High CD44 expression mediates p62-associated NFE2L2/NRF2 activation in breast cancer stem cell-like cells: Implications for cancer stem cell resistance

Reactive oxygen species: Role in carcinogenesis, cancer cell signaling and tumor progression

Cancer is one of the major causes of death in the world and its global burden is expected to continue increasing. In several types of cancers, reactive oxygen species (ROS) have been extensively linked to carcinogenesis and cancer progression. However, studies have reported conflicting evidence regarding the role of ROS in cancer, mostly dependent on the cancer type or the step of the tumorigenic process. We review recent studies describing diverse aspects of the interplay of ROS with cancer in the different stages of cancer progression, with a special focus on their role in carcinogenesis, their importance for cancer cell signaling and their relationship to the most prevalent cancer risk factors.

Diallyl disulfide and diallyl trisulfide in garlic as novel therapeutic agents to overcome drug resistance in breast cancer

Breast cancer is one of the leading causes of cancer-related deaths in women worldwide. It is a cancer that originates from the mammary ducts and involves mutations in multiple genes. Recently, the treatment of breast cancer has become increasingly challenging owing to the increase in tumor heterogeneity and aggressiveness, which gives rise to therapeutic resistance. Epidemiological, population-based, and hospital-based case-control studies have demonstrated an association between high intake of certain Allium vegetables and a reduced risk in the development of breast cancer. Diallyl disulfide (DADS) and diallyl trisulfide (DATS) are the main allyl sulfur compounds present in garlic, and are known to exhibit anticancer activity as they interfere with breast cancer cell proliferation, tumor metastasis, and angiogenesis. The present review highlights multidrug resistance mechanisms and their signaling pathways in breast cancer. This review discusses the potential anticancer activities of DADS and DATS, with emphasis on drug resistance in triple-negative breast cancer (TNBC). Understanding the anticancer activities of DADS and DATS provides insights into their potential in targeting drug resistance mechanisms of TNBC, especially in clinical studies.

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The review describes the causes of drug resistance in TNBC.

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The effects of DADS and DATS on drug resistance mechanisms in TNBC are presented.

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The impacts of DADS and DATS on metastasis of TNBC are discussed.

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Antitumor immune activities of DADS and DATS against TNBC are illustrated.

Alcohol Metabolism Enriches Squamous Cell Carcinoma Cancer Stem Cells That Survive Oxidative Stress via Autophagy

BACKGROUND

Alcohol (ethanol) consumption is a major risk factor for head and neck and esophageal squamous cell carcinomas (SCCs). However, how ethanol (EtOH) affects SCC homeostasis is incompletely understood.

METHODS

We utilized three-dimensional (3D) organoids and xenograft tumor transplantation models to investigate how EtOH exposure influences intratumoral SCC cell populations including putative cancer stem cells defined by high CD44 expression (CD44H cells).

RESULTS

Using 3D organoids generated from SCC cell lines, patient-derived xenograft tumors, and patient biopsies, we found that EtOH is metabolized via alcohol dehydrogenases to induce oxidative stress associated with mitochondrial superoxide generation and mitochondrial depolarization, resulting in apoptosis of the majority of SCC cells within organoids. However, CD44H cells underwent autophagy to negate EtOH-induced mitochondrial dysfunction and apoptosis and were subsequently enriched in organoids and xenograft tumors when exposed to EtOH. Importantly, inhibition of autophagy increased EtOH-mediated apoptosis and reduced CD44H cell enrichment, xenograft tumor growth, and organoid formation rate.

CONCLUSIONS

This study provides mechanistic insights into how EtOH may influence SCC cells and establishes autophagy as a potential therapeutic target for the treatment of EtOH-associated SCC.

Emerging role of ferroptosis in breast cancer: New dawn for overcoming tumor progression

Breast cancer has become a serious threat to women's health. Cancer progression is mainly derived from resistance to apoptosis induced by procedures or therapies. Therefore, new drugs or models that can overcome apoptosis resistance should be identified. Ferroptosis is a recently identified mode of cell death characterized by excess reactive oxygen species-induced lipid peroxidation. Since ferroptosis is distinct from apoptosis, necrosis and autophagy, its induction successfully eliminates cancer cells that are resistant to other modes of cell death. Therefore, ferroptosis may become a new direction around which to design breast cancer treatment. Unfortunately, the complete appearance of ferroptosis in breast cancer has not yet been fully elucidated. Furthermore, whether ferroptosis inducers can be used in combination with traditional anti- breast cancer drugs is still unknown. Moreover, a summary of ferroptosis in breast cancer progression and therapy is currently not available. In this review, we discuss the roles of ferroptosis-associated modulators glutathione, glutathione peroxidase 4, iron, nuclear factor erythroid-2 related factor-2, superoxide dismutases, lipoxygenase and coenzyme Q in breast cancer. Furthermore, we provide evidence that traditional drugs against breast cancer induce ferroptosis, and that ferroptosis inducers eliminate breast cancer cells. Finally, we put forward prospect of using ferroptosis inducers in breast cancer therapy, and predict possible obstacles and corresponding solutions. This review will deepen our understanding of the relationship between ferroptosis and breast cancer, and provide new insights into breast cancer-related therapeutic strategies.

Expression of Four Autophagy-Related Genes Accurately Predicts the Prognosis of Gastrointestinal Cancer in Asian Patients

Gastrointestinal (GI) cancers are among the most fatal diseases in the world. Numerous studies have demonstrated the relationship between autophagy and development of gastrointestinal cancers. However, whether autophagy-related genes can predict prognosis of GI cancers in individuals of Asian ancestry has not been defined. This study, evaluated the prognostic value of autophagy-related genes in gastrointestinal cancer. Expression profile of autophagy-related genes for 296 gastrointestinal cancer patients of Asian ancestry was downloaded from the TCGA database (TCGA-LIHC, TCGA-STAD, TCGA-ESCA, TCGA-PAAD, TCGA-COAD, TCGA-CHOL, and TCGA-READ). The prognostic value of the autophagy-related genes was evaluated using univariate Cox, LASSO, and multivariate Cox regression analyses. The risk score of the autophagy-related gene signature was calculated to assess its predictive prognostic value for GI cancers. Forty-seven differentially expressed autophagy-related genes, in Asian patients with gastrointestinal cancers, were identified. Of the 47 genes, 4 were associated with prognosis of GI cancer (SQSTM1, BIRC5, NRG3, and CXCR4). A prognostic model for GI cancer, based on the expression of the above 4 genes in the training set, showed that cancer patients were stratified into high-risk and low-risk groups (P < 0.05). The utility of the model for overall survival (OS) of GI cancer patients was consistent across the entire set, training set, and test set (entire set: P = 4.568 × 10⁻⁴; train set: P = 5.718 × 10⁻³; test set: P = 3.516 × 10⁻²). The sensitivity and specificity of the ROC curve of the above prognostic model in predicting the 5-year prognosis of GI cancer was satisfactory (entire set: 0.728; train set: 0.727; test set: 0.733). Analysis of clinical samples validated the overexpression of the 4 genes (SQSTM1, BIRC5, NRG3, and CXCR4) in tumor tissues relative to paired normal tissues, consistent with bioinformatic findings. Expression of the 4 autophagy-related genes (SQSTM1, BIRC5, NRG3, and CXCR4) can accurately predict the prognosis of gastrointestinal tumors in Asian patients.

Sanhuang Decoction Controls Tumor Microenvironment by Ameliorating Chronic Stress in Breast Cancer: A Report of Ninety Cases

Long-term endocrine treatment which results in estrogen deprivation causes chronic stress associated with a series of uncomfortable symptoms leading not only to a decrease in quality of life but also to cancer recurrence, which may be mediated primarily through the enhanced expression of angiogenic factors, as well as a series of inflammatory microenvironmental changes that favor tumor progression. In this study, we designed a clinical trial and aimed to explore the effects of Sanhuang Decoction (SHD) treatment on chronic stress, inflammatory factors, and breast cancer recovery. A total of 90 patients with breast cancer who met the inclusion/exclusion criteria were randomly allocated to a treatment or control group. The treatment group received the standard endocrine treatment and the traditional Chinese medicine decoction known as SHD. The control group received the standard endocrine treatment only. The treatment period was 6 months. The modified Kupperman Menopausal Index, the self-rating anxiety scale, and the self-rating depression scale were evaluated once per month. The body microenvironment plasma indices related to chronic stress, such as oxidative and antioxidative stress markers, inflammatory factors, hemorheology, coagulation, lipid and D-dimer, immunologic functions, tumor biomarkers, and angiogenic factors of the vascular endothelial growth factor (VEGF) were measured before and after 6 months of treatment. After treatment for 5 months, the scores in the treatment group decreased to nearly normal levels and the control group showed no significant improvement. After treatment for 6 months, all indices related to the body microenvironment, as well as the tumor biomarkers and carcinoembryonic antigen, carbohydrate antigen 153, and angiogenic factor VEGF levels improved significantly to normal levels in the treatment group. Our primary research showed that treatment with SHD effectively improved the quality of life of breast cancer patients by facilitating a change in the body microenvironment that controlled tumor growth and prevented drug resistance.

Targeted Inhibition of Anti-Inflammatory Regulator Nrf2 Results in Breast Cancer Retardation In Vitro and In Vivo

Nuclear factor erythroid-2 related factor-2 (Nrf2) is an oxidative stress-response transcriptional activator that promotes carcinogenesis through metabolic reprogramming, tumor promoting inflammation, and therapeutic resistance. However, the extension of Nrf2 expression and its involvement in regulation of breast cancer (BC) responses to chemotherapy remain largely unclear. This study determined the expression of Nrf2 in BC tissues (n = 46) and cell lines (MDA-MB-453, MCF-7, MDA-MB-231, MDA-MB-468) with diverse phenotypes. Immunohistochemical (IHC)analysis indicated lower Nrf2 expression in normal breast tissues, compared to BC samples, although the difference was not found to be significant. However, pharmacological inhibition and siRNA-induced downregulation of Nrf2 were marked by decreased activity of NADPH quinone oxidoreductase 1 (NQO1), a direct target of Nrf2. Silenced or inhibited Nrf2 signaling resulted in reduced BC proliferation and migration, cell cycle arrest, activation of apoptosis, and sensitization of BC cells to cisplatin in vitro. Ehrlich Ascites Carcinoma (EAC) cells demonstrated elevated levels of Nrf2 and were further tested in experimental mouse models in vivo. Intraperitoneal administration of pharmacological Nrf2 inhibitor brusatol slowed tumor cell growth. Brusatol increased lymphocyte trafficking towards engrafted tumor tissue in vivo, suggesting activation of anti-cancer effects in tumor microenvironment. Further large-scale BC testing is needed to confirm Nrf2 marker and therapeutic capacities for chemo sensitization in drug resistant and advanced tumors.

Sequestosome 1/p62: A multitasker in the regulation of malignant tumor aggression (Review)

Sequestosome 1 (SQSTM1)/p62 is an adapter protein mainly involved in the transportation, degradation and destruction of various proteins that cooperates with components of autophagy and the ubiquitin-proteasome degradation pathway. Numerous studies have shown that SQSTM1/p62 functions at multiple levels, including involvement in genetic stability or modification, post-transcriptional regulation and protein function. As a result, SQSTM1/p62 is a versatile protein that is a critical core regulator of tumor cell genetic stability, autophagy, apoptosis and other forms of cell death, malignant growth, proliferation, migration, invasion, metastasis and chemoradiotherapeutic response, and an indicator of patient prognosis. SQSTM1/p62 regulates these processes via its distinct molecular structure, through which it participates in a variety of activating or inactivating tumor-related and tumor microenvironment-related signaling pathways, particularly positive feedback loops and epithelial-mesenchymal transition-related pathways. Therefore, functioning as a proto-oncogene or tumor suppressor gene in various types of cancer and tumor-associated microenvironments, SQSTM1/p62 is capable of promoting or retarding malignant tumor aggression, giving rise to immeasurable effects on tumor occurrence and development, and on patient treatment and prognosis.

Mechanisms involved in selecting and maintaining neuroblastoma cancer stem cell populations, and perspectives for therapeutic targeting

Pediatric neuroblastomas (NBs) are heterogeneous, aggressive, therapy-resistant embryonal tumours that originate from cells of neural crest (NC) origin and in particular neuroblasts committed to the sympathoadrenal progenitor cell lineage. Therapeutic resistance, post-therapeutic relapse and subsequent metastatic NB progression are driven primarily by cancer stem cell (CSC)-like subpopulations, which through their self-renewing capacity, intermittent and slow cell cycles, drug-resistant and reversibly adaptive plastic phenotypes, represent the most important obstacle to improving therapeutic outcomes in unfavourable NBs. In this review, dedicated to NB CSCs and the prospects for their therapeutic eradication, we initiate with brief descriptions of the unique transient vertebrate embryonic NC structure and salient molecular protagonists involved NC induction, specification, epithelial to mesenchymal transition and migratory behaviour, in order to familiarise the reader with the embryonic cellular and molecular origins and background to NB. We follow this by introducing NB and the potential NC-derived stem/progenitor cell origins of NBs, before providing a comprehensive review of the salient molecules, signalling pathways, mechanisms, tumour microenvironmental and therapeutic conditions involved in promoting, selecting and maintaining NB CSC subpopulations, and that underpin their therapy-resistant, self-renewing metastatic behaviour. Finally, we review potential therapeutic strategies and future prospects for targeting and eradication of these bastions of NB therapeutic resistance, post-therapeutic relapse and metastatic progression.

The Pathways Underlying the Multiple Roles of p62 in Inflammation and Cancer

p62 is a highly conserved, multi-domain, and multi-functional adaptor protein critically involved in several important cellular processes. Via its pronounced domain architecture, p62 binds to numerous interaction partners, thereby influencing key pathways that regulate tissue homeostasis, inflammation, and several common diseases including cancer. Via binding of ubiquitin chains, p62 acts in an anti-inflammatory manner as an adaptor for the auto-, xeno-, and mitophagy-dependent degradation of proteins, pathogens, and mitochondria. Furthermore, p62 is a negative regulator of inflammasome complexes. The transcription factor Nrf2 regulates expression of a bundle of ROS detoxifying genes. p62 activates Nrf2 by interaction with and autophagosomal degradation of the Nrf2 inhibitor Keap1. Moreover, p62 activates mTOR, the central kinase of the mTORC1 sensor complex that controls cell proliferation and differentiation. Through different mechanisms, p62 acts as a positive regulator of the transcription factor NF-κB, a central player in inflammation and cancer development. Therefore, p62 represents not only a cargo receptor for autophagy, but also a central signaling hub, linking several important pro- and anti-inflammatory pathways. This review aims to summarize knowledge about the molecular mechanisms underlying the roles of p62 in health and disease. In particular, different types of tumors are characterized by deregulated levels of p62. The elucidation of how p62 contributes to inflammation and cancer progression at the molecular level might promote the development of novel therapeutic strategies.

Elevation of Chemosensitivity of Lung Adenocarcinoma A549 Spheroid Cells by Claudin-2 Knockdown through Activation of Glucose Transport and Inhibition of Nrf2 Signal

Claudin-2 (CLDN2), a tight junctional protein, is involved in the chemoresistance in a three-dimensional spheroid culture model of human lung adenocarcinoma A549 cells. However, the mechanism has not been fully clarified. We found that the knockdown of CLDN2 expression by siRNA in the spheroid reduces the expression of glucose transporters and metabolic enzymes. In a two-dimensional culture model, the expression of these proteins was increased by glucose deprivation or fasentin, an inhibitor of glucose transporter. In addition, the expression levels of nuclear factor erythroid 2-related factor 2 (Nrf2) and antioxidant enzymes including heme oxygenase-1, NAD(P)H:quinone oxidoreductase-1, and a glutamate–cysteine ligase modifier subunit were increased by fasentin. The fluorescence intensities of JC-1, a probe of mitochondrial membrane potential, and MitoROS 580, a probe of mitochondrial superoxide production, were increased by fasentin. These results suggest that mitochondrial production of reactive oxygen species is increased by glucose deficiency. The knockdown of CLDN2 enhanced the flux of 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-D-glucose (2-NBDG), a fluorescent deoxyglucose derivative, in a transwell assay, and the accumulation of glucose and 2-NBDG in spheroid cells. The expression of Nrf2 was decreased by CLDN2 knockdown, which was inhibited by fasentin and sulforaphane, a typical Nrf2 activator, in spheroid cells. The sensitivity of spheroid cells to doxorubicin, an anthracycline antitumor antibiotic, was enhanced by CLDN2 knockdown, which was inhibited by fasentin and sulforaphane. We suggest that CLDN2 induces chemoresistance in spheroid cells mediated through the inhibition of glucose transport and activation of the Nrf2 signal.

Cancer stem cells, epithelial-mesenchymal transition, ATP and their roles in drug resistance in cancer

The cancer stem cell (CSC) state and epithelial-mesenchymal transition (EMT) activation are tightly interconnected. Cancer cells that acquire the EMT/CSC phenotype are equipped with adaptive metabolic changes to maintain low reactive oxygen species levels and stemness, enhanced drug transporters, anti-apoptotic machinery and DNA repair system. Factors present in the tumor microenvironment such as hypoxia and the communication with non-cancer stromal cells also promote cancer cells to enter the EMT/CSC state and display related resistance. ATP, particularly the high levels of intratumoral extracellular ATP functioning through both signaling pathways and ATP internalization, induces and regulates EMT and CSC. The three of them work together to enhance drug resistance. New findings in each of these factors will help us explore deeper into mechanisms of drug resistance and suggest new resistance-associated markers and therapeutic targets.

Autophagy Blockade by Ai Du Qing Formula Promotes Chemosensitivity of Breast Cancer Stem Cells Via GRP78/β-Catenin/ABCG2 Axis

Accumulating evidence suggests that the root of drug chemoresistance in breast cancer is tightly associated with subpopulations of cancer stem cells (CSCs), whose activation is largely dependent on taxol-promoting autophagy. Our pilot study identified GRP78 as a specific marker for chemoresistance potential of breast CSCs by regulating Wnt/β-catenin signaling. Ai Du Qing (ADQ) is a traditional Chinese medicine formula that has been utilized in the treatment cancer, particularly during the consolidation phase. In the present study, we investigated the regulatory effects and molecular mechanisms of ADQ in promoting autophagy-related breast cancer chemosensitivity. ADQ with taxol decreasing the cell proliferation and colony formation of breast cancer cells, which was accompanied by suppressed breast CSC ratio, limited self-renewal capability, as well as attenuated multi-differentiation. Furthermore, autophagy in ADQ-treated breast CSCs was blocked by taxol via regulation of β-catenin/ABCG2 signaling. We also validated that autophagy suppression and chemosensitizing activity of this formula was GRP78-dependent. In addition, GRP78 overexpression promoted autophagy-inducing chemoresistance in breast cancer cells by stabilizing β-catenin, while ADQ treatment downregulated GRP78, activated the Akt/GSK3β-mediated proteasome degradation of β-catenin via ubiquitination activation, and consequently attenuated the chemoresistance-promoted effect of GRP78. In addition, both mouse breast cancer xenograft and zebrafish xenotransplantation models demonstrated that ADQ inhibited mammary tumor growth, and the breast CSC subpopulation showed obscure adverse effects. Collectively, this study not only reveals the chemosensitizating mechanism of ADQ in breast CSCs, but also highlights the importance of GRP78 in mediating autophagy-promoting drug resistance via β-catenin/ABCG2 signaling.

The role of CD44 in cancer chemoresistance: A concise review

CD44 is a cell surface adhesion molecule, which is overexpressed on cancer stem cells. The interaction of CD44 with hyaluronan is responsible for tumor development, metastasis, and expression of the chemoresistant phenotype. The overexpression of CD44 impedes the cytotoxic effect of chemotherapy medications in various cancers. Therefore, the high expression of CD44 is associated with a poor prognosis in affected patients. This high expression of CD44 in various cancers has provided an ample opportunity for the treatment of patients with chemoresistant malignancy. This review aims to demonstrate the various cross-talk between CD44 and intracellular and extracellular factors and highlight its role in developing chemoresistant tumors in some troublesome cancers.

Involvement of NRF2 in Breast Cancer and Possible Therapeutical Role of Polyphenols and Melatonin

Oxidative stress is defined as a disturbance in the prooxidant/antioxidant balance in favor of the former and a loss of control over redox signaling processes, leading to potential biomolecular damage. It is involved in the etiology of many diseases, varying from diabetes to neurodegenerative diseases and cancer. Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor and reported as one of the most important oxidative stress regulators. Due to its regulatory role in the expression of numerous cytoprotective genes involved in the antioxidant and anti-inflammatory responses, the modulation of NRF2 seems to be a promising approach in the prevention and treatment of cancer. Breast cancer is the prevalent type of tumor in women and is the leading cause of death among female cancers. Oxidative stress-related mechanisms are known to be involved in breast cancer, and therefore, NRF2 is considered to be beneficial in its prevention. However, its overactivation may lead to a negative clinical impact on breast cancer therapy by causing chemoresistance. Some known “oxidative stress modulators”, such as melatonin and polyphenols, are suggested to play an important role in the prevention and treatment of cancer, where the activation of NRF2 is reported as a possible underlying mechanism. In the present review, the potential involvement of oxidative stress and NRF2 in breast cancer will be reviewed, and the role of the NRF2 modulators—namely, polyphenols and melatonin—in the treatment of breast cancer will be discussed.

The multifaceted role of NRF2 in cancer progression and cancer stem cells maintenance

The transcription factor nuclear factor erythroid 2-like 2 (NEF2L2; NRF2) plays crucial roles in the defense system against electrophilic or oxidative stress by upregulating an array of genes encoding antioxidant proteins, electrophile/reactive oxygen species (ROS) detoxifying enzymes, and drug efflux transporters. In contrast to the protective roles in normal cells, the multifaceted role of NRF2 in tumor growth and progression, resistance to therapy and intratumoral stress, and metabolic adaptation is rapidly expanding, and the complex association of NRF2 with cancer signaling networks is being unveiled. In particular, the implication of NRF2 signaling in cancer stem cells (CSCs), a small population of tumor cells responsible for therapy resistance and tumor relapse, is emerging. Here, we described the dark side of NRF2 signaling in cancers discovered so far. A particular focus was put on the role of NRF2 in CSCs maintenance and therapy resistance, showing that low ROS levels and refractory drug response of CSCs are mediated by the activation of NRF2 signaling. A better understanding of the roles of the NRF2 pathway in CSCs will allow us to develop a novel therapeutic approach to control tumor relapse after therapy.

Nrf2 Signaling Pathway in Chemoprotection and Doxorubicin Resistance: Potential Application in Drug Discovery

Doxorubicin (DOX) is extensively applied in cancer therapy due to its efficacy in suppressing cancer progression and inducing apoptosis. After its discovery, this chemotherapeutic agent has been frequently used for cancer therapy, leading to chemoresistance. Due to dose-dependent toxicity, high concentrations of DOX cannot be administered to cancer patients. Therefore, experiments have been directed towards revealing underlying mechanisms responsible for DOX resistance and ameliorating its adverse effects. Nuclear factor erythroid 2-related factor 2 (Nrf2) signaling is activated to increase levels of reactive oxygen species (ROS) in cells to protect them against oxidative stress. It has been reported that Nrf2 activation is associated with drug resistance. In cells exposed to DOX, stimulation of Nrf2 signaling protects cells against cell death. Various upstream mediators regulate Nrf2 in DOX resistance. Strategies, both pharmacological and genetic interventions, have been applied for reversing DOX resistance. However, Nrf2 induction is of importance for alleviating side effects of DOX. Pharmacological agents with naturally occurring compounds as the most common have been used for inducing Nrf2 signaling in DOX amelioration. Furthermore, signaling networks in which Nrf2 is a key player for protection against DOX adverse effects have been revealed and are discussed in the current review.

Nanosized Drug Delivery Systems for Breast Cancer Stem Cell Targeting

Breast cancer stem cells (BCSCs), also known as breast cancer initiating cells, are reported to be responsible for the initiation, progression, therapeutic resistance, and relapse of breast cancer. Conventional therapeutic agents mainly kill the bulk of breast tumor cells and fail to eliminate BCSCs, even enhancing the fraction of BCSCs in breast tumors sometimes. Therefore, it is essential to develop specific and effective methods of eliminating BCSCs that will enhance the efficacy of killing breast tumor cells and thereby, increase the survival rates and quality of life of breast cancer patients. Despite the availability of an increasing number of anti-BCSC agents, their clinical translations are hindered by many issues, such as instability, low bioavailability, and off-target effects. Nanosized drug delivery systems (NDDSs) have the potential to overcome the drawbacks of anti-BCSC agents by providing site-specific delivery and enhancing of the stability and bioavailability of the delivered agents. In this review, we first briefly introduce the strategies and agents used against BCSCs and then highlight the mechanism of action and therapeutic efficacy of several state-of-the-art NDDSs that can be used to treat breast cancer by eliminating BCSCs.

Targeting cancer stem cells for reversing therapy resistance: mechanism, signaling, and prospective agents

Cancer stem cells (CSCs) show a self-renewal capacity and differentiation potential that contribute to tumor progression and therapy resistance. However, the underlying processes are still unclear. Elucidation of the key hallmarks and resistance mechanisms of CSCs may help improve patient outcomes and reduce relapse by altering therapeutic regimens. Here, we reviewed the identification of CSCs, the intrinsic and extrinsic mechanisms of therapy resistance in CSCs, the signaling pathways of CSCs that mediate treatment failure, and potential CSC-targeting agents in various tumors from the clinical perspective. Targeting the mechanisms and pathways described here might contribute to further drug discovery and therapy.

Pro-Tumoral Functions of Autophagy Receptors in the Modulation of Cancer Progression

Cancer progression involves a variety of pro-tumorigenic biological processes including cell proliferation, migration, invasion, and survival. A cellular pathway implicated in these pro-tumorigenic processes is autophagy, a catabolic route used for recycling of cytoplasmic components to generate macromolecular building blocks and energy, under stress conditions, to remove damaged cellular constituents to adapt to changing nutrient conditions and to maintain cellular homeostasis. During autophagy, cells form a double-membrane sequestering a compartment termed the phagophore, which matures into an autophagosome. Following fusion with the lysosome, the cargo is degraded inside the autolysosomes and the resulting macromolecules released back into the cytosol for reuse. Cancer cells use this recycling system during cancer progression, however the key autophagy players involved in this disease is unclear. Accumulative evidences show that autophagy receptors, crucial players for selective autophagy, are overexpressed during cancer progression, yet the mechanisms whereby pro-tumorigenic biological processes are modulated by these receptors remains unknown. In this review, we summarized the most important findings related with the pro-tumorigenic role of autophagy receptors p62/SQSTM1, NBR1, NDP52, and OPTN in cancer progression. In addition, we showed the most relevant cargos degraded by these receptors that have been shown to function as critical regulators of pro-tumorigenic processes. Finally, we discussed the role of autophagy receptors in the context of the cellular pathways implicated in this disease, such as growth factors signaling, oxidative stress response and apoptosis. In summary, we highlight that autophagy receptors should be considered important players of cancer progression, which could offer a niche for the development of novel diagnosis and cancer treatment strategies.

Role of Hypoxia-Mediated Autophagy in Tumor Cell Death and Survival

Programmed cell death or type I apoptosis has been extensively studied and its contribution to the pathogenesis of disease is well established. However, autophagy functions together with apoptosis to determine the overall fate of the cell. The cross talk between this active self-destruction process and apoptosis is quite complex and contradictory as well, but it is unquestionably decisive for cell survival or cell death. Autophagy can promote tumor suppression but also tumor growth by inducing cancer-cell development and proliferation. In this review, we will discuss how autophagy reprograms tumor cells in the context of tumor hypoxic stress. We will illustrate how autophagy acts as both a suppressor and a driver of tumorigenesis through tuning survival in a context dependent manner. We also shed light on the relationship between autophagy and immune response in this complex regulation. A better understanding of the autophagy mechanisms and pathways will undoubtedly ameliorate the design of therapeutics aimed at targeting autophagy for future cancer immunotherapies.

Breast cancer stem cells: A review of their characteristics and the agents that affect them

The evolving concept that cancer stem cells (CSCs) are the driving element in cancer development, evolution and heterogeneity, has overridden the previous model of a tumor consisting of cells all with similar sequentially acquired mutations and a similar potential for renewal, invasion and metastasis. This paradigm shift has focused attention on therapeutically targeting CSCs directly as a means of eradicating the disease. In breast cancers, CSCs can be identified by cell surface markers and are characterized by their ability to self-renew and differentiate, resist chemotherapy and radiation, and initiate new tumors upon serial transplantation in xenografted mice. These functional properties of CSCs are regulated by both intracellular and extracellular factors including pluripotency-related transcription factors, intracellular signaling pathways and external stimuli. Several classes of natural products and synthesized compounds have been studied to target these regulatory elements and force CSCs to lose stemness and/or terminally differentiate and thereby achieve a therapeutic effect. However, realization of an effective treatment for breast cancers, focused on the biological effects of these agents on breast CSCs, their functions and signaling, has not yet been achieved. In this review, we delineate the intrinsic and extrinsic factors identified to date that control or promote stemness in breast CSCs and provide a comprehensive compilation of potential agents that have been studied to target breast CSCs, transcription factors and stemness-related signaling. Our aim is to stimulate further study of these agents that could become the basis for their use as stand-alone treatments or components of combination therapies effective against breast cancers.

Trailblazing perspectives on targeting breast cancer stem cells

Breast cancer (BCa) is one of the most prevalent malignant tumors affecting women's health worldwide. The recurrence and metastasis of BCa have made it a long-standing challenge to achieve remission-persistent or disease-undetectable clinical outcomes. Cancer stem cells (CSCs) possess the ability to self-renew and generate heterogeneous tumor bulk. The existence of CSCs has been found to be vital in the initiation, metastasis, therapy resistance, and recurrence of tumors across cancer types. Because CSCs grow slowly in their dormant state, they are insensitive to conventional chemotherapies; however, when CSCs emerge from their dormant state and become clinically evident, they usually acquire genetic traits that make them resistant to existing therapies. Moreover, CSCs also show evidence of acquired drug resistance in synchrony with tumor relapses. The concept of CSCs provides a new treatment strategy for BCa. In this review, we highlight the recent advances in research on breast CSCs and their association with epithelial-mesenchymal transition (EMT), circulating tumor cells (CTCs), plasticity of tumor cells, tumor microenvironment (TME), T-cell modulatory protein PD-L1, and non-coding RNAs. On the basis that CSCs are associated with multiple dysregulated biological processes, we envisage that increased understanding of disease sub-classification, selected combination of conventional treatment, molecular aberration directed therapy, immunotherapy, and CSC targeting/sensitizing strategy might improve the treatment outcome of patients with advanced BCa. We also discuss novel perspectives on new drugs and therapeutics purposing the potent and selective expunging of CSCs.

Keap1-Nrf2 Pathway Regulates ALDH and Contributes to Radioresistance in Breast Cancer Stem Cells

Tumor recurrence after radiotherapy due to the presence of breast cancer stem cells (BCSCs) is a clinical challenge, and the mechanism remains unclear. Low levels of ROS and enhanced antioxidant defenses are shown to contribute to increasing radioresistance. However, the role of Nrf2-Keap1-Bach1 signaling in the radioresistance of BCSCs remains elusive. Fractionated radiation increased the percentage of the ALDH-expressing subpopulation and their sphere formation ability, promoted mesenchymal-to-epithelial transition and enhanced radioresistance in BCSCs. Radiation activated Nrf2 via Keap1 silencing and enhanced the tumor-initiating capability of BCSCs. Furthermore, knockdown of Nrf2 suppressed ALDH+ population and stem cell markers, reduced radioresistance by decreasing clonogenicity and blocked the tumorigenic ability in immunocompromised mice. An underlying mechanism of Keap1 silencing could be via miR200a, as we observed a significant increase in its expression, and the promoter methylation of Keap1 or GSK-3β did not change. Our data demonstrate that ALDH+ BCSC population contributes to breast tumor radioresistance via the Nrf2-Keap1 pathway, and targeting this cell population with miR200a could be beneficial but warrants detailed studies. Our results support the notion that Nrf2-Keap1 signaling controls mesenchymal-epithelial plasticity, regulates tumor-initiating ability and promotes the radioresistance of BCSCs.

CD44 as a tumor biomarker and therapeutic target

CD44, a complex transmembrane glycoprotein, exists in multiple molecular forms, including the standard isoform CD44s and CD44 variant isoforms. CD44 participates in multiple physiological processes, and aberrant expression and dysregulation of CD44 contribute to tumor initiation and progression. CD44 represents a common biomarker of cancer stem cells, and promotes epithelial-mesenchymal transition. CD44 is involved in the regulation of diverse vital signaling pathways that modulate cancer proliferation, invasion, metastasis and therapy-resistance, and it is also modulated by a variety of molecules in cancer cells. In addition, CD44 can serve as an adverse prognostic marker among cancer population. The pleiotropic roles of CD44 in carcinoma potentially offering new molecular target for therapeutic intervention. Preclinical and clinical trials for evaluating the pharmacokinetics, efficacy and drug-related toxicity of CD44 monoclonal antibody have been carried out among tumors with CD44 expression. In this review, we focus on current data relevant to CD44, and outline CD44 structure, the regulation of CD44, functional properties of CD44 in carcinogenesis and cancer progression as well as the potential CD44-targeting therapy for cancer management.

Enhancer remodeling promotes tumor-initiating activity in NRF2-activated non-small cell lung cancers

Transcriptional dysregulation, which can be caused by genetic and epigenetic alterations, is a fundamental feature of many cancers. A key cytoprotective transcriptional activator, NRF2, is often aberrantly activated in non-small cell lung cancers (NSCLCs) and supports both aggressive tumorigenesis and therapeutic resistance. Herein, we find that persistently activated NRF2 in NSCLCs generates enhancers at gene loci that are not normally regulated by transiently activated NRF2 under physiological conditions. Elevated accumulation of CEBPB in NRF2-activated NSCLCs is found to be one of the prerequisites for establishment of the unique NRF2-dependent enhancers, among which the NOTCH3 enhancer is shown to be critical for promotion of tumor-initiating activity. Enhancer remodeling mediated by NRF2-CEBPB cooperativity promotes tumor-initiating activity and drives malignancy of NRF2-activated NSCLCs via establishment of the NRF2-NOTCH3 regulatory axis.

Aberrant activation of NRF2 in cancer cells contributes to tumorigenicity and therapeutic resistance. Here, the authors show that NRF2 cooperates with CEBPB and remodels enhancers to confer tumor-initiating activity on NRF2- activated non-small cell lung cancers.

Nuclear Factor Erythroid 2-Related Factor 2 in Regulating Cancer Metabolism

Significance: Nuclear factor erythroid 2 (NFE2)-related factor 2 (NFE2L2, or NRF2) is a transcription factor predominantly affecting the expression of antioxidant genes. NRF2 plays a significant role in the control of redox balance, which is crucial in cancer cells. NRF2 activation regulates numerous cancer hallmarks, including metabolism, cancer stem cell characteristics, tumor aggressiveness, invasion, and metastasis formation. We review the molecular characteristics of the NRF2 pathway and discuss its interactions with the cancer hallmarks previously listed. Recent Advances: The noncanonical activation of NRF2 was recently discovered, and members of this pathway are involved in carcinogenesis. Further, cancer-related changes (e.g., metabolic flexibility) that support cancer progression were found to be redox- and NRF2 dependent. Critical Issues: NRF2 undergoes Janus-faced behavior in cancers. The pro- or antineoplastic effects of NRF2 are context dependent and essentially based on the specific molecular characteristics of the cancer in question. Therefore, systematic investigation of NRF2 signaling is necessary to clarify its role in cancer etiology. The biggest challenge in the NRF2 field is to determine which cancers can be targeted for better clinical outcomes. Further, large-scale genomic and transcriptomic studies are missing to correlate the clinical outcome with the activity of the NRF2 system. Future Directions: To exploit NRF2 in a clinical setting in the future, the druggable members of the NRF2 pathway should be identified. In addition, it will be important to study how the modulation of the NRF2 system interferes with cytostatic drugs and their combinations.

Role of Nrf2 and mitochondria in cancer stem cells; in carcinogenesis, tumor progression, and chemoresistance

Cancer stem cells (CSCs) are rare sub-population in tumor mass with self-renewal and differentiation abilities; CSCs are considered as the main cells which are responsible for tumor metastasis, cancer recurrence, and chemo/radio-resistance. CSCs are believed to contain low mitochondria in quantity, high concentration of nuclear factor erythroid 2-related factor 2 (Nrf2), and low reactive oxygen species (ROS) levels. Mitochondria regulate certain cellular functions, including controlling of cellular energetics, calcium signaling, cell growth and cell differentiation, cell cycle regulation, and cell death. Also, mitochondria are the main sources of intrinsic ROS production. Dysfunction of CSCs mitochondria due to oxidative phosphorylation is reported in several pathological conditions, including metabolic disorders, age-related diseases, and various types of cancers. ROS levels play a significant role in cellular signal transduction and CSCs' identity and differentiation capability. Nrf2 is a master transcription factor that plays critical functions in maintaining cellular redox hemostasis by regulating several antioxidant and detoxification pathways. Recently, the critical function of Nrf2 in CSCs has been revealed by several studies. Nrf2 is an essential molecule in the maintenance of CSCs' stemness and self-renewal in response to different oxidative stresses such as chemotherapy-induced elevation of ROS. Nrf2 enables these cells to recover from chemotherapy damages, and promotes establishment of invasion and dissemination. In this study, we have summarized the role of Nrf2 and mitochondria function CSCs, which promote cancer development. The significant role of Nrf2 in the regulation of mitochondrial function and ROS levels suggests this molecule as a potential target to eradicate CSCs.

Ferroptosis: An emerging approach for targeting cancer stem cells and drug resistance

Resistance to chemotherapeutic agents remains a major challenge in the fierce battle against cancer. Cancer stem cells (CSCs) are a small population of cells in tumors that possesses the ability to self-renew, initiate tumors, and cause resistance to conventional anticancer agents. Targeting this population of cells was proven as a promising approach to eliminate cancer recurrence and improve the clinical outcome. CSCs are less susceptible to death by classical anticancer agents inducing apoptosis. CSCs can be eradicated by ferroptosis, which is a non-apoptotic-regulated mechanism of cell death. The induction of ferroptosis is an attractive strategy to eliminate tumors due to its ability to selectively target aggressive CSCs. The current review critically explored the crosstalk and regulatory pathways controlling ferroptosis, which can selectively induce CSCs death. In addition, successful chemotherapeutic agents that achieve better therapeutic outcomes through the induction of ferroptosis in CSCs were discussed to highlight their promising clinical impact.

Chemotherapeutic resistance of head and neck squamous cell carcinoma is mediated by EpCAM induction driven by IL-6/p62 associated Nrf2-antioxidant pathway activation

Overexpression of epithelial cell adhesion molecule (EpCAM) has been associated with chemotherapeutic resistance, leads to aggressive tumor behavior, and results in an adverse clinical outcome. The molecular mechanism by which EpCAM enrichment is linked to therapeutic resistance via Nrf2, a key regulator of antioxidant genes is unknown. We have investigated the link between EpCAM and the Nrf2 pathway in light of therapeutic resistance using head and neck squamous cell carcinoma (HNSCC) patient tumor samples and cell lines. We report that EpCAM was highly expressed in Nrf2-positive and HPV-negative HNSCC cells. In addition, cisplatin-resistant tumor cells consisted of a higher proportion of EpCAM^high cells compared to the cisplatin sensitive counterpart. EpCAM^high populations exhibited resistance to cisplatin, a higher efficiency in colony formation, sphere growth and invasion capacity, and demonstrated reduced reactive oxygen species (ROS) activity. Furthermore, Nrf2 expression was significantly higher in EpCAM^high populations. Mechanistically, expression of Nrf2 and its target genes were most prominently observed in EpCAM^high populations. Silencing of EpCAM expression resulted in the attenuation of expressions of Nrf2 and SOD1 concomitant with a reduction of Sox2 expression. On the other hand, silencing of Nrf2 expression rendered EpCAM^high populations sensitive to cisplatin treatment accompanied by the inhibition of colony formation, sphere formation, and invasion efficiency and increased ROS activity. The molecular mechanistic link between EpCAM expression and activation of Nrf2 was found to be a concerted interaction of interleukin-6 (IL-6) and p62. Silencing of p62 expression in EpCAM^high populations resulted in the attenuation of Nrf2 pathway activation suggesting that Nrf2 pathway activation promoted resistance to cisplatin in EpCAM^high populations. We propose that therapeutic targeting the Nrf2-EpCAM axis might be an excellent approach to modulate stress resistance and thereby survival of HNSCC patients enriched in EpCAM^high populations.

Role of CYP4F2 as a novel biomarker regulating malignant phenotypes of liver cancer cells via the Nrf2 signaling axis

Hepatocellular carcinoma (HCC) is one of the most prevalent types of cancer worldwide. The present study attempted to identify a prognostic biomarker for HCC. RNA sequencing data from the GSE63863 dataset were downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) were identified based on a protein-protein interaction (PPI) network, and prognostic evaluation was subsequently conducted. Following lentiviral transfection, the migratory, proliferative and apoptotic abilities of cells were evaluated using wound healing, Cell Counting Kit-8, Transwell migration and apoptosis assays. A total of 192 DEGs were identified from 11 pairs of HCC and matched non-tumor samples. The PPI network revealed the top three modules, and eight genes were identified from these modules. The expression levels of cytochrome P450 family 4 subfamily F member 2 (CYP4F2) were downregulated in 50 HCC samples from The Cancer Genome Atlas and in the HCC Hep3B cell line. Low CYP4F2 expression was associated with a lower overall survival time. Functional studies revealed that CYP4F2 overexpression inhibited HCC cell proliferation and migration, and induced apoptosis. Furthermore, CYP4F2 overexpression repressed the expression of genes in the nuclear factor, erythroid 2 like 2 (Nrf2) signaling pathway, including Nrf2, heme oxygenase-1 and ferritin heavy chain 1, while increasing NAD(P)H quinone dehydrogenase 1 expression, suggesting that CYP4F2 overexpression reversed the antioxidant response of liver cancer cells. Overall, the present findings indicated that CYP4F2 may be a potential prognostic biomarker for predicting tumorigenesis and long-term survival rates in patients with HCC.

p62: Friend or Foe? Evidences for OncoJanus and NeuroJanus Roles

p62 is a versatile protein involved in the delicate balance between cell death and survival, which is fundamental for cell fate decision in the context of both cancer and neurodegenerative diseases. As an autophagy adaptor, p62 recognizes polyubiquitin chains and interacts with LC3, thereby targeting the selected cargo to the autophagosome with consequent autophagic degradation. Beside this function, p62 behaves as an interactive hub in multiple signalling including those mediated by Nrf2, NF-κB, caspase-8, and mTORC1. The protein is thus crucial for the control of oxidative stress, inflammation and cell survival, apoptosis, and metabolic reprogramming, respectively. As a multifunctional protein, p62 falls into the category of those factors that can exert opposite roles in the cells. Chronic p62 accumulation was found in many types of tumors as well as in stress granules present in different forms of neurodegenerative diseases. However, the protein seems to have a Janus behaviour since it may also serve protective functions against tumorigenesis or neurodegeneration. This review describes the diversified roles of p62 through its multiple domains and interactors and specifically focuses on its oncoJanus and neuroJanus roles.

Nuclear Factor Erythroid-Derived 2-Like 2-Induced Reductive Stress Favors Self-Renewal of Breast Cancer Stem-Like Cells via the FoxO3a-Bmi-1 Axis

Aims: A subpopulation of cancer cells, termed cancer stem cells (CSCs), has stemness properties, such as self-renewal and differentiation, which drive cancer recurrence and tumor resistance. CSCs possess enhanced protection capabilities to maintain reduced intracellular levels of reactive oxygen species (ROS) compared with nonstem-like cancer cells. This study investigated whether reductive stress could regulate self-renewal activity in breast CSCs. Results: We found that manifestation of stemness in breast cancer stem-like cells was associated with an elevated production of reduced glutathione (GSH) maintained by upregulation of glutamate cysteine ligase catalytic subunit (GCLC) and consequently, lowered ROS levels. This was accompanied by upregulation of phospho-AMP-activated protein kinase, FoxO3a, and Bmi-1. Notably, expression of nuclear factor erythroid-derived 2-like 2 (Nrf2) protein was substantially increased in cells undergoing sphere formation. We noticed that expression of Bmi-1 was inhibited after introduction of Nrf2 short interfering RNA into MCF-7 mammosphere cells. Silencing of Nrf2 expression suppressed the xenograft growth of subcutaneously or orthotopically injected human breast cancer cells. Innovation: Association between Nrf2 and self-renewal signaling in CSCs has been reported, but the underlying molecular mechanism remains largely unresolved. This study demonstrates the Nrf2-mediated signaling pathway in maintenance of reductive stress in breast CSCs. Conclusion: Nrf2 overactivation in breast CSCs upregulates GCLC expression and consequently enhances GSH biosynthesis with concurrent reduction in intracellular ROS accumulation, thereby provoking the reductive stress. The consequent upregulation of nuclear FoxO3a and its binding to the promoter of the gene encoding Bmi-1 account for the self-renewal activity of breast cancer stem-like cells and their growth in a xenograft mouse model.

Loss of Xanthine Oxidoreductase Potentiates Propagation of Hepatocellular Carcinoma Stem Cells

BACKGROUND AND AIMS

Liver cancer stem cells (CSCs) exist in the tumor environment and are critically involved in the initiation and progression of hepatocellular carcinoma (HCC). However, the underlying molecular mechanisms of self-renewal and maintenance of liver CSCs remain poorly understood.

APPROACH AND RESULTS

We identified that xanthine oxidoreductase (XOR), which was expressed at low levels in human HCC samples and liver CSCs, restrained HCC formation and chemoresistance by attenuating liver CSC propagation. Mechanistically, XOR physically interacts with ubiquitin-specific peptidase 15 (USP15), thereby promoting deubiquitination of Kelch-like ECH associated protein 1 (KEAP1) to stabilize its expression, which leads to degradation of Nrf2 (nuclear factor erythroid 2-related factor 2) through ubiquitination and subsequently reactive oxygen species accumulation in liver CSCs. Finally, our data reveal that XOR promotes USP15-mediated Nrf2-KEAP1 signaling to block liver CSCs and tumor propagation.

CONCLUSION

We identified that XOR may represent a potential therapeutic target for clinical intervention in HCC driven by liver CSCs.

Biomarkers of tumor invasiveness in proteomics (Review)

Over the past two decades, quantitative proteomics has emerged as an important tool for deciphering the complex molecular events involved in cancers. The number of references involving studies on the cancer metastatic process has doubled since 2010, while the last 5 years have seen the development of novel technologies combining deep proteome coverage capabilities with quantitative consistency and accuracy. To highlight key findings within this huge amount of information, the present review identified a list of tumor invasive biomarkers based on both the literature and data collected on a biocollection of experimental cell lines, tumor models of increasing invasiveness and tumor samples from patients with colorectal or breast cancer. Crossing these different data sources led to 76 proteins of interest out of 1,245 mentioned in the literature. Information on these proteins can potentially be translated into clinical prospects, since they represent potential targets for the development and evaluation of innovative therapies, alone or in combination. Herein, a systematical review of the biology of each of these proteins, including their specific subcellular/extracellular or multiple localizations is presented. Finally, as an important advantage of quantitative proteomics is the ability to provide data on all these molecules simultaneously in cell pellets, body fluids or paraffin‑embedded sections of tumors/invaded tissues, the significance of some of their interconnections is discussed.

Tumor Cell Associated Hyaluronan-CD44 Signaling Promotes Pro-Tumor Inflammation in Breast Cancer

Cancer has been conceptualized as a chronic wound with a predominance of tumor promoting inflammation. Given the accumulating evidence that the microenvironment supports tumor growth, we investigated hyaluronan (HA)-CD44 interactions within breast cancer cells, to determine whether this axis directly impacts the formation of an inflammatory microenvironment. Our results demonstrate that breast cancer cells synthesize and fragment HA and express CD44 on the cell surface. Using RNA sequencing approaches, we found that loss of CD44 in breast cancer cells altered the expression of cytokine-related genes. Specifically, we found that production of the chemokine CCL2 by breast cancer cells was significantly decreased after depletion of either CD44 or HA. In vivo, we found that CD44 deletion in breast cancer cells resulted in a delay in tumor formation and localized progression. This finding was accompanied by a decrease in infiltrating CD206+ macrophages, which are typically associated with tumor promoting functions. Importantly, our laboratory results were supported by human breast cancer patient data, where increased HAS2 expression was significantly associated with a tumor promoting inflammatory gene signature. Because high levels of HA deposition within many tumor types yields a poorer prognosis, our results emphasize that HA-CD44 interactions potentially have broad implications across multiple cancers.

New insights into small-cell lung cancer development and therapy

Small‐cell lung cancer (SCLC) accounts for approximately 15% of lung cancer cases; however, it is characterized by easy relapse and low survival rate, leading to one of the most intractable diseases in clinical practice. Despite decades of basic and clinical research, little progress has been made in the management of SCLC. The current standard first‐line regimens of SCLC still remain to be cisplatin or carboplatin combined with etoposide, and the adverse events of chemotherapy are by no means negligible. Besides, the immunotherapy on SCLC is still in an early stage and novel studies are urgently needed. In this review, we describe SCLC development and current therapy, aiming at providing useful advices on basic research and clinical strategy.

Long noncoding RNA TUG1 regulates prostate cancer cell proliferation, invasion and migration via the Nrf2 signaling axis

BACKGROUND

Long noncoding RNAs (lncRNAs) have been identified to modulate the development and progression of prostate cancer (PCa) via the regulation of their target genes. However, the biological function underlying the effect of lncRNA TUG1 in PCa remains unclear.

METHODS

Reverse transcription-quantitative polymerase chain reaction (qRT-PCR) and Western blotting analysis were used to assess the mRNA expression of TUG1 and protein expression levels of Nrf2 pathway members, respectively. The migration, invasion, and proliferation abilities of cells were assessed by the wound-healing, Transwell migration/invasion, and CCK8 assays, respectively.

RESULTS

TUG1 was strikingly upregulated in PCa cells compared with non-tumorigenic human prostate epithelial cells. The LncTar Web Server, which is a bioinformatics tool, was used to predict the target association between TUG1 and Nrf2. Moreover, the expression of TUG1 showed a strikingly positive correlation with that of Nrf2 in TCGA PCa RNA-Seq data (r = 0.26,P = 4.63E-09). Subsequently, inhibition of TUG1 using siRNA resulted in deceased proliferation, migration, and invasion of PCa cells; however, these effects were reversed by treatment with oltipraz (an activator of Nrf2). Finally, we evaluated the Nrf2 pathway to reveal the underlying mechanism of TUG1 in PCa cells, and found that TUG1 knockdown decreased the protein expression of Nrf2 downstream members (e.g., HO-1, FTH1, and NQO1).

CONCLUSIONS

LncRNA TUG1 plays an oncogenic role in human PCa cells by promoting the cell proliferation and invasion in PCa cell lines, at least partly via the Nrf2 signaling pathway.

IL-1-conferred gene expression pattern in ERα+ BCa and AR+ PCa cells is intrinsic to ERα- BCa and AR- PCa cells and promotes cell survival

Background

Breast (BCa) and prostate (PCa) cancers are hormone receptor (HR)-driven cancers. Thus, BCa and PCa patients are given therapies that reduce hormone levels or directly block HR activity; but most patients eventually develop treatment resistance. We have previously reported that interleukin-1 (IL-1) inflammatory cytokine downregulates ERα and AR mRNA in HR-positive (HR⁺) BCa and PCa cell lines, yet the cells can remain viable. Additionally, we identified pro-survival proteins and processes upregulated by IL-1 in HR⁺ BCa and PCa cells, that are basally high in HR⁻ BCa and PCa cells. Therefore, we hypothesize that IL-1 confers a conserved gene expression pattern in HR⁺ BCa and PCa cells that mimics conserved basal gene expression patterns in HR⁻ BCa and PCa cells to promote HR-independent survival and tumorigenicity.

Methods

We performed RNA sequencing (RNA-seq) for HR⁺ BCa and PCa cell lines exposed to IL-1 and for untreated HR⁻ BCa and PCa cell lines. We confirmed expression patterns of select genes by RT-qPCR and used siRNA and/or drug inhibition to silence select genes in the BCa and PCa cell lines. Finally, we performed Ingenuity Pathway Analysis (IPA) and used the gene ontology web-based tool, GOrilla, to identify signaling pathways encoded by our RNA-seq data set.

Results

We identified 350 genes in common between BCa and PCa cells that are induced or repressed by IL-1 in HR⁺ cells that are, respectively, basally high or low in HR⁻ cells. Among these genes, we identified Sequestome-1 (SQSTM1/p62) and SRY (Sex-Determining Region Y)-Box 9 (SOX9) to be essential for survival of HR⁻ BCa and PCa cell lines. Analysis of publicly available data indicates that p62 and SOX9 expression are elevated in HR-independent BCa and PCa sublines generated in vitro, suggesting that p62 and SOX9 have a role in acquired hormone receptor independence and treatment resistance. We also assessed HR⁻ cell line viability in response to the p62-targeting drug, verteporfin, and found that verteporfin is cytotoxic for HR⁻ cell lines.

Conclusions

Our 350 gene set can be used to identify novel therapeutic targets and/or biomarkers conserved among acquired (e.g. due to inflammation) or intrinsic HR-independent BCa and PCa.

Anti-Tumor Effects of Osthole on Different Malignant Tissues: A Review of Molecular Mechanisms

Cancer management and/or treatment require a comprehensive understanding of the molecular and signaling pathways involved. Recently, much attention has been directed to these molecular and signaling pathways, and it has been suggested that a number of biomolecules/players involved in such pathways, such as PI3K/Akt, NF-kB, STAT, and Nrf2 contribute to the progression, invasion, proliferation, and metastasis of malignant cells. Synthetic anti-tumor agents and chemotherapeutic drugs have been a mainstay in cancer therapy and are widely used to suppress the progression and, hopefully, halt the proliferation of malignant cells. However, these agents have some undesirable side-effects and, therefore, naturally-occurring compounds with high potency and fewer side-effects are now of great interest. Osthole is a plant-derived chemical compound that can inhibit the proliferation of malignant cells and provide potent anti-cancer effects in various tissues. Therefore, in this review, we presented the main findings concerning the potential anti-tumor effects of osthole and its derivatives and described possible molecular mechanisms by which osthole may suppress malignant cell proliferation in different tissues.

Molecular Mechanisms Underlying Autophagy-Mediated Treatment Resistance in Cancer

Despite advances in diagnostic tools and therapeutic options, treatment resistance remains a challenge for many cancer patients. Recent studies have found evidence that autophagy, a cellular pathway that delivers cytoplasmic components to lysosomes for degradation and recycling, contributes to treatment resistance in different cancer types. A role for autophagy in resistance to chemotherapies and targeted therapies has been described based largely on associations with various signaling pathways, including MAPK and PI3K/AKT signaling. However, our current understanding of the molecular mechanisms underlying the role of autophagy in facilitating treatment resistance remains limited. Here we provide a comprehensive summary of the evidence linking autophagy to major signaling pathways in the context of treatment resistance and tumor progression, and then highlight recently emerged molecular mechanisms underlying autophagy and the p62/KEAP1/NRF2 and FOXO3A/PUMA axes in chemoresistance.

Magnoflorine improves sensitivity to doxorubicin (DOX) of breast cancer cells via inducing apoptosis and autophagy through AKT/mTOR and p38 signaling pathways

Breast cancer is a leading cause of cancer death among women worldwide. Doxorubicin (DOX) is a broad-spectrum anti-breast cancer agent, but its clinical use is restricted due to irreversible tissue toxicity. Thereby, new therapeutic approaches are urgently required to promote the sensitivity of breast cancer cells to DOX. Magnoflorine (Mag), a quaternary alkaloid isolated from Chinese herb Magnolia or Aristolochia, has various biological activities, such as anti-inflammation, anti-cancer, and anti-anxiety. In the study, we explored the effects Mag on the sensitivity of breast cancer cells to DOX. We demonstrated that Mag strongly promoted DOX-induced anti-proliferative effects in breast cancer cells while not in normal cells. Mag addition markedly promoted the effects of DOX on the inhibition of migration and invasion in breast cancer cells. DOX-triggered DNA damage in breast cancer cells was further accelerated by combination with Mag. DOX-induced cell distribution in G2/M phase was markedly elevated when co-treated with Mag. Additionally, DOX/Mag combinational treatment significantly induced apoptosis in breast cancer cells when compared to DOX alone group through inducing Caspase-3 cleavage. Moreover, Mag markedly promoted the role of DOX in autophagy induction by elevating light chain 3 (LC3)-II expression. Combination treatment with DOX and Mag significantly inhibited the activation of phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) signaling, and promoted p38 mitogen-activated protein kinase (MAPK) pathway. In addition, treatment with wortmannin (Wor, a blocker of autophagosome formation) markedly reduced DOX/Mag-induced p38 MAPK activation and LC3 conversion in breast cancer cells. Further, in MCF-7 xenograft model, DOX combined with Mag displayed a significant anti-tumor effect with little toxicity to organs such as liver, heart, kidney and spleen. These findings suggested that Mag promoted the anti-cancer effects of DOX to induce cellular apoptosis and autophagy in breast cancer cells.

The Role of Nrf2 signaling in cancer stem cells: From stemness and self-renewal to tumorigenesis and chemoresistance

Cancer stem cells (CSCs) are subpopulation of tumor mass with exclusive abilities in self-renewing, stemness maintaining, and differentiation into the various non-stem cancer cells to provoke tumorigenesis, metastasis dissemination, drug-resistant, and cancer recurrence. Reactive oxygen species (ROS) impair cellular function by oxidizing cell components containing proteins, lipids, and DNA. Tumor oxidant status is elevated due to high metabolic activity under influence of abnormal growth factors, cytokines and function ROS-producing enzymes, including nitric oxide synthases, cyclooxygenases, and lipoxygenases. Nuclear factor-erythroid 2-related factor 2 (NRF2) is a transcriptional master regulator element which is believed to recognize cellular oxidative stress followed by binding to promoter of cyto-protective and anti-oxidative genes to maintain cellular redox status through promoting antioxidant response participants (glutathione peroxidase, glutathione reductase, thioredoxin reductase, ferritin, NADPH: quinone oxidoreductase 1). However, Nrf2 signaling protects malignant cells from ROS damage against tumor growth and chemoresistance. In addition, Nrf2 is able to participate in differentiation of certain stem cells by modulating autophagy procedure, also NRF2 provokes DNA damage response and facilitates drug metabolism and drug resistance by controlling of downstream enzyme and transporter members. In this review, we discuss the role of NRF2 in stemness, self-renewal ability, tumorigenesis and chemoresistance of CSCs.

Lithocholic Acid, a Metabolite of the Microbiome, Increases Oxidative Stress in Breast Cancer

In breast cancer patients, the diversity of the microbiome decreases, coinciding with decreased production of cytostatic bacterial metabolites like lithocholic acid (LCA). We hypothesized that LCA can modulate oxidative stress to exert cytostatic effects in breast cancer cells. Treatment of breast cancer cells with LCA decreased nuclear factor-2 (NRF2) expression and increased Kelch-like ECH associating protein 1 (KEAP1) expression via activation of Takeda G-protein coupled receptor (TGR5) and constitutive androstane receptor (CAR). Altered NRF2 and KEAP1 expression subsequently led to decreased expression of glutathione peroxidase 3 (GPX3), an antioxidant enzyme, and increased expression of inducible nitric oxide synthase (iNOS). The imbalance between the pro- and antioxidant enzymes increased cytostatic effects via increased levels of lipid and protein oxidation. These effects were reversed by the pharmacological induction of NRF2 with RA839, tBHQ, or by thiol antioxidants. The expression of key components of the LCA-elicited cytostatic pathway (iNOS and 4HNE) gradually decreased as the breast cancer stage advanced. The level of lipid peroxidation in tumors negatively correlated with the mitotic index. The overexpression of iNOS, nNOS, CAR, KEAP1, NOX4, and TGR5 or the downregulation of NRF2 correlated with better survival in breast cancer patients, except for triple negative cases. Taken together, LCA, a metabolite of the gut microbiome, elicits oxidative stress that slows down the proliferation of breast cancer cells. The LCA-oxidative stress protective pathway is lost as breast cancer progresses, and the loss correlates with poor prognosis.

Hyaluronan-CD44 axis orchestrates cancer stem cell functions

The prominent role of CD44 in tumor cell signaling together with its establishment as a cancer stem cell (CSC) marker for various tumor entities imply a key role for CD44 in CSC functional properties. Hyaluronan, the main ligand of CD44, is a major constituent of CSC niche and, therefore, the hyaluronan-CD44 signaling axis is of functional importance in this special microenvironment. This review aims to provide recent advances in the importance of hyaluronan-CD44 interactions in the acquisition and maintenance of a CSC phenotype. Hyaluronan-CD44 axis has a substantial impact on stemness properties of CSCs and drug resistance through induction of EMT program, oxidative stress resistance, secretion of extracellular vesicles/exosomes and epigenetic control. Potential therapeutic approaches targeting CSCs based on the hyaluronan-CD44 axis are also presented.

Osthole promotes the suppressive effects of cisplatin on NRF2 expression to prevent drug-resistant cervical cancer progression

Cervical cancer is one of the most commonly diagnosed lethal malignancies among gynecological malignant tumors worldwide. Chemo-resistance is one of the key causal factors in cervical cancer death. Osthole (OST), a natural compound, exhibits various pharmacological activities, including anti-tumor effects. However, its involvement in the chemoresistance of human cervical cancer has not been reported. In the study, we aimed to clarify the role of OST in regulating the chemoresistance of human cervical cancer. The results indicated that cisplatin (CDDP) combined with OST markedly reduced the cell proliferation and induced cervical cancer cells undergoing apoptosis when compared to CDDP alone treatment. In CDDP-resistant cervical cancer cells, OST significantly decreased nuclear factor, erythroid 2 like 2 (NRF2), heme oxygenase-1 (HO-1), NAD(P)H quinone dehydrogenase 1 (NQO1) and glutamate-cysteine ligase catalytic subunit (GCLC) expression levels from mRNA or protein levels. Additionally, through combination with CDDP, OST dose- and time-dependently reduced NRF2 expression in CDDP-resistant cervical cancer cells. Moreover, we found that CDDP co-treated with OST significantly blocked phosphatidylinositol-3 kinase (PI3K)/AKT signaling pathway. Importantly, CDDP combined with LY294002, inhibitor of phosphoinositide 3-kinase (PI3K)/AKT serine/threonine kinase (AKT) signaling, markedly decreased the expression of NRF2, HO-1, NQO1 and GCLC in drug-resistant cervical cancer cells. The in vivo study also suggested that OST in combination obviously reduced tumor growth in comparison to the CDDP alone group. Taken together, these findings indicated that OST could be used as a potential sensitizer to reverse chemoresistance of cisplatin-resistant cervical cancer to cisplatin through repressing NRF2 expression partly associated with PI3K/AKT blockage.

Curcumin induces p53-independent inactivation of Nrf2 during oxidative stress-induced apoptosis

The transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2) is a master regulator of a battery of antioxidant and detoxificant genes with cytoprotective function. Since Nrf2 inactivation is necessary for the complete execution of apoptosis in the presence of extensive cellular damage caused by oxidative stress, constant activation of Nrf2 may protect tumoral cells from apoptosis. The tumor suppressor gene p53 has been suggested to participate in apoptosis-related repression of Nrf2. Thus, we studied the inactivation of Nrf2 during oxidant-induced apoptosis in a p53 dysfunctional cellular model. Using curcumin dose-response assay and time-response assay in an immortalized lymphoblastoid cell line (control line 45), we observed a time-dependent increase in apoptotic markers such as deoxyribonucleic acid (DNA) fragmentation, phosphatidylserine exposure, and caspase-3, caspase-9 and poly (ADP-ribose) polymerases (PARP) cleavage. Interestingly, at early times of exposure to a proapoptotic dose of curcumin (15 μM), we observed nuclear accumulation of Nrf2 and the expression of Nrf2 target genes, whereas at late exposure times we found a reduction of total and nuclear protein levels of Nrf2 as well as downregulation of Nrf2 target genes in the absence of p53 activation. These data suggest that apoptosis-related inactivation of Nrf2 could occur in a p53 dysfunctional background, opening the possible occurrence of p53-independent mechanism to explain Nrf2 inactivation during apoptosis.

Hyaluronan-Stabilized Redox-Sensitive Nanoassembly for Chemo-Gene Therapy and Dual T1/T2 MR Imaging in Drug-Resistant Breast Cancer Cells

Tailoring combinatorial therapies along with real-time monitoring strategies has been the major focus of overcoming multidrug resistance in cancer. However, attempting to develop a multifunctional nanoplatform in a single construct leads to compromising therapeutic outcomes. Herein, we developed a simple, theranostic nanoassembly containing a hyaluronic acid-stabilized redox-sensitive (HART) polyethylenimine polyplex composed of a doxorubicin (DOX) intercalated Bcl-2 shRNA encoded plasmid along with a green-synthesized hausmannite (Mn₃O₄) and hematite (Fe₃O₄) nanoparticle (GMF). The highly stable HART nanoassembly has enhanced CD44-mediated intracellular uptake along with hyaluronidase (hylase) and redox-responsive drug-gene release. With Bcl-2 gene silencing induced by the successful delivery of HART in multidrug-resistant MCF7 breast cancer cells, the synergistic cytotoxic effect of Bcl-2 silencing and DOX was achieved. In addition, the HART nanoassembly containing GMF exhibited excellent dual MRI contrast (T1/T2) by reducing artifact signals. Overall, the HART nanoassembly with its enhanced theranostic properties has the potential to improve the therapeutic efficacy in future preclinical and clinical trials.

Potential Applications of NRF2 Inhibitors in Cancer Therapy

The NRF2/KEAP1 pathway represents one of the most important cell defense mechanisms against exogenous or endogenous stressors. Indeed, by increasing the expression of several cytoprotective genes, the transcription factor NRF2 can shelter cells and tissues from multiple sources of damage including xenobiotic, electrophilic, metabolic, and oxidative stress. Importantly, the aberrant activation or accumulation of NRF2, a common event in many tumors, confers a selective advantage to cancer cells and is associated to malignant progression, therapy resistance, and poor prognosis. Hence, in the last years, NRF2 has emerged as a promising target in cancer treatment and many efforts have been made to identify therapeutic strategies aimed at disrupting its prooncogenic role. By summarizing the results from past and recent studies, in this review, we provide an overview concerning the NRF2/KEAP1 pathway, its biological impact in solid and hematologic malignancies, and the molecular mechanisms causing NRF2 hyperactivation in cancer cells. Finally, we also describe some of the most promising therapeutic approaches that have been successfully employed to counteract NRF2 activity in tumors, with a particular emphasis on the development of natural compounds and the adoption of drug repurposing strategies.