Sulfasalazine-induced cystine starvation: potential use for prostate cancer therapy

Unlocking ferroptosis in prostate cancer - the road to novel therapies and imaging markers

Ferroptosis is a distinct form of regulated cell death that is predominantly driven by the build-up of intracellular iron and lipid peroxides. Ferroptosis suppression is widely accepted to contribute to the pathogenesis of several tumours including prostate cancer. Results from some studies reported that prostate cancer cells can be highly susceptible to ferroptosis inducers, providing potential for an interesting new avenue of therapeutic intervention for advanced prostate cancer. In this Perspective, we describe novel molecular underpinnings and metabolic drivers of ferroptosis, analyse the functions and mechanisms of ferroptosis in tumours, and highlight prostate cancer-specific susceptibilities to ferroptosis by connecting ferroptosis pathways to the distinctive metabolic reprogramming of prostate cancer cells. Leveraging these novel mechanistic insights could provide innovative therapeutic opportunities in which ferroptosis induction augments the efficacy of currently available prostate cancer treatment regimens, pending the elimination of major bottlenecks for the clinical translation of these treatment combinations, such as the development of clinical-grade inhibitors of the anti-ferroptotic enzymes as well as non-invasive biomarkers of ferroptosis. These biomarkers could be exploited for diagnostic imaging and treatment decision-making.

Exploring Amino Acid Transporters as Therapeutic Targets for Cancer: An Examination of Inhibitor Structures, Selectivity Issues, and Discovery Approaches

Amino acid transporters are abundant amongst the solute carrier family and have an important role in facilitating the transfer of amino acids across cell membranes. Because of their impact on cell nutrient distribution, they also appear to have an important role in the growth and development of cancer. Naturally, this has made amino acid transporters a novel target of interest for the development of new anticancer drugs. Many attempts have been made to develop inhibitors of amino acid transporters to slow down cancer cell growth, and some have even reached clinical trials. The purpose of this review is to help organize the available information on the efforts to discover amino acid transporter inhibitors by focusing on the amino acid transporters ASCT2 (SLC1A5), LAT1 (SLC7A5), xCT (SLC7A11), SNAT1 (SLC38A1), SNAT2 (SLC38A2), and PAT1 (SLC36A1). We discuss the function of the transporters, their implication in cancer, their known inhibitors, issues regarding selective inhibitors, and the efforts and strategies of discovering inhibitors. The goal is to encourage researchers to continue the search and development within the field of cancer treatment research targeting amino acid transporters.

Ferroptosis in head and neck squamous cell carcinoma: from pathogenesis to treatment

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common malignant tumor worldwide, with high morbidity and mortality. Surgery and postoperative chemoradiotherapy have largely reduced the recurrence and fatality rates for most HNSCCs. Nonetheless, these therapeutic approaches result in poor prognoses owing to severe adverse reactions and the development of drug resistance. Ferroptosis is a kind of programmed cell death which is non-apoptotic. Ferroptosis of tumor cells can inhibit tumor development. Ferroptosis involves various biomolecules and signaling pathways, whose expressions can be adjusted to modulate the sensitivity of cells to ferroptosis. As a tool in the fight against cancer, the activation of ferroptosis is a treatment that has received much attention in recent years. Therefore, understanding the molecular mechanism of ferroptosis in HNSCC is an essential strategy with therapeutic potential. The most important thing to treat HNSCC is to choose the appropriate treatment method. In this review, we discuss the molecular and defense mechanisms of ferroptosis, analyze the role and mechanism of ferroptosis in the inhibition and immunity against HNSCC, and explore the therapeutic strategy for inducing ferroptosis in HNSCC including drug therapy, radiation therapy, immunotherapy, nanotherapy and comprehensive treatment. We find ferroptosis provides a new target for HNSCC treatment.

Targeting ferroptosis in melanoma: cancer therapeutics

Melanoma is an aggressive kind of skin cancer; its rate has risen rapidly over the past few decades. Melanoma reports for only about 1% of skin cancers but leads to a high majority of skin cancer deaths. Thus, new useful therapeutic approaches are currently required, to state effective treatments to consistently enhance the overall survival rate of melanoma patients. Ferroptosis is a recently identified cell death process, which is different from autophagy, apoptosis, necrosis, and pyroptosis in terms of biochemistry, genetics, and morphology which plays an important role in cancer treatment. Ferroptosis happens mostly by accumulating iron and lipid peroxides in the cell. Recently, studies have revealed that ferroptosis has a key role in the tumor's progression. Especially, inducing ferroptosis in cells can inhibit the tumor cells' growth, leading to back warding tumorigenesis. Here, we outline the ferroptosis characteristics from its basic role in melanoma cancer and mention its possible applications in melanoma cancer treatment. Video Abstract.

Targeting ferroptosis opens new avenues for the development of novel therapeutics

Ferroptosis is an iron-dependent form of regulated cell death with distinct characteristics, including altered iron homeostasis, reduced defense against oxidative stress, and abnormal lipid peroxidation. Recent studies have provided compelling evidence supporting the notion that ferroptosis plays a key pathogenic role in many diseases such as various cancer types, neurodegenerative disease, diseases involving tissue and/or organ injury, and inflammatory and infectious diseases. Although the precise regulatory networks that underlie ferroptosis are largely unknown, particularly with respect to the initiation and progression of various diseases, ferroptosis is recognized as a bona fide target for the further development of treatment and prevention strategies. Over the past decade, considerable progress has been made in developing pharmacological agonists and antagonists for the treatment of these ferroptosis-related conditions. Here, we provide a detailed overview of our current knowledge regarding ferroptosis, its pathological roles, and its regulation during disease progression. Focusing on the use of chemical tools that target ferroptosis in preclinical studies, we also summarize recent advances in targeting ferroptosis across the growing spectrum of ferroptosis-associated pathogenic conditions. Finally, we discuss new challenges and opportunities for targeting ferroptosis as a potential strategy for treating ferroptosis-related diseases.

Immunotherapy against the Cystine/Glutamate Antiporter xCT Improves the Efficacy of APR-246 in Preclinical Breast Cancer Models

Breast cancer is the most frequent cancer in women. Despite recent clinical advances, new therapeutic approaches are still required. The cystine-glutamate antiporter xCT, encoded by the SLC7A11 gene, which imports cystine in exchange with glutamate, is a potentially new target for breast cancer therapy, being involved in tumor cell redox balance and resistance to therapies. xCT expression is regulated by the oncosuppressor p53, which is mutated in many breast cancers. Indeed, mutant p53 (mut-p53) can induce xCT post-transcriptional down modulation, rendering mut-p53 tumors susceptible to oxidative damage. Interestingly, the drug APR-246, developed to restore the wild-type function of p53 in tumors harboring its mutation, alters the cell redox balance in a p53-independent way, possibly rendering the cells more sensitive to xCT inhibition. Here, we propose a combinatorial treatment based on xCT immunetargeting and APR-246 treatment as a strategy for tackling breast cancer. We demonstrate that combining the inhibition of xCT with the APR-246 drug significantly decreased breast cancer cell viability in vitro and induced apoptosis and affected cancer stem cells' self-renewal compared to the single treatments. Moreover, the immunetargeting of xCT through DNA vaccination in combination with APR-246 treatment synergistically hinders tumor progression and prevents lung metastasis formation in vivo. These effects can be mediated by the production of anti-xCT antibodies that are able to induce the antibody dependent cellular cytotoxicity of tumor cells. Overall, we demonstrate that DNA vaccination against xCT can synergize with APR-246 treatment and enhance its therapeutic effect. Thus, APR-246 treatment in combination with xCT immunetargeting may open new perspectives in the management of breast cancer.

Targeting amino acid metabolism in cancer

Metabolic rewiring is a characteristic hallmark of cancer cells. This phenomenon sustains uncontrolled proliferation and resistance to apoptosis by increasing nutrients and energy supply. However, reprogramming comes together with vulnerabilities that can be used against tumor and can be applied in targeted therapy. In the last years, the genetic background of tumors has been identified thoroughly and new therapies targeting those mutations tested. Nevertheless, we propose that targeting the phenotype of cancer cells could be another way of treatment aiming to avoid drug resistance and non-responsiveness of cancer patients. Amino acid metabolism is part of the altered processes in cancer cells. Amino acids are building blocks and also sensors of signaling pathways regulating main biological processes. In this comprehensive review, we described four amino acids (asparagine, arginine, methionine, and cysteine) which have been actively investigated as potential targets for anti-tumor therapy. Asparagine depletion is successfully used for decades in the treatment of acute lymphoblastic leukemia and there is a strong implication to apply it to other types of tumors. Arginine auxotrophic tumors are great candidates for arginine-starvation therapy. Higher requirement for essential amino acids such as methionine and cysteine point out promising targetable weaknesses of cancer cells.

Cancer suppression by ferroptosis and its role in digestive system tumors

Cancer is the second leading cause of death worldwide, and digestive system tumors remain the leading malignancy in China, seriously endangering national health and imposing a huge economic burden. Ferroptosis is a form of cell death characterized by increased intracellular reduced iron and accumulated lipid peroxide. Recent studies have revealed that ferroptosis is closely related to the occurrence and treatment of cancer. Therefore, this paper reviews the studies on ferroptosis and cancer to explore the potential of ferroptosis in the treatment of malignant tumors, especially digestive system tumors, and to provide a new direction for developing treatment options.

Oral Conventional Synthetic Disease-Modifying Antirheumatic Drugs with Antineoplastic Potential: a Review

There is an increasing trend of malignancy worldwide. Disease-modifying antirheumatic drugs (DMARDs) are the cornerstones for the treatment of immune-mediated inflammatory diseases (IMIDs), but risk of malignancy is a major concern for patients receiving DMARDs. In addition, many IMIDs already carry higher background risks of neoplasms. Recently, the black box warning of malignancies has been added for Janus kinase inhibitors. Also, the use of biologic DMARDs in patients with established malignancies is usually discouraged owing to exclusion of such patients in pivotal studies and, hence, lack of evidence. In contrast, some conventional synthetic DMARDs (csDMARDs) have been reported to show antineoplastic properties and can be beneficial for patients with cancer. Among the csDMARDs, chloroquine and hydroxychloroquine have been the most extensively studied, and methotrexate is an established chemotherapeutic agent. Even cyclosporine A, a well-known drug associated with cancer risk, can potentiate the effect of some chemotherapeutic agents. We review the possible mechanisms behind and clinical evidence of the antineoplastic activities of csDMARDs, including chloroquine and hydroxychloroquine, cyclosporine, leflunomide, mycophenolate mofetil, mycophenolic acid, methotrexate, sulfasalazine, and thiopurines. This knowledge may guide physicians in the choice of csDMARDs for patients with concurrent IMIDs and malignancies.

Pharmacologic approaches to amino acid depletion for cancer therapy

Cancer cells undergo metabolic reprogramming to support increased demands in bioenergetics and biosynthesis and to maintain reactive oxygen species at optimum levels. As metabolic alterations are broadly observed across many cancer types, metabolic reprogramming is considered a hallmark of cancer. A metabolic alteration commonly seen in cancer cells is an increased demand for certain amino acids. Amino acids are involved in a wide range of cellular functions, including proliferation, redox balance, bioenergetic and biosynthesis support, and homeostatic functions. Thus, targeting amino acid dependency in cancer is an attractive strategy for a number of cancers. In particular, pharmacologically mediated amino acid depletion has been evaluated as a cancer treatment option for several cancers. Amino acids that have been investigated for the feasibility of drug-induced depletion in preclinical and clinical studies for cancer treatment include arginine, asparagine, cysteine, glutamine, lysine, and methionine. In this review, we will summarize the status of current research on pharmacologically mediated amino acid depletion as a strategy for cancer treatment and potential chemotherapeutic combinations that synergize with amino acid depletion to further inhibit tumor growth and progression.

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.

Therapeutic Potential of Selenium in Glioblastoma

Little progress has been made in the long-term management of malignant brain tumors, leaving patients with glioblastoma, unfortunately, with a fatal prognosis. Glioblastoma remains the most aggressive primary brain cancer in adults. Similar to other cancers, glioblastoma undergoes a cellular metabolic reprogramming to form an oxidative tumor microenvironment, thereby fostering proliferation, angiogenesis and tumor cell survival. Latest investigations revealed that micronutrients, such as selenium, may have positive effects in glioblastoma treatment, providing promising chances regarding the current limitations in surgical treatment and radiochemotherapy outcomes. Selenium is an essential micronutrient with anti-oxidative and anti-cancer properties. There is additional evidence of Se deficiency in patients suffering from brain malignancies, which increases its importance as a therapeutic option for glioblastoma therapy. It is well known that selenium, through selenoproteins, modulates metabolic pathways and regulates redox homeostasis. Therefore, selenium impacts on the interaction in the tumor microenvironment between tumor cells, tumor-associated cells and immune cells. In this review we take a closer look at the current knowledge about the potential of selenium on glioblastoma, by focusing on brain edema, glioma-related angiogenesis, and cells in tumor microenvironment such as glioma-associated microglia/macrophages.

HDAC6 inhibitors sensitize non-mesenchymal triple-negative breast cancer cells to cysteine deprivation

Triple-negative breast cancer (TNBC) is a highly malignant type of breast cancer and lacks effective therapy. Targeting cysteine-dependence is an emerging strategy to treat the mesenchymal TNBC. However, many TNBC cells are non-mesenchymal and unresponsive to cysteine deprivation. To overcome such resistance, three selective HDAC6 inhibitors (Tubacin, CAY10603, and Tubastatin A), identified by epigenetic compound library screening, can synergize with cysteine deprivation to induce cell death in the non-mesenchymal TNBC. Despite the efficacy of HDAC6 inhibitor, knockout of HDAC6 did not mimic the synthetic lethality induced by its inhibitors, indicating that HDAC6 is not the actual target of HDAC6 inhibitor in this context. Instead, transcriptomic profiling showed that tubacin triggers an extensive gene transcriptional program in combination with erastin, a cysteine transport blocker. Notably, the zinc-related gene response along with an increase of labile zinc was induced in cells by the combination treatment. The disturbance of zinc homeostasis was driven by PKCγ activation, which revealed that the PKCγ signaling pathway is required for HDAC6 inhibitor-mediated synthetic lethality. Overall, our study identifies a novel function of HDAC6 inhibitors that function as potent sensitizers of cysteine deprivation and are capable of abolishing cysteine-independence in non-mesenchymal TNBC.

Chronic Sulfasalazine Treatment in Mice Induces System xc - - Independent Adverse Effects

Despite ample evidence for the therapeutic potential of inhibition of the cystine/glutamate antiporter system x_c⁻ in neurological disorders and in cancer, none of the proposed inhibitors is selective. In this context, a lot of research has been performed using the EMA- and FDA-approved drug sulfasalazine (SAS). Even though this molecule is already on the market for decades as an anti-inflammatory drug, serious side effects due to its use have been reported. Whereas for the treatment of the main indications, SAS needs to be cleaved in the intestine into the anti-inflammatory compound mesalazine, it needs to reach the systemic circulation in its intact form to allow inhibition of system x_c⁻. The higher plasma levels of intact SAS (or its metabolites) might induce adverse effects, independent of its action on system x_c⁻. Some of these effects have however been attributed to system x_c⁻ inhibition, calling into question the safety of targeting system x_c⁻. In this study we chronically treated system x_c⁻ - deficient mice and their wildtype littermates with two different doses of SAS (160 mg/kg twice daily or 320 mg/kg once daily, i.p.) and studied some of the adverse effects that were previously reported. SAS had a negative impact on the survival rate, the body weight, the thermoregulation and/or stress reaction of mice of both genotypes, and thus independent of its inhibitory action on system x_c⁻. While SAS decreased the total distance travelled in the open-field test the first time the mice encountered the test, it did not influence this parameter on the long-term and it did not induce other behavioral changes such as anxiety- or depressive-like behavior. Finally, no major histological abnormalities were observed in the spinal cord. To conclude, we were unable to identify any undesirable system x_c⁻-dependent effect of chronic administration of SAS.

Cysteine metabolic circuitries: druggable targets in cancer

To enable survival in adverse conditions, cancer cells undergo global metabolic adaptations. The amino acid cysteine actively contributes to cancer metabolic remodelling on three different levels: first, in its free form, in redox control, as a component of the antioxidant glutathione or its involvement in protein s-cysteinylation, a reversible post-translational modification; second, as a substrate for the production of hydrogen sulphide (H2S), which feeds the mitochondrial electron transfer chain and mediates per-sulphidation of ATPase and glycolytic enzymes, thereby stimulating cellular bioenergetics; and, finally, as a carbon source for epigenetic regulation, biomass production and energy production. This review will provide a systematic portrayal of the role of cysteine in cancer biology as a source of carbon and sulphur atoms, the pivotal role of cysteine in different metabolic pathways and the importance of H2S as an energetic substrate and signalling molecule. The different pools of cysteine in the cell and within the body, and their putative use as prognostic cancer markers will be also addressed. Finally, we will discuss the pharmacological means and potential of targeting cysteine metabolism for the treatment of cancer.

Ferroptosis: a new unexpected chance to treat metastatic melanoma?

Human skin melanoma is one of the most aggressive and difficult to treat human malignancies, with an increasing incidence over the years. While the resection of the early diagnosed primary tumor remains the best clinical approach, advanced/metastatic melanoma still remains with a poor prognosis. Indeed, although enormous progress in the therapeutic treatment of human tumors has been made in recent years, patients affected by metastatic melanoma are still poorly affected by these clinical advances. Therefore, new valuable therapeutic approaches are urgently needed, to design and define effective treatments to consistently increase the overall survival rate of patients affected by this malignancy. In this review we summarize the main signaling pathways studied to kill human skin melanoma, and introduce the ferroptotic cell death as a new pathway to be explored to eradicate this tumor.

Together we stand, apart we fall: how cell-to-cell contact/interplay provides resistance to ferroptosis

Contextualisation of the new type of cell death called “ferroptosis” opened a completely new avenue for the development of anti-cancer therapies. Cumulative fundamental research dating back to the mid-20th century, crowned by the extraordinary work of the group led by Dr. Stockwell from Columbia University in 2012, finally got its candidature to be applied in the clinical settings. Although the potential for clinical importance is undoubtedly growing every day, as showed by the increasing number of papers dealing with ferroptosis and its applications, long experience of cancer research and treatment taught us that caution is still necessary. The plasticity of the tumour cells, particularly acute, along with its involvement in the resistance mechanisms, that have been seen, to greater or lesser extent, for almost all currently used therapies, represents the biggest fascinations in biomedical research field and also the biggest challenge to achieving cures in cancer patients. Accordingly, the main features of fundamental research have to be vigilance and anticipation. In this review, we tried to summarize the literature data, accumulated in the past couple of years, which point out the pitfalls in which “ferroptosis inducers” can fall if used prematurely in the clinical settings, but at the same time can provide a great advantage in the exhausting battle with cancer resistance. This is the first comprehensive review focusing on the effects of the cell-to-cell contact/interplay in the development of resistance to ferroptosis, while the contribution of cell-born factors has been summarized previously so here we just listed them.

xCT: A Critical Molecule That Links Cancer Metabolism to Redox Signaling

System x_c^- cystine/glutamate antiporter, composed of a light-chain subunit (xCT, SLC7A11) and a heavy-chain subunit (CD98hc, SLC3A2), is mainly responsible for the cellular uptake of cystine in exchange for intracellular glutamate. In recent years, the xCT molecule has been found to play an important role in tumor growth, progression, metastasis, and multidrug resistance in various types of cancer. Interestingly, xCT also exhibits an essential function in regulating tumor-associated ferroptosis. Despite significant progress in targeting the system x_c^- transporter in cancer treatment, the underlying mechanisms still remain elusive. It is also unclear why solid tumors are more sensitive to xCT inhibitors such as sulfasalazine, as compared to hematological malignancies. This review mainly focuses on the role of xCT cystine/glutamate transporter in regard to tumor growth, chemoresistance, tumor-selective ferroptosis, and the mechanisms regulating xCT gene expression. The potential therapeutic implications of targeting the system x_c^- and its combination with chemotherapeutic agents or immunotherapy to suppress tumor growth and overcome drug resistance are also discussed.

Ferroptosis: An emerging therapeutic opportunity for cancer

Ferroptosis, a new form of non-apoptotic, regulated cell death characterized by iron dependency and lipid peroxidation, is involved in many pathological conditions such as neurodegenerative diseases, heart ischemia/reperfusion injury, acute renal failure, and cancer. While metabolic dysfunctions can lead to excessive lipid peroxidation culminating in ferroptotic cell death, glutathione peroxidase 4 (GPX4) resides in the center of a network that functions to prevent lipid hydroperoxides from accumulation, thereby suppressing ferroptosis. Indeed, RSL3 and other small-molecule GPX4 inhibitors can induce ferroptosis in not only cultured cancer cells but also tumor xenografts implanted in mice. Similarly, erastin and other system Xc⁻ inhibitors can deplete intracellular glutathione required for GPX4 function, leading to lipid peroxidation and ferroptosis. As therapy-resistant cancer cells are sensitive to GPX4-targeted therapeutic regimens, the agents capable of inducing ferroptosis hold great promises to improve current cancer therapy. This review will outline the molecular basis of ferroptosis, but focus on the strategies and the agents developed in recent years for therapeutic induction of ferroptosis. The potentials of these ferroptosis-inducing agents, which include system Xc⁻ inhibitors, GPX4 inhibitors, and iron-based nanoparticles, in cancer therapy will be subsequently discussed.

xCT inhibitor sulfasalazine depletes paclitaxel-resistant tumor cells through ferroptosis in uterine serous carcinoma

Glutathione (GSH) is a primary antioxidant that protects cells against reactive oxygen species (ROS), and high levels of GSH promote cancer cell survival and resistance to chemotherapy. The glutamine transporter xCT is essential for the intracellular synthesis of GSH, whereby xCT determines the intracellular redox balance. However, whether xCT inhibition can overcome GSH-mediated resistance to chemotherapeutic agents in uterine serous carcinoma (USC) remains unclear. Thus, the present study investigated the effect of the xCT inhibitor, sulfasalazine (SAS) on cytotoxicity in paclitaxel-sensitive and -resistant USC cell lines. The molecular mechanism by which SAS induces ferroptotic cell death in paclitaxel-resistant cells was assessed. The results of the cytotoxicity assay demonstrated that SAS was more cytotoxic in paclitaxel-resistant cells compared with in -sensitive cells; however, paclitaxel cytotoxicity was not enhanced in either of the USC cell lines. Immunoblotting analysis and the cell death assays performed using ferroptosis inhibitors indicated that SAS-mediated cell death was induced through ferroptosis, and not apoptosis, in paclitaxel-resistant cells. Furthermore, ROS production was increased in paclitaxel-resistant but not in -sensitive cells, even at low SAS concentration, and JNK was activated, which is a downstream target in the Ras signaling pathway. Knockdown of JNK reversed the inhibitory effect of SAS on cell proliferation and cell death. The synthetic lethal interaction between ROS accumulation and Ras effector JNK activation may be critical for enhancing the sensitivity to ferroptotic cell death mediated by xCT inhibitor, SAS. Taken together, the results of the present study suggest that xCT inhibition may be an effective treatment for patients with recurrent paclitaxel-resistant USC.

Breakdown of an Ironclad Defense System: The Critical Role of NRF2 in Mediating Ferroptosis

Ferroptosis is a non-apoptotic mode of regulated cell death that is iron and lipid peroxidation dependent. As new mechanistic insight into ferroptotic effectors and how they are regulated in different disease contexts is uncovered, our understanding of the physiological and pathological relevance of this mode of cell death continues to grow. Along these lines, a host of pharmacological modulators of this pathway have been identified, targeting proteins involved in iron homeostasis; the generation and reduction of lipid peroxides; or cystine import and glutathione metabolism. Also, of note, many components of the ferroptosis cascade are target genes of the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2), indicating its critical role in mediating the ferroptotic response. In this review, we discuss the in vitro, in vivo, and clinical evidence of ferroptosis in disease, including a brief discussion of targeting upstream mediators of this cascade, including NRF2, to treat ferroptosis-driven diseases.

Spatial modeling of prostate cancer metabolic gene expression reveals extensive heterogeneity and selective vulnerabilities

Spatial heterogeneity is a fundamental feature of the tumor microenvironment (TME), and tackling spatial heterogeneity in neoplastic metabolic aberrations is critical for tumor treatment. Genome-scale metabolic network models have been used successfully to simulate cancer metabolic networks. However, most models use bulk gene expression data of entire tumor biopsies, ignoring spatial heterogeneity in the TME. To account for spatial heterogeneity, we performed spatially-resolved metabolic network modeling of the prostate cancer microenvironment. We discovered novel malignant-cell-specific metabolic vulnerabilities targetable by small molecule compounds. We predicted that inhibiting the fatty acid desaturase SCD1 may selectively kill cancer cells based on our discovery of spatial separation of fatty acid synthesis and desaturation. We also uncovered higher prostaglandin metabolic gene expression in the tumor, relative to the surrounding tissue. Therefore, we predicted that inhibiting the prostaglandin transporter SLCO2A1 may selectively kill cancer cells. Importantly, SCD1 and SLCO2A1 have been previously shown to be potently and selectively inhibited by compounds such as CAY10566 and suramin, respectively. We also uncovered cancer-selective metabolic liabilities in central carbon, amino acid, and lipid metabolism. Our novel cancer-specific predictions provide new opportunities to develop selective drug targets for prostate cancer and other cancers where spatial transcriptomics datasets are available.

Amino acids in cancer

Over 90 years ago, Otto Warburg's seminal discovery of aerobic glycolysis established metabolic reprogramming as one of the first distinguishing characteristics of cancer¹. The field of cancer metabolism subsequently revealed additional metabolic alterations in cancer by focusing on central carbon metabolism, including the citric acid cycle and pentose phosphate pathway. Recent reports have, however, uncovered substantial non-carbon metabolism contributions to cancer cell viability and growth. Amino acids, nutrients vital to the survival of all cell types, experience reprogrammed metabolism in cancer. This review outlines the diverse roles of amino acids within the tumor and in the tumor microenvironment. Beyond their role in biosynthesis, they serve as energy sources and help maintain redox balance. In addition, amino acid derivatives contribute to epigenetic regulation and immune responses linked to tumorigenesis and metastasis. Furthermore, in discussing the transporters and transaminases that mediate amino acid uptake and synthesis, we identify potential metabolic liabilities as targets for therapeutic intervention.

Metabolic Remodeling as a Way of Adapting to Tumor Microenvironment (TME), a Job of Several Holders

The microenvironment depends and generates dependence on all the cells and structures that share the same niche, the biotope. The contemporaneous view of the tumor microenvironment (TME) agrees with this idea. The cells that make up the tumor, whether malignant or not, behave similarly to classes of elements within a living community. These elements inhabit, modify and benefit from all the facilities the microenvironment has to offer and that will contribute to the survival and growth of the tumor and the progression of the disease.The metabolic adaptation to microenvironment is a crucial process conducting to an established tumor able to grow locally, invade and metastasized. The metastatic cancer cells are reasonable more plastic than non-metastatic cancer cells, because the previous ones must survive in the microenvironment where the primary tumor develops and in addition, they must prosper in the microenvironment in the metastasized organ.The metabolic remodeling requires not only the adjustment of metabolic pathways per se but also the readjustment of signaling pathways that will receive and obey to the extracellular instructions, commanding the metabolic adaptation. Many diverse players are pivotal in cancer metabolic fitness from the initial signaling stimuli, going through the activation or repression of genes, until the phenotype display. The new phenotype will permit the import and consumption of organic compounds, useful for energy and biomass production, and the export of metabolic products that are useless or must be secreted for a further recycling or controlled uptake. In the metabolic network, three subsets of players are pivotal: (1) the organic compounds; (2) the transmembrane transporters, and (3) the enzymes.This chapter will present the "Pharaonic" intent of diagraming the interplay between these three elements in an attempt of simplifying and, at the same time, of showing the complex sight of cancer metabolism, addressing the orchestrating role of microenvironment and highlighting the influence of non-cancerous cells.

Targeting extracellular nutrient dependencies of cancer cells

Background

Cancer cells rewire their metabolism to meet the energetic and biosynthetic demands of their high proliferation rates and environment. Metabolic reprogramming of cancer cells may result in strong dependencies on nutrients that could be exploited for therapy. While these dependencies may be in part due to the nutrient environment of tumors, mutations or expression changes in metabolic genes also reprogram metabolic pathways and create addictions to extracellular nutrients.

Scope of review

This review summarizes the major nutrient dependencies of cancer cells focusing on their discovery and potential mechanisms by which metabolites become limiting for tumor growth. We further detail available therapeutic interventions based on these metabolic features and highlight opportunities for restricting nutrient availability as an anti-cancer strategy.

Major conclusions

Strategies to limit nutrients required for tumor growth using dietary interventions or nutrient degrading enzymes have previously been suggested for cancer therapy. The best clinical example of exploiting cancer nutrient dependencies is the treatment of leukemia with l-asparaginase, a first-line chemotherapeutic that depletes serum asparagine. Despite the success of nutrient starvation in blood cancers, it remains unclear whether this approach could be extended to other solid tumors. Systematic studies to identify nutrient dependencies unique to individual tumor types have the potential to discover targets for therapy.

The Xc- inhibitor sulfasalazine improves the anti-cancer effect of pharmacological vitamin C in prostate cancer cells via a glutathione-dependent mechanism

PURPOSE

Traditional treatment regimens for advanced prostate cancer, especially castration-resistant prostate cancer, result in low survival times with severe side effects. Therefore, new treatment options are required. Vitamin C (VC) has been identified as a promising anti-cancer agent of which the effects depend on the accumulation of H₂O₂ that is produced through autoxidation. Sulfasalazine (SAS), a cystine transporter (X_c^-) inhibitor, is known to suppress cellular glutathione (GSH) biosynthesis. Here, we hypothesized that targeting the X_c^- transporter via SAS may improve the anti-cancer activity of VC through regulating GSH biosynthesis, which in turn may result in the accumulation of reactive oxygen species (ROS).

METHODS

The anti-cancer effect of VC and/or SAS on prostate cancer cells was assessed using WST-8, colony formation and annexin V-FITC/PI FACS assays. Changes in cellular ROS and GSH levels were determined to verify our hypothesis. Finally, BALB/c nude mice bearing prostate cancer xenografts were used to assess the anti-cancer effects of single or combined VC and SAS therapies.

RESULTS

We found that SAS could potentiate the short- and long-term cytotoxicity of VC in prostate cancer cells. We also found that the synergistic effect of SAS and VC led to significant cellular GSH depletion, resulting in increased ROS accumulation. This synergistic effect could be reversed by the antioxidant N-acetyl-L-cysteine (NAC). The synergistic effect of SAS and VC was also noted in prostate cancer xenografts and correlated with immunohistochemistry results.

CONCLUSIONS

Our results strongly indicate that SAS, a relatively non-toxic drug that targets cystine transporters, in combination with VC may be superior to their single applications in the treatment of prostate cancer.

Spinal microglia contribute to cancer-induced pain through system xC --mediated glutamate release

Introduction:

Microglial cells, the resident macrophages of the central nervous system, are a key contributor to the generation and maintenance of cancer-induced pain (CIP). In healthy organisms, activated microglia promote recovery through the release of trophic and anti-inflammatory factors to clear toxins and pathogens and support neuronal survival. Chronically activated microglia, however, release toxic substances, including excess glutamate, causing cytotoxicity. Accordingly, rising attention is given to microglia for their role in abnormal physiology and in mediating neurotoxicity.

Objectives:

To examine the nociceptive relationship between peripherally-released glutamate and microglial xCT.

Methods:

A validated murine model of 4T1 carcinoma cell–induced nociception was used to assess the effect of peripheral tumour on spinal microglial activation and xCT expression. Coculture systems were then used to investigate the direct effect of glutamate released by wildtype and xCT knockdown MDA-MB-231 carcinoma cells on microglial activation, functional system x_C⁻ activity, and protein levels of interferon regulatory factor 8 (IRF8), a transcription factor implicated in microglia-mediated nociception.

Results:

Blockade of system x_C⁻ with sulfasalazine (SSZ) in vivo attenuated nociception in a 4T1 murine model of CIP and attenuates tumour-induced microglial activation in the dorsal horn of the spinal cord. Furthermore, knockdown of xCT in MDA-MB-231 cells mitigated tumour cell–induced microglial activation and functional system x_C⁻ activity in vitro.

Conclusions:

These data collectively demonstrate that the system xCT antiporter is functionally implicated in CIP and may be particularly relevant to pain progression through microglia. Upregulated xCT in chronically activated spinal microglia may be one pathway to central glutamate cytotoxicity. Microglial xCT may therefore be a valuable target for mitigating CIP.

Small-Molecule Ferroptotic Agents with Potential to Selectively Target Cancer Stem Cells

Effective management of advanced cancer requires systemic treatment including small molecules that target unique features of aggressive tumor cells. At the same time, tumors are heterogeneous and current evidence suggests that a subpopulation of tumor cells, called tumor initiating or cancer stem cells, are responsible for metastatic dissemination, tumor relapse and possibly drug resistance. Classical apoptotic drugs are less effective against this critical subpopulation. In the course of generating a library of open-chain epothilones, we discovered a new class of small molecule anticancer agents that has no effect on tubulin but instead kills selected cancer cell lines by harnessing reactive oxygen species to induce ferroptosis. Interestingly, we find that drug sensitivity is highest in tumor cells with a mesenchymal phenotype. Furthermore, these compounds showed enhanced toxicity towards mesenchymal breast cancer populations with cancer stem cell properties in vitro. In summary, we have identified a new class of small molecule ferroptotic agents that warrant further investigation.

Sulfasalazine could modulate the CD44v9-xCT system and enhance cisplatin-induced cytotoxic effects in metastatic bladder cancer

The prognostic role of CD44v9, a variant isoform of CD44 and a new cell surface marker of cancer stem cells, remains unclear in bladder cancer (BC) patients. Furthermore, limited information is available on the functional role of sulfasalazine (SSZ), which could modulate the CD44v9‐xCT system in order to enhance cisplatin (CDDP)‐induced cytotoxicity and inhibit the metastatic potential of BC. CD44v9 protein expression was examined immunohistochemically in 63 muscle invasive BC (MIBC) patients who underwent radical cystectomy. CD44v9 expression was independently associated with disease recurrence and cancer‐specific death in MIBC. Cytotoxic effects, glutathione levels, and reactive oxygen species production by SSZ and CD44v9 and phospho‐p38^MAPK protein expression by SSZ with or without CDDP were assessed in MBT‐2V cells with highly metastatic potential. Sulfasalazine exerted cytotoxic effects against MBT‐2V cells by inhibiting glutathione levels and inducing the production of reactive oxygen species. Sulfasalazine in combination with CDDP appeared to exert strong cytotoxic effects against MBT‐2V cells by inhibiting CD44v9 expression and upregulating phospho‐p38^MAPK expression. The inhibitory effects of SSZ with or without CDDP were also investigated using an MBT‐2V lung metastatic model. In the murine lung metastatic BC model, SSZ significantly prolonged animal survival. Furthermore, the combination of SSZ with CDDP exerted stronger inhibitory effects on the establishment of lung tumor nodules than SSZ or CDDP alone. CD44v9 expression could be a clinical biomarker for predicting poor outcomes in MIBC patients. Sulfasalazine in combination with CDDP has potential as a novel therapy against metastatic BC.

Assessment of Tumor Redox Status through (S)-4-(3-[18F]fluoropropyl)-L-Glutamic Acid PET Imaging of System xc - Activity

The cell's endogenous antioxidant system is vital to maintenance of redox homeostasis. Despite its central role in normal and pathophysiology, no noninvasive tools exist to measure this system in patients. The cystine/glutamate antiporter system xc - maintains the balance between intracellular reactive oxygen species and antioxidant production through the provision of cystine, a key precursor in glutathione biosynthesis. Here, we show that tumor cell retention of a system xc --specific PET radiotracer, (S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG), decreases in proportion to levels of oxidative stress following treatment with a range of redox-active compounds. The decrease in [18F]FSPG retention correlated with a depletion of intracellular cystine resulting from increased de novo glutathione biosynthesis, shown through [U-13C6, U-15N2]cystine isotopic tracing. In vivo, treatment with the chemotherapeutic doxorubicin decreased [18F]FSPG tumor uptake in a mouse model of ovarian cancer, coinciding with markers of oxidative stress but preceding tumor shrinkage and decreased glucose utilization. Having already been used in pilot clinical trials, [18F]FSPG PET could be rapidly translated to the clinic as an early redox indicator of tumor response to treatment. SIGNIFICANCE: [18F]FSPG PET imaging provides a sensitive noninvasive measure of tumor redox status and provides an early marker of tumor response to therapy.See related commentary by Lee et al., p. 701.

Sulfosalicylic acid/Fe3+ based nanoscale coordination polymers for effective cancer therapy by the Fenton reaction: an inspiration for understanding the role of aspirin in the prevention of cancer

A new type of pH-responsive nanoscale coordination polymer for ROS-enhanced cancer therapy.

Overexpression of cysteine-glutamate transporter and CD44 for prediction of recurrence and survival in patients with oral cavity squamous cell carcinoma

BACKGROUND

This study analyzed the expression of CD44 and cystine-glutamate transporter SLC7A11 (xCT) in primary oral cavity squamous cell carcinoma (SCC) and the relationships of expression to tumor recurrence and patient survival.

METHODS

Associations between CD44 and xCT expression and clinicopathologic results were analyzed in 231 patients with oral cavity SCC. Cox proportional hazard analyses were used to identify factors associated with recurrence-free survival (RFS), disease-specific survival (DSS), and overall survival (OS).

RESULTS

Overexpression of CD44 and/or xCT was associated with advanced T classification, perineural invasion, and lymphovascular invasion (P < .05 each). High expression of xCT was also associated with nodal metastasis and depth of invasion (P < .01 each). Multivariate analysis indicated that high expression of xCT and both xCT and CD44 were independent predictors of poor RFS, DSS, and OS (P < .05 each).

CONCLUSION

Overexpression of xCT or xCT plus CD44 may predict posttreatment recurrence and survival in patients with oral cavity SCC.

CISD2 inhibition overcomes resistance to sulfasalazine-induced ferroptotic cell death in head and neck cancer

Sulfasalazine has been repurposed to induce ferroptotic cancer cell death via inhibition of x_c^--cystine/glutamate antiporter (xCT). However, cancer cells are capable of developing mechanisms to evade cell death. Therefore, we sought to determine the molecular mechanisms underlying resistance to sulfasalazine-induced ferroptosis in head and neck cancer (HNC). The effects of sulfasalazine and pioglitazone were tested in various HNC cell lines. The effects of these drugs and inhibition and overexpression of CISD2 gene were determined by evaluating viability, cell death, lipid ROS production, mitochondrial iron, and mouse tumor xenograft models. SAS induced ferroptotic cell death in HNC at different levels. CISD2 expression showed an association between its expression and ferroptosis resistance. CISD2 overexpression conferred resistance to ferroptosis by sulfasalazine. Silencing CISD2 gene rendered resistant HNC cells susceptible to sulfasalazine-induced ferroptosis, with increased levels of lipid ROS and mitochondrial ferrous iron. Pioglitazone induced over-accumulation of mitochondrial iron and ROS and sensitized resistant HNC cells to sulfasalazine treatment in vitro and in a mouse tumor-xenograft model. CISD2 inhibition overcomes HNC resistance to ferroptotic cell death induced by sulfasalazine via increased accumulation of mitochondrial ferrous iron and lipid ROS.

Pushing the Limits of Cancer Therapy: The Nutrient Game

The standard cancer treatments include chemotherapy, radiotherapy, or their combination, which are generally associated with a multitude of side effects ranging from discomfort to the development of secondary tumors and severe toxicity to multiple systems including immune system. Mounting evidence has highlighted that the fine-tuning of nutrients may selectively sensitize cancer cells to conventional cancer therapies, while simultaneously protecting normal cells from their side effects. Nutrient modulation through diet also improves cancer immunesurveillance in a way that severe immunosuppression could be avoided or even the immune response or immune-based cancer therapies be potentiated also through patient microbiota remodeling. Here, we review recent advances in cancer therapy focusing on the effects of adjuvant dietary interventions (e.g., ketogenic diets, fasting) on the metabolic pathways within cancer cells and tumor environment (e.g., microbiota, immune system, tumor microenvironment) that are involved in cancer progression and resistance as well as cancer cell death. Finally, based on the overall literature data, we designed a nutritional intervention consisting in a plant-based moderate ketogenic diet that could be exploited for future preclinical research in cancer therapy.

Novel impact of the DNMT3A R882H mutation on GSH metabolism in a K562 cell model established by TALENs

DNA methyltransferase 3A (DNMT3A) mutations occurred in 18%~23% of acute myeloid leukemia (AML) patients, and were considered to be an adverse prognostic factor for adult de novo AML cases. However, the relevant molecular mechanism of the mutation in AML pathogenesis remains obscure. In this study, we established K562 and SKM1 cell model carrying the DNMT3A R882H mutation via transcription activator-like effector nuclease (TALEN) and Clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) technology, and discovered that mutated DNMT3A could promote the proliferative capability of malignant cell clones. Further RNA microarray analysis revealed that some genes crucial for glutathione (GSH) synthesis, including CTH, PSPH, PSAT1 and especially SLC7A11 (the cysteine/glutamate transporter) were significantly up-regulated, which resulted in significant elevation of intracellular GSH levels. A subsequent experiment demonstrated that the mutant clones are resistant to chemotherapy as well as SLC7A11-inhibitorsBy shRNA induced SLC7A11 silencing, we discovered profoundly decreased cellular GSH and cell proliferative ability of DNMT3A mutated clones. Our results provided novel insight into the role of the DNMT3A R882H mutation in AML pathogenesis and suggested that targeting the cellular GSH synthetic pathway could enhance the current therapy for AML patients with the DNMT3A R882H mutation.

Extracellular redox state shift: A novel approach to target prostate cancer invasion

AIM

Extracellular superoxide dismutase (ECSOD) and the cysteine/glutamate transporter (Cys)/(xCT) are tumor microenvironment (TME) redox state homeostasis regulators. Altered expression of ECSOD and xCT can lead to imbalance of the TME redox state and likely have a profound effect on cancer invasion. In the present study, we investigated whether ECSOD and xCT could be therapeutic targets for prostate cancer (PCa) invasion.

RESULTS

Immunohistochemistry of tumor microarray PCa tissues (N = 165) with high Gleason scores indicated that xCT protein expression is significantly increased while ECSOD protein expression is significantly decreased. Metastatic PCa indicated ECSOD protein expression is significantly decreased in epithelial area whereas xCT protein expression is significantly increased in stromal area. Furthermore, inhibition of extracellular O₂^•- by overexpression of ECSOD or alteration of the extracellular Cys/CySS ratio by knockdown of xCT protein inhibited PCa cell invasion. Simultaneous overexpression of ECSOD and knockdown xCT inhibited PCa cell invasion more than overexpression of ECSOD or knockdown of xCT alone. In the co-culturing system, simultaneous overexpression of ECSOD and knockdown of xCT in prostate stromal WPMY-1 cells inhibited PCa cell invasiveness more than overexpression of ECSOD alone. The decrease in PCa invasion correlated with increased of extracellular H₂O₂ levels. Notably, overexpression of catalase in TME reversed the inhibitory effect of ECSOD on cancer cell invasion.

CONCLUSION

Impaired ECSOD activity and an upregulated of xCT protein expression may be clinical features of an aggressive PCa, particularly metastatic cancers and/or those with a high Gleason score. Therefore, shifting the extracellular redox state toward an oxidizing status by targeted modulation of ECSOD and xCT, in both cancer and stromal cells, may provide a greater strategy for potential therapeutic interventions of aggressive PCa.

The Central Role of Amino Acids in Cancer Redox Homeostasis: Vulnerability Points of the Cancer Redox Code

A fine balance in reactive oxygen species (ROS) production and removal is of utmost importance for homeostasis of all cells and especially in highly proliferating cells that encounter increased ROS production due to enhanced metabolism. Consequently, increased production of these highly reactive molecules requires coupling with increased antioxidant defense production within cells. This coupling is observed in cancer cells that allocate significant energy reserves to maintain their intracellular redox balance. Glutathione (GSH), as a first line of defense, represents the most important, non-enzymatic antioxidant component together with the NADPH/NADP⁺ couple, which ensures the maintenance of the pool of reduced GSH. In this review, the central role of amino acids (AAs) in the maintenance of redox homeostasis in cancer, through GSH synthesis (cysteine, glutamate, and glycine), and nicotinamide adenine dinucleotide (phosphate) production (serine, and glutamine/glutamate) are illustrated. Special emphasis is placed on the importance of AA transporters known to be upregulated in cancers (such as system x_c-light chain and alanine-serine-cysteine transporter 2) in the maintenance of AA homeostasis, and thus indirectly, the redox homeostasis of cancer cells. The role of the ROS varies (often described as a "two-edged sword") during the processes of carcinogenesis, metastasis, and cancer treatment. Therefore, the context-dependent role of specific AAs in the initiation, progression, and dissemination of cancer, as well as in the redox-dependent sensitivity/resistance of the neoplastic cells to chemotherapy are highlighted.

ROS in Cancer: The Burning Question

An unanswered question in human health is whether antioxidation prevents or promotes cancer. Antioxidation has historically been viewed as chemopreventive, but emerging evidence suggests that antioxidants may be supportive of neoplasia. We posit this contention to be rooted in the fact that ROS do not operate as one single biochemical entity, but as diverse secondary messengers in cancer cells. This cautions against therapeutic strategies to increase ROS at a global level. To leverage redox alterations towards the development of effective therapies necessitates the application of biophysical and biochemical approaches to define redox dynamics and to functionally elucidate specific oxidative modifications in cancer versus normal cells. An improved understanding of the sophisticated workings of redox biology is imperative to defeating cancer.

Identification of capsazepine as a novel inhibitor of system xc- and cancer-induced bone pain

The cystine/glutamate antiporter has been implicated in a variety of cancers as a major mediator of redox homeostasis. The excess glutamate secreted by this transporter in aggressive cancer cells has been associated with cancer-induced bone pain (CIBP) from distal breast cancer metastases. High-throughput screening of small molecule inhibitors of glutamate release from breast cancer cells identified several potential compounds. One such compound, capsazepine (CPZ), was confirmed to inhibit the functional unit of system x_c^- (xCT) through its ability to block uptake of its radiolabeled substrate, cystine. Blockade of this antiporter induced production of reactive oxygen species (ROS) within 4 hours and induced cell death within 48 hours at concentrations exceeding 25 μM. Furthermore, cell death and ROS production were significantly reduced by co-treatment with N-acetylcysteine, suggesting that CPZ toxicity is associated with ROS-induced cell death. These data suggest that CPZ can modulate system x_c^- activity in vitro and this translates into antinociception in an in vivo model of CIBP where systemic administration of CPZ successfully delayed the onset and reversed CIBP-induced nociceptive behaviors resulting from intrafemoral MDA-MB-231 tumors.

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

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

18F-5-Fluoroaminosuberic Acid as a Potential Tracer to Gauge Oxidative Stress in Breast Cancer Models

The cystine transporter (system x_C^-) is an antiporter of cystine and glutamate. It has relatively low basal expression in most tissues and becomes upregulated in cells under oxidative stress (OS) as one of the genes expressed in response to the antioxidant response element promoter. We have developed ¹⁸F-5-fluoroaminosuberic acid (FASu), a PET tracer that targets system x_C^- The goal of this study was to evaluate ¹⁸F-FASu as a specific gauge for system x_C^- activity in vivo and its potential for breast cancer imaging. Methods:¹⁸F-FASu specificity toward system x_C^- was studied by cell inhibition assay, cellular uptake after OS induction with diethyl maleate, with and without anti-xCT small interfering RNA knockdown, in vitro uptake studies, and in vivo uptake in a system x_C^--transduced xenograft model. In addition, radiotracer uptake was evaluated in 3 breast cancer models: MDA-MB-231, MCF-7, and ZR-75-1. Results: Reactive oxygen species-inducing diethyl maleate increased glutathione levels and ¹⁸F-FASu uptake, whereas gene knockdown with anti-xCT small interfering RNA led to decreased tracer uptake. ¹⁸F-FASu uptake was robustly inhibited by system x_C^- inhibitors or substrates, whereas uptake was significantly higher in transduced cells and tumors expressing xCT than in wild-type HEK293T cells and tumors (P < 0.0001 for cells, P = 0.0086 for tumors). ¹⁸F-FASu demonstrated tumor uptake in all 3 breast cancer cell lines studied. Among them, triple-negative breast cancer MDA-MB-231, which has the highest xCT messenger RNA level, had the highest tracer uptake (P = 0.0058 when compared with MCF-7; P < 0.0001 when compared with ZR-75-1). Conclusion:¹⁸F-FASu as a system x_C^- substrate is a specific PET tracer for functional monitoring of system x_C^- and OS imaging. By enabling noninvasive analysis of x_C^- responses in vivo, this biomarker may serve as a valuable target for the diagnosis and treatment monitoring of certain breast cancers.

Dynamic roles of p53-mediated metabolic activities in ROS-induced stress responses

The p53 tumor suppressor is a multifaceted polypeptide that impedes tumorigenesis by regulating a diverse array of cellular processes. Triggered by a wide variety of stress stimuli, p53 transcriptionally regulates genes involved in the canonical tumor suppression pathways of apoptosis, cell-cycle arrest, and senescence. We recently discovered a novel mechanism whereby p53 inhibits cystine uptake through repression of the SLC7A11 gene to mediate ferroptosis. Importantly, this p53-SLC7A11 axis is preserved in the p53(3KR) mutant, and contributes to its ability to suppress tumorigenesis in the absence of the classical tumor suppression mechanisms. Here, we report that wild type p53 can induce both apoptosis and ferroptosis upon reactive oxygen species (ROS)-induced stress. Furthermore, we demonstrate that p53's functional N-terminal domain is required for its capacity to regulate oxidative stress responses and ferroptosis. Notably, activated p53 dynamically modulates intracellular ROS, causing an initial reduction and a subsequent increase of ROS levels. Taken together, these data implicate ferroptosis as an additional component of the cell death program induced by wild type p53 in human cancer cells, and reveal a complex and dynamic role of p53 in oxidative stress responses.

Genomic analysis of xCT-mediated regulatory network: Identification of novel targets against AIDS-associated lymphoma

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of primary effusion lymphoma (PEL), a rapidly progressing malignancy mostly arising in HIV-infected patients. Even under conventional chemotherapy, PEL continues to portend nearly 100% mortality within several months, which urgently requires novel therapeutic strategies. We have previously demonstrated that targeting xCT, an amino acid transporter for cystine/glutamate exchange, induces significant PEL cell apoptosis through regulation of multiple host and viral factors. More importantly, one of xCT selective inhibitors, Sulfasalazine (SASP), effectively prevents PEL tumor progression in an immune-deficient xenograft model. In the current study, we use Illumina microarray to explore the profile of genes altered by SASP treatment within 3 KSHV⁺ PEL cell-lines, and discover that many genes involved in oxidative stress/antioxidant defense system, apoptosis/anti-apoptosis/cell death, and cellular response to unfolded proteins/topologically incorrect proteins are potentially regulated by xCT. We further validate 2 downstream candidates, OSGIN1 (oxidative stress-induced growth inhibitor 1) and XRCC5 (X-ray repair cross-complementing protein 5), and evaluate their functional relationship with PEL cell survival/proliferation and chemoresistance, respectively. Together, our data indicate that targeting these novel xCT-regulated downstream genes may represent a promising new therapeutic strategy against PEL and/or other AIDS-related lymphoma.

Alternative fuels for cancer cells

Tumor metabolism is significantly altered to support the various metabolic needs of tumor cells. The most prominent change is the increased tumor glycolysis that leads to increased glucose uptake and utilization. However, it has become obvious that many non-glucose nutrients, such as amino acids, lactate, acetate, and macromolecules, can serve as alternative fuels for cancer cells. This knowledge reveals an unexpected flexibility and evolutionarily conserved model in which cancer cells uptake nutrients from their external environment to fulfill their necessary energetic needs. Tumor cells may have evolved the ability to utilize different carbon sources because of the limited supply of nutrients in their microenvironment, which can be driven by oncogenic mutations or tumor microenvironmental stresses. In certain cases, these factors permanently alter the tumor cells' metabolism, causing certain nutrients to become indispensable and thus creating opportunities for therapeutic intervention to eradicate tumors by their metabolic vulnerabilities.

Immunotargeting of Antigen xCT Attenuates Stem-like Cell Behavior and Metastatic Progression in Breast Cancer

Resistance to therapy and lack of curative treatments for metastatic breast cancer suggest that current therapies may be missing the subpopulation of chemoresistant and radioresistant cancer stem cells (CSC). The ultimate success of any treatment may well rest on CSC eradication, but specific anti-CSC therapies are still limited. A comparison of the transcriptional profiles of murine Her2(+) breast tumor TUBO cells and their derived CSC-enriched tumorspheres has identified xCT, the functional subunit of the cystine/glutamate antiporter system xc(-), as a surface protein that is upregulated specifically in tumorspheres. We validated this finding by cytofluorimetric analysis and immunofluorescence in TUBO-derived tumorspheres and in a panel of mouse and human triple negative breast cancer cell-derived tumorspheres. We further show that downregulation of xCT impaired tumorsphere generation and altered CSC intracellular redox balance in vitro, suggesting that xCT plays a functional role in CSC biology. DNA vaccination based immunotargeting of xCT in mice challenged with syngeneic tumorsphere-derived cells delayed established subcutaneous tumor growth and strongly impaired pulmonary metastasis formation by generating anti-xCT antibodies able to alter CSC self-renewal and redox balance. Finally, anti-xCT vaccination increased CSC chemosensitivity to doxorubicin in vivo, indicating that xCT immunotargeting may be an effective adjuvant to chemotherapy.

Xc- inhibitor sulfasalazine sensitizes colorectal cancer to cisplatin by a GSH-dependent mechanism

Sulfasalazine (SSZ) is an anti-inflammatory drug that has been demonstrated to induce apoptosis and tumor regression through inhibition of plasma membrane cystine transporter xc(-). Cysteine is a rate-limiting precursor for intracellular glutathione (GSH) synthesis, which is vital for compound detoxification and maintaining redox balance. Platinum-based chemotherapy is an important regimen used in clinics for various cancers including colorectal cancer (CRC). We hypothesized that targeting xc(-) transporter by SSZ may annihilate cellular detoxification through interruption of GSH synthesis and may enhance the anti-cancer activity of cisplatin (CDDP) by increasing drug transport. In the present study, we revealed that xCT, the active subunit of xc(-), is highly expressed in CRC cell lines and human colorectal carcinoma tissues compared with their normal counterparts. SSZ effectively depleted cellular GSH, leading to significant accumulation of reactive oxygen species and growth inhibition in CRC cells. In contrast, the normal epithelial cells of colon origin were less sensitive to SSZ, showing a moderate ROS elevation. Importantly, SSZ effectively enhanced the intracellular platinum level and cytotoxicity of CDDP in CRC cells. The synergistic effect of SSZ and CDDP was reversed by antioxidant N-acetyl-L-cysteine (NAC). Together, these results suggest that SSZ, a relatively non-toxic drug that targets cystine transporter, may, in combination with CDDP, have effective therapy for colorectal cancer.