FOXM1 target genes such as Aurka, Plk1, and Ccnb1. These results indicate that HDACi sensitizes HepG2 cells to oxidative stress and results in G2/M cell cycle arrest via down-regulation of FOXM1, which plays a key role in progression of G2/M cell cycle.

Baicalein, as a Prooxidant, Triggers Mitochondrial Apoptosis in MCF-7 Human Breast Cancer Cells Through Mobilization of Intracellular Copper and Reactive Oxygen Species Generation

Background

Baicalein, a natural flavonoid derived from traditional Chinese herb Scutellaria baicalensis Georg (known as Huang Qin in Chinese), has been reported to exhibit notable antitumor activity in various cancer cells, including breast cancer. However, the detailed mechanisms underlying its induced apoptosis as a prooxidant in breast cancer cells are still unknown.

Materials and methods

In this study, we investigated the effect of endogenous copper on cytotoxic activity of baicalin against human breast cancer MCF-7 cells in vitro.

Results

Baicalein could remarkably reduce the cell viability in both dose- and time-dependent manners in MCF-7 cells but with lower cytotoxic effects on normal breast epithelial cells, MCF-10A. Such cell death could be prevented by pretreatment with Cu (I)-specific chelator neocuproine (Neo) and reactive oxygen species (ROS) scavengers. Meanwhile, baicalein could induce MCF-7 cell morphological changes, promote apoptotic cell death and increase the apoptotic cell number. Moreover, DCHF-DA staining, flow cytometry and Western blotting analyses proved that baicalein triggered the mitochondrial-dependent apoptotic pathway, as indicated by enhancement the level of intracellular ROS, disruption of mitochondrial membrane potential (ΔΨm), downregulation of anti-apoptotic protein Bcl-2, upregulation of pro-apoptotic protein Bax, release of cytochrome C and activation of caspase-9 and caspase-3 in MCF-7 cells. The pretreatment with Neo remarkably weakened these effects of baicalein. Furthermore, we confirmed that the prooxidant action of baicalein involved the direct production of hydroxyl radicals through redox recycling of copper ions.

Conclusion

These findings suggested that baicalein, acting as a prooxidant, could trigger apoptosis in MCF-7 cells occurs via the ROS-mediated intrinsic mitochondria-dependent pathway.

Developing glutathione-activated catechol-type diphenylpolyenes as small molecule-based and mitochondria-targeted prooxidative anticancer theranostic prodrugs

Developing concise theranostic prodrugs is highly desirable for personalized and precision cancer therapy. Herein we used the glutathione (GSH)-mediated conversion of 2,4-dinitrobenzenesulfonates to phenols to protect a catechol moiety and developed stable pro-catechol-type diphenylpolyenes as small molecule-based prooxidative anticancer theranostic prodrugs. These molecules were synthesized via a modular route allowing creation of various pro-catechol-type diphenylpolyenes. As a typical representative, PDHH demonstrated three unique advantages: (1) capable of exploiting increased levels of GSH in cancer cells to in situ release a catechol moiety followed by its in situ oxidation to o-quinone, leading to preferential redox imbalance (including generation of H₂O₂ and depletion of GSH) and final selective killing of cancer cells over normal cells, and is also superior to 5-fluorouracil and doxorubicin, the widely used chemotherapy drugs, in terms of its ability to kill preferentially human colon cancer SW620 cells (IC₅₀ = 4.3 μM) over human normal liver L02 cells (IC₅₀ = 42.3 μM) with a favourable in vitro selectivity index of 9.8; (2) permitting a turn-on fluorescent monitoring for its release, targeting mitochondria and therapeutic efficacy without the need of introducing additional fluorophores after its activation by GSH in cancer cells; (3) efficiently targeting mitochondria without the need of introducing additional mitochondria-directed groups.

A promising redox cycle-based strategy for designing a catechol-type diphenylbutadiene as a potent prooxidative anti-melanoma agent

Developing anti-melanoma agents with increased activity and specificity is highly desirable due to the increasing incidence, highly metastatic malignancy, and high mortality rate of melanoma. Abnormal redox characteristics such as higher levels of tyrosinase, NAD(P)H: quinone oxidoreductase-1 (NQO1) and reactive oxygen species (ROS) observed in melanoma cells than in other cancer cells and normal cells illustrate their redox vulnerability and have opened a window for developing prooxidative anti-melanoma agents (PAAs) to target the vulnerability. However, how to design PAAs which promote selectively the ROS accumulation in melanoma cells remains a challenge. This work describes a promising redox cycle-based strategy for designing a catechol-type diphenylbutadiene as such type of PAA. This molecule is capable of constructing an efficient catalytic redox cycle with tyrosinase and NQO1 in melanoma B16F1 cells to induce selectively the ROS (mainly including hydrogen peroxide, H₂O₂) accumulation in the cells, resulting in highly selective suppression of melanoma B16F1 cells over tyrosinase-deficient HeLa and normal L-02 cells.

Designing salicylaldehyde isonicotinoyl hydrazones as Cu(II) ionophores with tunable chelation and release of copper for hitting redox Achilles heel of cancer cells

Higher levels of copper, reduced glutathione (GSH) and reactive oxygen species (ROS) observed in cancer cells than in normal cells, favor the idea of developing copper ionophores as prooxidative anticancer agents (PAAs) to hit the altered redox homeostasis (redox Achilles heel) of cancer cells. In this work, we used salicylaldehyde isonicotinoyl hydrazone (SIH-1) as a basic scaffold to design Cu(II) ionophores with tunable chelation and release of Cu(II) by introducing electron-withdrawing nitro and electron-donating methoxyl groups in the para position to phenolic hydroxyl, or by blocking the phenolic hydroxyl site using methyl. These molecules were used to probe how chelation and release of copper influence their ionophoric role and ability to target redox Achilles heel of cancer cells. Among these molecules, SIH-1 was identified as the most potent Cu(II) ionophore to kill preferentially HepG2 cells over HUVEC cells, and also superior to clioquinol, a copper ionophore evaluated in clinical trials, in terms of its relatively higher cytotoxicity and better selectivity. Higher oxidative potential, despite of lower stability constant, of the Cu(II) complex formed by SIH-1 than by the other molecules, is responsible for its stronger ability in releasing copper by GSH, inducing redox imbalance and triggering mitochondria-mediated apoptosis of HepG2 cells. This work gives useful information on how to design copper ionophores as PAAs for selective killing of cancer cells.