ABCG2: the key to chemoresistance in cancer stem cells?

Natural products for combating multidrug resistance in cancer

Cancer cells frequently develop resistance to chemotherapeutic therapies and targeted drugs, which has been a significant challenge in cancer management. With the growing advances in technologies in isolation and identification of natural products, the potential of natural products in combating cancer multidrug resistance has received substantial attention. Importantly, natural products can impact multiple targets, which can be valuable in overcoming drug resistance from different perspectives. In the current review, we will describe the well-established mechanisms underlying multidrug resistance, and introduce natural products that could target these multidrug resistant mechanisms. Specifically, we will discuss natural compounds such as curcumin, resveratrol, baicalein, chrysin and more, and their potential roles in combating multidrug resistance. This review article aims to provide a systematic summary of recent advances of natural products in combating cancer drug resistance, and will provide rationales for novel drug discovery.

Lycium barbarum polysaccharide reverses drug resistance in oxaliplatin-resistant colon cancer cells by inhibiting PI3K/AKT-dependent phosphomannose isomerase

Objective: Here, we aimed to explore the effect of LBP in combination with Oxaliplatin (OXA) on reversing drug resistance in colon cancer cells through in vitro and in vivo experiments. We also aimed to explore the possible mechanism underlying this effect. Finally, we aimed to determine potential targets of Lycium barbarum polysaccharide (LBP) in colon cancer (CC) through network pharmacology and molecular docking. Methods: The invasion ability of colon cancer cells was assessed using the invasion assay. The migration ability of these cells was assessed using the migration assay and wound healing assay. Cell cycle analysis was carried out using flow cytometry. The expression levels of phosphomannose isomerase (PMI) and ATP-binding cassette transport protein of G2 (ABCG2) proteins were determined using immunofluorescence and western blotting. The expression levels of phosphatidylinositol3-kinase (PI3K), protein kinase B (AKT), B-cell lymphoma 2 (Bcl-2), and BCL2-Associated X (Bax) were determined using western blotting. Forty BALB/c nude mice purchased from Weitong Lihua, Beijing, for the in vivo analyses. The mice were randomly divided into eight groups. They were administered HCT116 and HCT116-OXR cells to prepare colon cancer xenograft models and then treated with PBS, LBP (50 mg/kg), OXA (10 mg/kg), or LBP + OXA (50 mg/kg + 10 mg/kg). The tumor weight and volume of treated model mice were measured, and organ toxicity was evaluated using hematoxylin and eosin staining. The expression levels of PMI, ABCG2, PI3K, and AKT proteins were then assessed using immunohistochemistry. Moreover, PMI and ABCG2 expression levels were analyzed using immunofluorescence and western blotting. The active components and possible targets of LBP in colon cancer were explored using in silico analysis. GeneCards was used to identify CC targets, and an online Venn analysis tool was used to determine intersection targets between these and LBP active components. The PPI network for intersection target protein interactions and the PPI network for interactions between the intersection target proteins and PMI was built using STRING and Cytoscape. To obtain putative targets of LBP in CC, we performed GO function enrichment and KEGG pathway enrichment analyses. Results: Compared with the HCT116-OXR blank treatment group, both invasion and migration abilities of HCT116-OXR cells were inhibited in the LBP + OXA (2.5 mg/mL LBP, 10 μΜ OXA) group (p < 0.05). Cells in the LBP + OXA (2.5 mg/mL LBP, 10 μΜ OXA) group were found to arrest in the G1 phase of the cell cycle. Knockdown of PMI was found to downregulate PI3K, AKT, and Bcl-2 (p < 0.05), while it was found to upregulate Bax (p < 0.05). After treatment with L. barbarum polysaccharide, 40 colon cancer subcutaneous tumor models showed a decrease in tumor size. There was no difference in the liver index after LBP treatment (p > 0.05). However, the spleen index decreased in the OXA and LBP + OXA groups (p < 0.05), possibly as a side effect of oxaliplatin. Immunohistochemistry, immunofluorescence, and western blotting showed that LBP + OXA treatment decreased PMI and ABCG2 expression levels (p < 0.05). Moreover, immunohistochemistry showed that LBP + OXA treatment decreased the expression levels of PI3K and AKT (p < 0.05). Network pharmacology analysis revealed 45 active LBP components, including carotenoids, phenylpropanoids, quercetin, xanthophylls, and other polyphenols. It also revealed 146 therapeutic targets of LBP, including AKT, SRC, EGFR, HRAS, STAT3, and MAPK3. KEGG pathway enrichment analysis showed that the LBP target proteins were enriched in pathways, including cancer-related signaling pathways, PI3K/AKT signaling pathway, and IL-17 signaling pathways. Finally, molecular docking experiments revealed that the active LBP components bind well with ABCG2 and PMI. conclusion: Our in vitro experiments showed that PMI knockdown downregulated PI3K, AKT, and Bcl-2 and upregulated Bax. This finding confirms that PMI plays a role in drug resistance by regulating the PI3K/AKT pathway and lays a foundation to study the mechanism underlying the reversal of colon cancer cell drug resistance by the combination of LBP and OXA. Our in vivo experiments showed that LBP combined with oxaliplatin could inhibit tumor growth. LBP showed no hepatic or splenic toxicity. LBP combined with oxaliplatin could downregulate the expression levels of PMI, ABCG2, PI3K, and AKT; it may thus have positive significance for the treatment of advanced metastatic colon cancer. Our network pharmacology analysis revealed the core targets of LBP in the treatment of CC as well as the pathways they are enriched in. It further verified the results of our in vitro and in vivo experiments, showing the involvement of multi-component, multi-target, and multi-pathway synergism in the drug-reversing effect of LBP in CC. Overall, the findings of the present study provide new avenues for the future clinical treatment of CC.

Drug repurposing: A novel strategy to target cancer stem cells and therapeutic resistance

Chemotherapy is an effortless and frequently used approach in cancer therapy. However, in most cases, it can only prolong life expectancy and does not guarantee a complete cure. Furthermore, chemotherapy is associated with severe adverse effects, one of the major complications of effective cancer therapy. In addition, newly published research outputs show that cancer stem cells are involved in cancer disease progression, drug resistance, metastasis, and recurrence and that they are functional in the trans-differentiation capacity of cancer stem cells to cancer cells in response to treatments. Novel strategies are therefore required for better management of cancer therapy. The prime approach would be to synthesize and develop novel drugs that need extensive resources, time, and endurance to be brought into therapeutic use. The subsequent approach would be to screen the anti-cancer activity of available non-cancerous drugs. This concept of repurposing non-cancer drugs as an alternative to current cancer therapy has become popular in recent years because using existing anticancer drugs has several adverse effects. Micronutrients have also been investigated for cancer therapy due to their significant anti-cancer effects with negligible or no side effects and availability in food sources. In this paper, we discuss an ideal hypothesis for screening available non-cancerous drugs with anticancer activity, with a focus on cancer stem cells and their clinical application for cancer treatment. Further, drug repurposing and the combination of micronutrients that can target both cancers and cancer stem cells may result in a better therapeutic approach leading to maximum tumor growth control.

Self-Renewal Inhibition in Breast Cancer Stem Cells: Moonlight Role of PEDF in Breast Cancer

Breast cancer is the leading cause of death among females in developed countries. Although the implementation of screening tests and the development of new therapies have increased the probability of remission, relapse rates remain high. Numerous studies have indicated the connection between cancer-initiating cells and slow cellular cycle cells, identified by their capacity to retain long labeling (LT+). In this study, we perform new assays showing how stem cell self-renewal modulating proteins, such as PEDF, can modify the properties, percentage of biomarker-expressing cells, and carcinogenicity of cancer stem cells. The PEDF signaling pathway could be a useful tool for controlling cancer stem cells' self-renewal and therefore control patient relapse, as PEDF enhances resistance in breast cancer patient cells' in vitro culture. We have designed a peptide consisting of the C-terminal part of this protein, which acts by blocking endogenous PEDF in cell culture assays. We demonstrate that it is possible to interfere with the self-renewal capacity of cancer stem cells, induce anoikis in vivo, and reduce resistance against docetaxel treatment in cancer patient cells in in vitro culture. We have also demonstrated that this modified PEDF protein produces a significant decrease in the percentage of expressed cancer stem cell markers.

Extirpating the cancer stem cell hydra: Differentiation therapy and Hyperthermia therapy for targeting the cancer stem cell hierarchy

Ever since the discovery of cancer stem cells (CSCs), they have progressively attracted more attention as a therapeutic target. Like the mythical hydra, this subpopulation of cells seems to contribute to cancer immortality, spawning more cells each time that some components of the cancer cell hierarchy are destroyed. Traditional modalities focusing on cancer treatment have emphasized apoptosis as a route to eliminate the tumor burden. A major problem is that cancer cells are often in varying degrees of dedifferentiation contributing to what is known as the CSCs hierarchy and cells which are known to be resistant to conventional therapy. Differentiation therapy is an experimental therapeutic modality aimed at the conversion of malignant phenotype to a more benign one. Hyperthermia therapy (HT) is a modality exploiting the changes induced in cells by the application of heat produced to aid in cancer therapy. While differentiation therapy has been successfully employed in the treatment of acute myeloid leukemia, it has not been hugely successful for other cancer types. Mounting evidence suggests that hyperthermia therapy may greatly augment the effects of differentiation therapy while simultaneously overcoming many of the hard-to-treat facets of recurrent tumors. This review summarizes the progress made so far in integrating hyperthermia therapy with existing modules of differentiation therapy. The focus is on studies related to the successful application of both hyperthermia and differentiation therapy when used alone or in conjunction for hard-to-treat cancer cell niche with emphasis on combined approaches to target the CSCs hierarchy.

Natural killer cell therapy targeting cancer stem cells: Old wine in a new bottle

A small proportion of cancer cells that have stem cell-like properties are known as cancer stem cells (CSCs). They can be used to identify malignant tumor phenotypes and patients with poor prognosis. Targeting these cells has been shown to improve the effectiveness of cancer therapies. Owing to the nature of CSCs, they are resistant to conventional treatment methods such as radio- and chemotherapy. Therefore, more effective anti-CSC therapies are required. Immunotherapy, including natural killer (NK) and T cell therapy, has demonstrated the ability to eliminate CSCs. NK cells have demonstrated superior anti-CSC capabilities compared to T cells in recognizing low levels of major histocompatibility complex (MHC) class I expression. However, CSC escape also occurs during NK cell therapy. It is important to determine CSC-specific immune evasion mechanisms and find out potential solutions to optimize NK cell function. Therefore, this review discusses promising strategies that can improve the efficiency of NK cell therapy in treating CSCs, and aims to provide a reference for future research.

Cancer stem cells: an insight into the development of metastatic tumors and therapy resistance

The term "cancer stem cells" (CSCs) refers to cancer cells that exhibit traits parallel to normal stem cells, namely the potential to give rise to every type of cell identified in a tumor microenvironment. It has been found that CSCs usually develops from other neoplastic cells or non-cancerous somatic cells by acquiring stemness and malignant characteristics through particular genetic modifications. A trivial number of CSCs, identified in solid and liquid cancer, can give rise to an entire tumor population with aggressive anticancer drug resistance, metastasis, and invasiveness. Besides, cancer stem cells manipulate their intrinsic and extrinsic features, regulate the metabolic pattern of the cell, adjust efflux-influx efficiency, modulate different signaling pathways, block apoptotic signals, and cause genetic and epigenetic alterations to retain their pluripotency and ability of self-renewal. Notably, to keep the cancer stem cells' ability to become malignant cells, mesenchymal stem cells, tumor-associated fibroblasts, immune cells, etc., interact with one another. Furthermore, CSCs are characterized by the expression of particular molecular markers that carry significant diagnostic and prognostic significance. Because of this, scientific research on CSCs is becoming increasingly imperative, intending to understand the traits and behavior of cancer stem cells and create more potent anticancer therapeutics to fight cancer at the CSC level. In this review, we aimed to elucidate the critical role of CSCs in the onset and spread of cancer and the characteristics of CSCs that promote severe resistance to targeted therapy.

ABC transporters affects tumor immune microenvironment to regulate cancer immunotherapy and multidrug resistance

Multidrug resistance (MDR) is the phenomenon in which cancer cells simultaneously develop resistance to a broad spectrum of structurally and mechanistically unrelated drugs. MDR severely hinders the effective treatment of cancer and is the major cause of chemotherapy failure. ATP-binding cassette (ABC) transporters are extensively expressed in various body tissues, and actively transport endogenous and exogenous substrates through biological membranes. Overexpression of ABC transporters is frequently observed in MDR cancer cells, which promotes efflux of chemotherapeutic drugs and reduces their intracellular accumulation. Increasing evidence suggests that ABC transporters regulate tumor immune microenvironment (TIME) by transporting various cytokines, thus controlling anti-tumor immunity and sensitivity to anticancer drugs. On the other hand, the expression of various ABC transporters is regulated by cytokines and other immune signaling molecules. Targeted inhibition of ABC transporter expression or function can enhance the efficacy of immune checkpoint inhibitors by promoting anticancer immune microenvironment. This review provides an update on the recent research progress in this field.

A Pan-Cancer Landscape of ABCG2 across Human Cancers: Friend or Foe?

Emerging evidence from research or clinical studies reported that ABCG2 (ATP-binding cassette sub-family G member 2) interrelates with multidrug resistance (MDR) development in cancers. However, no comprehensive pan-cancer analysis is available at present. Therefore, we explore multiple databases, such as TCGA to investigate the potential therapeutic roles of ABCG2 across 33 different tumors. ABCG2 is expressed on a lower level in most cancers and shows a protective effect. For example, a lower expression level of ABCG2 was detrimental to the survival of adrenocortical carcinoma (TCGA-ACC), glioblastoma multiforme (GBM), and kidney renal clear cell carcinoma (KIRC) patients. Distinct associations exist between ABCG2 expression and stemness scores, microenvironmental scores, microsatellite instability (MSI), and tumor mutational burden (TMB) of tumor patients. We observed a significant positive correlation between the ABCG2 mutation site and prognosis in uterine corpus endometrial carcinoma (UCEC) patients. Moreover, transmembrane transporter activity and hormone biosynthetic-associated functions were found to be involved in the functionality of ABCG2 and its related genes. The cDNAs of cancer cell lines were collected to detect exon mutation sequences and to analyze ABCG2 mRNA expression. The mRNA expression level of ABCG2 showed a significant difference among spheres and drug-resistant cancer cell lines compared with their corresponding adherent cancer cell lines in six types of cancer. This pan-cancer study provides, for the first time, a comprehensive understanding of the multifunctionality of ABCG2 and unveils further details of the potential therapeutic role of ABCG2 in pan-cancer.

Interaction of crown ethers with the ABCG2 transporter and their implication for multidrug resistance reversal

Overexpression of ABC transporters, such as ABCB1 and ABCG2, plays an important role in mediating multidrug resistance (MDR) in cancer. This feature is also attributed to a subpopulation of cancer stem cells (CSCs), having enhanced tumourigenic potential. ABCG2 is specifically associated with the CSC phenotype, making it a valuable target for eliminating aggressive and resistant cells. Several natural and synthetic ionophores have been discovered as CSC-selective drugs that may also have MDR-reversing ability, whereas their interaction with ABCG2 has not yet been explored. We previously reported the biological activities, including ABCB1 inhibition, of a group of adamantane-substituted diaza-18-crown-6 (DAC) compounds that possess ionophore capabilities. In this study, we investigated the mechanism of ABCG2-inhibitory activity of DAC compounds and the natural ionophores salinomycin, monensin and nigericin. We used a series of functional assays, including real-time microscopic analysis of ABCG2-mediated fluorescent substrate transport in cells, and docking studies to provide comparative aspects for the transporter-compound interactions and their role in restoring chemosensitivity. We found that natural ionophores did not inhibit ABCG2, suggesting that their CSC selectivity is likely mediated by other mechanisms. In contrast, DACs with amide linkage in the side arms demonstrated noteworthy ABCG2-inhibitory activity, with DAC-3Amide proving to be the most potent. This compound induced conformational changes of the transporter and likely binds to both Cavity 1 and the NBD-TMD interface. DAC-3Amide reversed ABCG2-mediated MDR in model cells, without affecting ABCG2 expression or localization. These results pave the way for the development of new crown ether compounds with improved ABCG2-inhibitory properties.

Natural Products Targeting Cancer Stem Cells for Augmenting Cancer Therapeutics

Cancer stem cells (CSC) have been identified in several types of solid tumors. In some cases, CSC may be the source of all the tumor cells, the cause of the tumor's resistance to chemotherapeutic agents, and the source of metastatic cells. Thus, a combination therapy targeting non-CSC tumor cells as well as specifically targeting CSCs holds the potential to be highly effective. Natural products (NPs) have been a historically rich source of biologically active compounds and are known for their ability to influence multiple signaling pathways simultaneously with negligible side effects. In this review, we discuss the potential of NPs in targeting multiple signaling pathways in CSC and their potential to augment the efficacy of standard cancer therapy. Specifically, we focus on the anti-CSC activities of flavonoids, FDA-approved drugs originating from natural sources. Additionally, we emphasize the potential of NPs in targeting microRNA-mediated signaling, given the roles of microRNA in the maintenance of the CSC phenotype.

Combination of multifunctional ursolic acid with kinase inhibitors for anti-cancer drug carrier vesicles

A sterically stabilized unilamellar nanocarrier vesicle (SSV) system containing dipalmitoylphosphatidylcholine, cholesterol, ursolic acid and PEGylated phospholipid has been developed by exploiting the structural advantages of ursolic acid: by spontaneously attaching to the lipid head groups, it induces curvature at the outer side of the bilayers, allowing the preparation of size-limited vesicles without extrusion. Ursolic acid (UA) also interacts with the PEG chains, supporting steric stabilization even when the amount of PEGylated phospholipid is reduced. Using fluorescence immunohistochemistry, vesicles containing ursolic acid (UA-SSVs) were found to accumulate in the tumor in 3 h on xenografted mouse, suggesting the potential use of these vesicles for passive tumor targeting. Further on, mono- and combination therapy with UA and six different kinase inhibitors (crizotinib, erlotinib, foretinib, gefitinib, refametinib, trametinib) was tested on seven cancer cell-lines. In most combinations synergism was observed, in the case of trametinib even at very low concentration (0.001 μM), which targets the MAPK pathway most often activated in human cancers. The coupled intercalation of UA and trametinib (2:1 molar ratio) into vesicles causes further structural advantageous molecular interactions, promoting the formation of small vesicles. The high drug:lipid molar ratio (~0.5) in the novel type of co-delivery vesicles enables their direct medical application, possibly also overcoming the multidrug resistance effect.

UCH-L1 and UCH-L3 regulate the cancer stem cell-like properties through PI3 K/Akt signaling pathway in prostate cancer cells

Castration-resistant prostate cancer (CRPC) is a highly aggressive and advanced prostate cancer that is currently incurable with conventional therapies. The recurrence and chemotherapy-resistant properties of CRPC are attributed to prostate cancer stem cells (CSCs). On the other hand, the factors regulating the prostate CSC-like properties have not been studied extensively. Previously, ubiquitin C-terminal hydrolase-L1 (UCH-L1) and ubiquitin C-terminal hydrolase-L3 (UCH-L3) were reported to be involved in prostate cancer cell progression through the epithelial-to-mesenchymal transition (EMT) regulation. Here, the differential regulation on the CSC-like properties by UCH-L1 and UCH-L3 were identified in prostate cancer cells. The CSC-like characteristics, such as the expression of pluripotency markers, chemoresistance, and sphere-forming ability, were promoted by UCH-L1, whereas those were repressed by UCH-L3. Moreover, the modulation of CSC-like properties by UCH-L1 and UCH-L3 was through the PI3 K/Akt signaling pathway. The CSC-like properties induced by UCH-L1 overexpression or UCH-L3 depletion were suppressed by the PI3 K/Akt pathway inhibitor. In conclusion, UCH-L1 and UCH-L3 are novel regulators of the CSC-like properties and shed light on new therapeutic strategies to overcome CSCs in prostate cancers.

Casein Kinase-1-Alpha Inhibitor (D4476) Sensitizes Microsatellite Instable Colorectal Cancer Cells to 5-Fluorouracil via Authophagy Flux Inhibition

Adjuvant chemotherapy with 5-fluorouracil (5-FU) does not improve survival of patients suffering from a form of colorectal cancer (CRC) characterized by high level of microsatellite instability (MSI-H). Given the importance of autophagy and multi-drug-resistant (MDR) proteins in chemotherapy resistance, as well as the role of casein kinase 1-alpha (CK1α) in the regulation of autophagy, we tested the combined effect of 5-FU and CK1α inhibitor (D4476) on HCT116 cells as a model of MSI-H colorectal cancer. To achieve this goal, the gene expression of Beclin1 and MDR genes, ABCG2 and ABCC3 were analyzed using quantitative real-time polymerase chain reaction. We used immunoblotting to measure autophagy flux (LC3, p62) and flow cytometry to detect apoptosis. Our findings showed that combination treatment with 5-FU and D4476 inhibited autophagy flux. Moreover, 5-FU and D4476 combination therapy induced G2, S and G1 phase arrests and it depleted mRNA of both cell proliferation-related genes and MDR-related genes (ABCG2, cyclin D1 and c-myc). Hence, our data indicates that targeting of CK1α may increase the sensitivity of HCT116 cells to 5-FU. To our knowledge, this is the first description of sensitization of CRC cells to 5-FU chemotherapy by CK1α inhibitor.

Clinical Significance and the Role of Guanylate-Binding Protein 5 in Oral Squamous Cell Carcinoma

Guanylate binding protein 5 (GBP5) is the interferon (IFN)-inducible subfamily of guanosine triphosphatases (GTPases) and is involved in pathogen defense. However, the role played by GBP5 in cancer development, especially in oral squamous cell carcinoma (OSCC), is still unknown. Herein, next-generation sequencing analysis showed that the gene expression levels of GBP5 were significantly higher in OSCC tissues compared with those found in corresponding tumor adjacent normal tissues (CTAN) from two pairs of OSCC patients. Higher gene expression levels of GBP5 were also found in tumor tissues of 23 buccal mucosal squamous cell carcinoma (BMSCC)/14 tongue squamous cell carcinoma (TSCC) patients and 30 oral cancer patients from The Cancer Genome Atlas (TCGA) database compared with those in CTAN tissues. Immunohistochemical results showed that protein expression levels of GBP5 were also higher in the tumor tissues of 353 OSCC patients including 117 BMSCC, 187 TSCC, and 49 lip squamous cell carcinoma patients. Moreover, TCGA database analysis indicated that high gene expression levels of GBP5 were associated with poor overall survival in oral cancer patients with moderate/poor cell differentiation, and associated with poor disease-free survival in oral cancer patients with moderate/poor cell differentiation and lymph node metastasis. Furthermore, GBP5-knockdowned cells exhibited decreased cell growth, arrest at G1 phase, and decreased invasion/migration. The gene expression of markers for epithelial-mesenchymal transition and cancer stemness was also reduced in GBP5-silenced oral cancer cells. Taken together, GBP5 might be a potential biomarker and therapeutic target for OSCC patients, especially for those with poor cell differentiation and lymph node metastasis.

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.

Long non-coding RNAs in cancer stem cells

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Long non coding RNAs are involved in the regulation of multiple cellular processes.

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Cancer stemness and escape from immunological anti-cancer mechanisms are important mechanisms of resistance to anti-cancer agents and are pivotal in controlling cancer development and metastases.

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Long non coding RNAs have deep effects on the immune-modulation and on the control of cancer stem cells.

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Several pathways involved in immunological escape and cancer stemness are modulated by long non coding RNAs.

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Targeting long non coding RNAs is a potential new strategy to control tumor development and metastases.

In recent years, it has been evidenced that the human transcriptome includes several types of non-coding RNAs (ncRNAs) that are mainly involved in the regulation of different cellular processes. Among ncRNAs, long-non-coding RNAs (lncRNAs) are defined as longer than 200 nucleotides and have been shown to be involved in several physiological and pathological events, including immune system regulation and cancer. Cancer stem cells (CSCs) are defined as a population of cancer cells that possess characteristics, such as resistance to standard treatments, cancer initiation, ability to undergo epithelial-to-mesenchymal transition, and the ability to invade, spread, and generate metastases. The cancer microenvironment, together with genetic and epigenetic factors, is fundamental for CSC maintenance and tumor growth and progression. Unsurprisingly, lncRNAs have been involved in both CSC biology and cancer progression, prognosis and recurrence. Here we review the most recent literature on IncRNAs involvement in CSC biology and function.

Regulation of ABCG2 expression by Wnt5a through FZD7 in human pancreatic cancer cells

ATP-binding cassette subfamily G member 2 (ABCG2), a member of the ABC transporter superfamily, has been implicated in the development of chemotherapeutic drug resistance in cancer cells. However, the regulators of ABCG2 expression and their roles in anticancer drug resistance have not been fully characterized, especially in the context of pancreatic cancer. The aim of the present study was to investigate whether ABCG2 contributed to drug resistance in pancreatic cancer and to elucidate its regulatory molecular pathways. Using immunohistochemical analysis of pancreatic ductal adenocarcinoma and adjacent healthy tissue samples, the present study identified a positive correlation between ABCG2 and Wnt5a, a member of the Wnt family of secreted proteins. It was also determined that treatment with recombinant human Wnt5a protein could upregulate the expression of ABCG2 in the Capan-2 human pancreatic cancer cell line and enhance its resistance to gemcitabine. The upregulation of ABCG2 by Wnt5a was inhibited by small interfering RNA silencing of Frizzled class receptor 7 (FZD7) or by FZD7 inhibitors. Moreover, both FZD7 silencing or inhibition of its function attenuated gemcitabine resistance induced by Wnt5a in Capan-2 cells. Therefore, the present findings suggested that Wnt5a and FZD7 acted as upstream regulators of ABCG2 expression and that FZD7 may be an essential factor for Wnt5a-induced gemcitabine resistance in pancreatic cancer cells.

ABCG2 Overexpression Contributes to Pevonedistat Resistance

MLN4924 (pevonedistat) is a first-in-class NEDD8-activating enzyme (NAE) inhibitor in clinical trials for the treatment of solid tumors and hematologic malignancies. Despite the promising activity of MLN4924 observed in early trials, drug resistance has been noted in some patients. Identifying the underlying cause of treatment failure may help to better stratify patients that are most likely to benefit from this novel agent. Early preclinical studies revealed that the development of NAE mutations promotes resistance to MLN4924. However, these mutations have not been detected in patients that are relapsed/refractory to MLN4924, suggesting that other mechanisms are driving clinical resistance. To better understand the potential mechanisms of MLN4924 resistance, we generated MLN4924-resistant ovarian cancer cells. Interestingly, these cells did not develop mutations in NAE. Transcriptome analyses revealed that one of the most upregulated genes in resistant cells was ABCG2. This result was validated by quantitative real-time PCR and immunoblotting. Importantly, the sensitivity of MLN4924-resistant cells was restored by lentiviral short hairpin RNA (shRNA) targeting ABCG2. Further investigation using ABCG2-overexpressing NCI-H460/MX20 cells determined that these cells are resistant to the anticancer effects of MLN4924 and can be sensitized by co-treatment with the ABCG2 inhibitors YHO-13351 and fumitremorgin C. Finally, HEK293 models with overexpression of wild-type ABCG2 (R482) and variants (R482G and R482T) all demonstrated significant resistance to MLN4924 compared to wild-type cells. Overall, these findings define an important molecular resistance mechanism to MLN4924 and demonstrate that ABCG2 may be a useful clinical biomarker that predicts resistance to MLN4924 treatment.

Ovarian cancer stem cell: A potential therapeutic target for overcoming multidrug resistance

The cancer stem cell (CSC) model encompasses an advantageous paradigm that in recent decades provides a better elucidation for many important biological aspects of cancer initiation, progression, metastasis, and, more important, development of multidrug resistance (MDR). Such several other hematological malignancies and solid tumors and the identification and isolation of ovarian cancer stem cells (OV-CSCs) show that ovarian cancer also follows this hierarchical model. Gaining a better insight into CSC-mediated resistance holds promise for improving current ovarian cancer therapies and prolonging the survival of recurrent ovarian cancer patients in the future. Therefore, in this review, we will discuss some important mechanisms by which CSCs can escape chemotherapy, and then review the recent and growing body of evidence that supports the contribution of CSCs to MDR in ovarian cancer.

An ABCG2 non-substrate anticancer agent FL118 targets drug-resistant cancer stem-like cells and overcomes treatment resistance of human pancreatic cancer

Background

Pancreatic cancer is a deadly disease with a very low 5-year patient survival rate of 6–8%. The major challenges of eliminating pancreatic cancer are treatment resistance and stromal barriers to optimal drug access within the tumor. Therefore, effective molecular targeting drugs with high intra-tumor access and retention are urgently needed for managing this devastating disease in the clinic.

Methods

This study has used the following in vitro and in vivo techniques for the investigation of exceptional anticancer drug FL118’s efficacy in treatment of resistant pancreatic cancer: cell culture; immunoblotting analysis to test protein expression; DNA sub-G1 flow cytometry analyses to test cell death; MTT assay to test cell viability; pancreatic cancer stem cell assays (fluorescence microscopy tracing; matrigel assay; CD44-positive cell colony formation assay); human luciferase-labeled pancreatic tumor orthotopic animal model in vivo imaging; pancreatic cancer patient-derived xenograft (PDX) animal models; and toxicology studies with immune-competent BALB/cj mice and beagle dogs.

Results

Our studies found that FL118 alone preferentially killed cisplatin-resistant cancer cells, while a combination of FL118 with cisplatin synergistically killed resistant pancreatic cancer cells and reduced spheroid formation of treatment-resistant pancreatic cancer stem-like cells. Furthermore, using in vivo-imaging, we found that FL118 in combination with cisplatin strongly inhibited both drug-resistant pancreatic xenograft tumor growth and metastasis. In PDX model, we demonstrated that FL118 alone effectively eliminated PDX tumors, while FL118 in combination with gemcitabine eliminated PDX tumors that showed relative resistance (less sensitivity) to treatment with FL118. These FL118 efficacy results are consistent with our molecular-targeting data showing that FL118 inhibited the expression of multiple antiapoptotic proteins (survivin, Mcl-1, XIAP, cIAP2) and ERCC6, a critical regulator of DNA repair, in treatment-resistant pancreatic stem-like cancer cells. Furthermore, FL118 toxicity studies in BALB/cj mice and beagle dogs indicated that FL118 exhibits favorable hematopoietic and biochemical toxicities.

Conclusion

Together, our studies suggest that FL118 is a promising anticancer drug for further clinical development to effectively treat drug-resistant pancreatic cancer alone or in combination with other pancreatic cancer chemotherapeutic drugs.

Inhibition of pancreatic cancer stem cells by Rauwolfia vomitoria extract

The poor treatment outcomes of pancreatic cancer are linked to an enrichment of cancer stem cells (CSCs) in these tumors, which are resistant to chemotherapy and promote metastasis and tumor recurrence. The present study investigated an extract from the root of the medicinal plant Rauwolfia vomitoria (Rau) for its activity against pancreatic CSCs. In vitro tumor spheroid formation and CSC markers were tested, and in vivo tumorigenicity was evaluated in nude mice. Rau inhibited the overall proliferation of human pancreatic cancer cell lines with a 50% inhibitory concentration (IC₅₀) ranging between 125 and 325 µg/ml, and showed limited cytotoxicity towards normal epithelial cells. The pancreatic CSC population, identified using cell surface markers or a tumor spheroid formation assay, was significantly reduced, with an IC₅₀ value of ~100 µg/ml treatment for 48 h and ~27 µg/ml for long-term tumor spheroid formation. The levels of CSC-related gene Nanog and nuclear β-catenin were decreased, suggesting suppression of the Wnt/β-catenin signaling pathway. In vivo, 20 mg/kg of Rau administered five times per week by oral gavage significantly reduced the tumorigenicity of PANC-1 cells in immunocompromised mice. Taken together, these data showed that Rau preferentially inhibited pancreatic cancer stem cells. Further investigation is warranted to examine the potential of Rau as a novel treatment for pancreatic cancer.

Drug resistance of bladder cancer cells through activation of ABCG2 by FOXM1

Recurrence is a serious problem in patients with bladder cancer. The hypothesis for recurrence was that the proliferation of drug-resistant cells was reported, and this study focused on drug resistance due to drug efflux. Previous studies have identified FOXM1 as the key gene for recurrence. We found that FOXM1 inhibition decreased drug efflux activity and increased sensitivity to Doxorubicin. Therefore, we examined whether the expression of ABC transporter gene related to drug efflux is regulated by FOXM1. As a result, ABCG2, one of the genes involved in drug efflux, has been identified as a new target for FOXM1. We also demonstrated direct transcriptional regulation of ABCG2 by FOXM1 using ChIP assay. Consequently, in the presence of the drug, FOXM1 is proposed to directly activate ABCG2 to increase the drug efflux activation and drug resistance, thereby involving chemoresistance of bladder cancer cells. Therefore, we suggest that FOXM1 and ABCG2 may be useful targets and important parameters in the treatment of bladder cancer. [BMB Reports 2018; 51(2): 98-103].

Genotoxic stress induces Sca-1-expressing metastatic mammary cancer cells

We describe a cell damage-induced phenotype in mammary carcinoma cells involving acquisition of enhanced migratory and metastatic properties. Induction of this state by radiation required increased activity of the Ptgs2 gene product cyclooxygenase 2 (Cox2), secretion of its bioactive lipid product prostaglandin E2 (PGE2), and the activity of the PGE2 receptor EP4. Although largely transient, decaying to low levels in a few days to a week, this phenotype was cumulative with damage and levels of cell markers Sca-1 and ALDH1 increased with treatment dose. The Sca-1+ , metastatic phenotype was inhibited by both Cox2 inhibitors and PGE2 receptor antagonists, suggesting novel approaches to radiosensitization.

Extract of the Medicinal Plant Pao Pereira Inhibits Pancreatic Cancer Stem-Like Cell In Vitro and In Vivo

Pancreatic cancers are enriched with cancer stem-like cells (CSCs), which are resistant to chemotherapies, and responsible for tumor metastasis and recurrence. Here, we investigated the extract of a medicinal plant Pao Pereira (Pao) for its activity against pancreatic CSCs. Pao inhibited overall proliferation of human pancreatic cancer cell lines with IC₅₀ ranging from 125 to 325 μg/mL and had limited cytotoxicity to normal epithelial cells. Pancreatic CSC population, identified using surface markers CD24+ CD44+ EpCam+ or tumor spheroid formation assay, was significantly reduced, with IC₅₀s of ~100 μg/mL for 48 hours treatment, and ~27 μg/mL for long-term treatment. Nuclear β-catenin levels were decreased, suggesting suppression of Wnt/β-catenin signaling pathway. In vivo, Pao at 20 mg/kg, 5 times/week gavage, significantly reduced tumorigenicity of PANC-1 cells in immunocompromised mice, indicating inhibition of CSCs in vivo. Further investigation is warranted in using Pao as a novel treatment targeting pancreatic CSCs.

Salvianolic acid B reverses multidrug resistance in nude mice bearing human colon cancer stem cells

Salvianolic acid B (SalB) is a water-soluble phenolic compound, extractable from Salvia miltiorrhiza, and has previously been demonstrated to reverse tumor multidrug resistance (MDR) in colon cancer cells. Cancer stem cells (CSCs) are closely associated with drug resistance. Therefore, establishing a nude mouse model bearing human colon CSCs is important for the study of the mechanisms underlying colon cancer drug resistance as well as the reversal of drug resistance. The present study aimed to establish a nude mouse model bearing human colon CSCs and to investigate the effects of SalB on the drug resistance exhibited by the nude mouse model as well as determine its underlying mechanism. Cells from two colon cancer cell lines (LoVo and HCT-116) were cultured in serum-free medium to obtain CSCs-enriched spheroid cells. Following this, nude mice were transplanted with LoVo and HCT-116 colon CSCs to establish the CSC nude mouse model, which was subsequently demonstrated to exhibit MDR. The results of the present study revealed that following treatment with SalB, the chemotherapeutic drug resistance of xenografts was reversed to a certain extent. Western blot analysis was performed to investigate the expression levels of cluster of differentiation (CD)44, CD133, transcription factor sox-2 (SOX2) and ATP-binding cassette sub-family G member 2 (ABCG2) proteins, and the results demonstrated that treatment with SalB downregulated the expression of CD44, SOX2 and ABCG2 proteins in both LoVo and HCT-116 colon CSCs xenografts. In conclusion, the results of the present study revealed that a serum-free suspension method can be performed to successfully isolate colon CSCs. In addition, a nude mice bearing colon CSCs animal model was successfully established, and associated tumors were confirmed to exhibit MDR. Furthermore, SalB was demonstrated to successfully reverse MDR in nude mice bearing LoVo and HCT-116 colon CSCs, as well as suppress the expression of CD44, SOX2 and ABCG2 proteins.

Cancer stem cells as key drivers of tumour progression

BACKGROUND

Cancer stem cells (CSCs) are subpopulations of cancer cells sharing similar characteristics as normal stem or progenitor cells such as self-renewal ability and multi-lineage differentiation to drive tumour growth and heterogeneity. Throughout the cancer progression, CSC can further be induced from differentiated cancer cells via the adaptation and cross-talks with the tumour microenvironment as well as a response from therapeutic pressures, therefore contributes to their heterogeneous phenotypes. Challengingly, conventional cancer treatments target the bulk of the tumour and are unable to target CSCs due to their highly resistance nature, leading to metastasis and tumour recurrence.

MAIN BODY

This review highlights the roles of CSCs in tumour initiation, progression and metastasis with a focus on the cellular and molecular regulators that influence their phenotypical changes and behaviours in the different stages of cancer progression. We delineate the cross-talks between CSCs with the tumour microenvironment that support their intrinsic properties including survival, stemness, quiescence and their cellular and molecular adaptation in response to therapeutic pressure. An insight into the distinct roles of CSCs in promoting angiogenesis and metastasis has been captured based on in vitro and in vivo evidences.

CONCLUSION

Given dynamic cellular events along the cancer progression and contributions of resistance nature by CSCs, understanding their molecular and cellular regulatory mechanism in a heterogeneous nature, provides significant cornerstone for the development of CSC-specific therapeutics.

Ginsenoside-Rb1 targets chemotherapy-resistant ovarian cancer stem cells via simultaneous inhibition of Wnt/β-catenin signaling and epithelial-to-mesenchymal transition

Chemoresistance is a major clinical problem compromising the successful treatment of cancer. One exciting approach is the eradication of cancer stem/tumor-initiating cells (jointly CSCs), which account for tumor initiation, progression, and drug resistance. Here we show for the first time, with mechanism-based evidence, that ginsenoside-Rb1, a natural saponin isolated from the rhizome of Panax quinquefolius and notoginseng, exhibits potent cytotoxicity on CSCs. Rb1 and its metabolite compound K could effectively suppress CSC self-renewal without regrowth. Rb1 and compound K treatment also sensitized the CSCs to clinically relevant doses of cisplatin and paclitaxel. These effects were associated with the Wnt/β-catenin signaling pathway by downregulating β-catenin/T-cell factor-dependent transcription and expression of its target genes ATP-binding cassette G2 and P-glycoprotein. We also identified reversal of epithelial-to-mesenchymal transition as a new player in the Rb1 and compound K-mediated inhibition of CSCs. Rb1 and compound K treatment also inhibited the self-renewal of CSCs derived from ovarian carcinoma patients as well as in xenograft tumor model. Moreover, we did not observe toxicity in response to doses of Rb1 and compound K that produced an anti-CSC effect. Therefore, Rb1 should be explored further as a promising nutraceutical prototype of treating refractory tumors.

Concise Review: Resistance to Tyrosine Kinase Inhibitors in Non-Small Cell Lung Cancer: The Role of Cancer Stem Cells

Among the potential mechanisms involved in resistance to tyrosine kinase inhibitors (TKIs) in non-small cell lung cancer, the manifestation of stem-like properties in cancer cells seems to have a crucial role. Alterations involved in the development of TKI resistance may be acquired in a very early phase of tumorigenesis, supporting the hypothesis that these aberrations may be present in cancer stem cells (CSCs). In this regard, the characterization of tumor subclones in the initial phase and the identification of the CSCs may be helpful in planning a specific treatment to target selected biomarkers, suppress tumor growth, and prevent drug resistance. The aim of this review is to elucidate the role of CSCs in the development of resistance to TKIs and its implication for the management of patients. Stem Cells 2018;36:633-640.

Impact of Breast Cancer Resistance Protein Expression on the In Vitro Efficacy of Anticancer Drugs in Pancreatic Cancer Cell Lines

Breast cancer resistance protein (BCRP) overexpression confers multidrug resistance to cancer cells, and the efficacy of anticancer drugs has been reported to be significantly affected by BCRP in cell lines transfected with BCRP or selected with drugs. It is unclear whether the in vitro efficacy of anticancer drugs is affected by endogenous BCRP, although cancer cell line panels consisting of defined tumor cell lines with endogenous BCRP have been used to screen for anticancer drugs in the pharmaceutical industry. We assessed the impact of BCRP expression on efficacy of anticancer drugs using pancreatic cancer cell lines expressing varying levels of endogenous BCRP. Pancreatic cancer cell lines were selected from the Cancer Cell Line Encyclopedia (CCLE). The EC₅₀ of 7-ethyl-10-hydroxycamptothecin (SN-38), topotecan, and mitoxantrone decreased in the presence of a BCRP inhibitor in PANC-1 and AsPC-1 cells, which exhibit high BCRP expression. However, no significant alterations in EC₅₀ were observed in HPAF-II, SW 1990, and MIA PaCa-2, which show moderate or low BCRP expression. The shift of EC₅₀ of anticancer drugs with and without a BCRP inhibitor increased with an increase of BCRP mRNA expression levels; however, the shift was obvious only in cells highly expressing BCRP. Thus, the in vitro efficacy of anticancer drugs on cell proliferation may be minimally affected by BCRP in most pancreatic cancer cell lines, considering that 72% of pancreatic cancer cell lines in CCLE show moderate or low BCRP expression. The effect of BCRP should be carefully evaluated in pancreatic cell lines that highly express BCRP.

SN38-PEG-PLGA-verapamil nanoparticles inhibit proliferation and downregulate drug transporter ABCG2 gene expression in colorectal cancer cells

Nowadays, nanoparticle-based drug delivery systems are recognized to reduce the therapeutic side effects. One of the common problems in cancer treatment is cancer drug resistance, resulting from the over-expression of one energy-dependent transporter that enhances drug efflux. Irinotecan is used for metastatic colorectal cancer. The involvement of ABCG2 transporter in irinotecan resistance has been established. The current study was designed to characterize SN38-loaded pegylated (polyethylene glycol) PLGA [poly(lactic-co-glycolic acid)]-verapamil nanoparticles (NPs), and to distinguish the cytotoxic effect of SN38-PEG-PLGA-Ver NPs and the ability of SN38-PEG-PLGA-Ver NPs to inhibit drug resistance through the inhibition of ABCG2 expression. The surface morphology of nanoparticles was determined by scanning electron microscopy. The drug cytotoxicity of SN38-PEG-PLGA-verapamil nanoparticles was measured by MTT assay with desired concentrations and SN38-PEG-PLGA-Ver at different incubation times. Real-time PCR was used to determine the mRNA level of ABCG2, BAX, and BCL2. The cellular uptake assay was performed to show the cellular uptake of nanoparticles. The size of NPs used in this study was about 179 nm with surface charge of -17.1 mV. MTT assay results showed that 1 μmol/L of free drug and 3 μmol/L of NPs could reduce HT29 cells by half (IC50) after 48 and 96 h, respectively. An increase in expression of BAX and a decrease in expression of ABCG2 were observed according to the real-time PCR. No significant change was detected in expression of BCL2. In conclusion, sufficient uptake of SN38-PEG-PLGA-Ver NPs and a significant decrease in expression of ABCG2 and an increase in expression of BAX and BAX/BCL2 ratio was observed after treatment with nanoparticles compared with free SN38. These results reveal that SN38-PEG-PLGA-Ver NPs can be an effective therapeutic method in colon cancer treatments and also may prevent anticancer drug resistance.

NFE2L2/NRF2 silencing-inducible miR-206 targets c-MET/EGFR and suppresses BCRP/ABCG2 in cancer cells

The nuclear factor (erythroid-derived 2)-like 2 (NFE2L2/NRF2) plays a critical role in the expression of multiple antioxidant and detoxifying enzymes. Herein, we provide evidence of the molecular links between NRF2 and oncogenic signaling hepatocyte growth factor receptor (HGFR/c-MET) and epidermal growth factor receptor (EGFR). Interfering RNA-induced stable inhibition of NRF2 in ovarian carcinoma SKOV3 and renal carcinoma A498 reduced the levels of c-MET and EGFR. MicroRNA-206 (miR-206) that was increased in both NRF2-silenced cells was predicted as a dual regulator of c-MET and EGFR. As experimental evidence, miR-206 decreased c-MET and EGFR levels through a direct binding to the 3′-untranslated region of the c-MET and EGFR genes. The treatment of NRF2-knockdown cells with the miR-206 inhibitor could restore c-MET and EGFR levels. The miR-206-mediated c-MET/EGFR repression resulted in two outcomes. First, presumably through the inhibition of c-MET/EGFR-dependent cell proliferation, overexpression of miR-206 inhibited tumor growth in SKOV3-inoculated nude mice. Second, reduced c-MET/EGFR in NRF2-silenced cells affected breast cancer resistance protein (BCRP/ABCG2) levels. The pharmacological and genetic inhibition of c-MET or EGFR, as well as the miR-206 mimic treatment, repressed BCRP levels and increased cellular accumulation of doxorubicin. In line with these, treatment of NRF2-silenced SKOV3 with the miR-206 inhibitor elevated BCRP levels and consequently made these cells more resistant to doxorubicin treatment. Collectively, our results demonstrated that the NRF2 silencing-inducible miR-206 targeted both c-MET and EGFR, and subsequently suppressed the BCRP level in cancer cells.

The RARS-MAD1L1 Fusion Gene Induces Cancer Stem Cell-like Properties and Therapeutic Resistance in Nasopharyngeal Carcinoma

Purpose: Nasopharyngeal carcinoma (NPC) is the most common head and neck cancer in Southeast Asia. Because local recurrence and distant metastasis are still the main causes of NPC treatment failure, it is urgent to identify new tumor markers and therapeutic targets for advanced NPC.Experimental Design: RNA sequencing (RNA-seq) was applied to look for interchromosome translocation in NPC. PCR, FISH, and immunoprecipitation were used to examine the fusion gene expression at RNA, DNA, and protein levels in NPC biopsies. MTT assay, colony formation assay, sphere formation assay, co-immunoprecipitation, chromatin immunoprecipitation assay, and in vivo chemoresistance assay were applied to explore the function of RARS-MAD1L1 in NPC.Results: We demonstrated that RARS-MAD1L1 was present in 10.03% (35/349) primary NPC biopsies and 10.7% (9/84) in head and neck cancer (HNC) samples. RARS-MAD1L1 overexpression increased cell proliferation, colony formation, and tumorigenicity in vitro, and the silencing of endogenous RARS-MAD1L1 reduced cancer cell growth and colony formation in vitro In addition, RARS-MAD1L1 increased the side population (SP) ratio and induced chemo- and radioresistance. Furthermore RARS-MAD1L1 interacted with AIMP2, which resulted in activation of FUBP1/c-Myc pathway. The silencing of FUBP1 or the administration of a c-Myc inhibitor abrogated the cancer stem cell (CSC)-like characteristics induced by RARS-MAD1L1. The expression of c-Myc and ABCG2 was higher in RARS-MAD1L1-positive HNC samples than in negative samples.Conclusions: Our findings indicate that RARS-MAD1L1 might contribute to tumorigenesis, CSC-like properties, and therapeutic resistance, at least in part, through the FUBP1/c-Myc axis, implying that RARS-MAD1L1 might serve as an attractive target for therapeutic intervention for NPC. Clin Cancer Res; 24(3); 659-73. ©2017 AACR.

Targeting oral cancer stemness and chemoresistance by isoliquiritigenin-mediated GRP78 regulation

Cancer stem cells (CSCs) are cells that drive tumorigenesis, contributing to metastasis and cancer recurrence as well as resistance to chemotherapy of oral squamous cell carcinomas (OSCC). Therefore, approaches to target CSCs become the subject of intense research for cancer therapy. In this study, we demonstrated that isoliquiritigenin, a chalcone-type flavonoid isolated from licorice root, exhibited more toxicity in oral cancer stem cells (OSCC-CSCs) compared to normal cells. Treatment of isoliquiritigenin not only inhibited the self-renewal ability but also reduced the expression of CSC markers, including the ALDH1 and CD44. In addition, the capacities of OSCC-CSCs to invade, metastasize and grow into a colony were suppressed by isoliquiritigenin. Most importantly, we showed that isoliquiritigenin potentiated chemotherapy along with downregulated expression of an ABC transporter that is associated with drug resistance, ABCG2. Moreover, a combination of isoliquiritigenin and Cisplatin significantly repressed the invasion and colony formation abilities of OSCC-CSCs. Our results suggested that administration of isoliquiritigenin reduced the protein expression of mRNA and membrane GRP78, a critical mediator of tumor biology. Overexpression of GRP78 reversed the inhibitory effect of isoliquiritigenin on OSCC-CSCs. Furthermore, isoliquiritigenin retarded the tumor growth in nude mice bearing OSCC xenografts. Taken together, these findings showed that isoliquiritigenin is an effective natural compound that can serve as an adjunct to chemotherapy for OSCC.

TNFα and TGFβ-1 synergistically increase the cancer stem cell properties of MiaPaCa-2 cells

Increased serum concentrations of tumor necrosis factor α (TNFα) and transforming growth factor β-1 (TGFβ-1) in the blood of patients with pancreatic cancer (PC) have previously been demonstrated. In addition, exogenous exposure to these cytokines promotes various cancer cell invasive and cancer stem cell (CSC) phenotypes. However, their importance in pancreatic CSCs remains elusive. In the present study, the effects of TNFα and TGFβ-1 on the human PC cell line MiaPaCa-2 were examined. Using flow cytometry, it was revealed that TNFα and TGFβ-1 synergistically increase cluster of differentiation (CD) 44v6, CD133 and ATP-binding cassette transporter G2 (ABCG2) expressing populations in adherent tumor cell culture conditions. Furthermore, a similar trend was observed in cells pretreated with these cytokines grown in sphere forming culture conditions. Similar to previous studies, TNFα treatment increased the proportion of epidermal growth factor receptor (EGFR) expressing cells in adherent culture, and this data was further supported by the results of the sphere formation assay, in which the subculture with a high proportion of EGFR expressing cells exhibited the most efficient sphere forming ability. However, the proportion of vascular endothelial growth factor receptor 1 (VEGFR1) expressing cells did not increase upon treatment with these cytokines individually or in combination. This data was subsequently supported by the results of the wound healing assay in which cytokine treatment did not increase the migration of cells. The MTT cell proliferation and cytotoxicity assay revealed that TNFα + TGFβ-1 treatment significantly increased cell proliferation and daunorubicin resistance, but not gemcitabine resistance. In conclusion, the data of the current study provide a mechanistic association between TNFα, TGFβ-1 and the CSC properties of MiaPaCa-2 cells. In addition, it suggests that targeting TNFα and TGFβ-1 is beneficial for improving the therapeutic efficacy of treatments for patients with PC.

Salinomycin's potential to eliminate glioblastoma stem cells and treat glioblastoma multiforme (Review)

Glioblastoma multiforme (GBM) is the most common and deadliest form of primary brain tumor. Despite treatment with surgery, radiotherapy, and chemotherapy with the drug temozolomide, the expected survival after diagnosis remains low. The median survival is only 14.6 months and the two-year survival is a mere 30%. One reason for this is the heterogeneity of GBM including the presence of glioblastoma cancer stem cells (GSCs). GSCs are a subset of cells with the unique ability to proliferate, differentiate, and create tumors. GSCs are resistant to chemotherapy and radiation and thought to play an important role in recurrence. In order to effectively treat GBM, a drug must be identified that can kill GSCs. The ionophore salinomycin has been shown to kill cancer stem cells and is therefore a promising future treatment for GBM. This study focuses on salinomycin's potential to treat GBM including its ability to reduce the CSC population, its toxicity to normal brain cells, its mechanism of action, and its potential for combination treatment.

Epigenetic Modifications and Head and Neck Cancer: Implications for Tumor Progression and Resistance to Therapy

Head and neck squamous carcinoma (HNSCC) is the sixth most prevalent cancer and one of the most aggressive malignancies worldwide. Despite continuous efforts to identify molecular markers for early detection, and to develop efficient treatments, the overall survival and prognosis of HNSCC patients remain poor. Accumulated scientific evidences suggest that epigenetic alterations, including DNA methylation, histone covalent modifications, chromatin remodeling and non-coding RNAs, are frequently involved in oral carcinogenesis, tumor progression, and resistance to therapy. Epigenetic alterations occur in an unsystematic manner or as part of the aberrant transcriptional machinery, which promotes selective advantage to the tumor cells. Epigenetic modifications also contribute to cellular plasticity during tumor progression and to the formation of cancer stem cells (CSCs), a small subset of tumor cells with self-renewal ability. CSCs are involved in the development of intrinsic or acquired therapy resistance, and tumor recurrences or relapse. Therefore, the understanding and characterization of epigenetic modifications associated with head and neck carcinogenesis, and the prospective identification of epigenetic markers associated with CSCs, hold the promise for novel therapeutic strategies to fight tumors. In this review, we focus on the current knowledge on epigenetic modifications observed in HNSCC and emerging Epi-drugs capable of sensitizing HNSCC to therapy.

Prostate Cancer Stem Cell Markers Drive Progression, Therapeutic Resistance, and Bone Metastasis

Metastatic or recurrent tumors are the primary cause of cancer-related death. For prostate cancer, patients diagnosed with local disease have a 99% 5-year survival rate; however, this 5-year survival rate drops to 28% in patients with metastatic disease. This dramatic decline in survival has driven interest in discovering new markers able to identify tumors likely to recur and in developing new methods to prevent metastases from occurring. Biomarker discovery for aggressive tumor cells includes attempts to identify cancer stem cells (CSCs). CSCs are defined as tumor cells capable of self-renewal and regenerating the entire tumor heterogeneity. Thus, it is hypothesized that CSCs may drive primary tumor aggressiveness, metastatic colonization, and therapeutic relapse. The ability to identify these cells in the primary tumor or circulation would provide prognostic information capable of driving prostate cancer treatment decisions. Further, the ability to target these CSCs could prevent tumor metastasis and relapse after therapy allowing for prostate cancer to finally be cured. Here, we will review potential CSC markers and highlight evidence that describes how cells expressing each marker may drive prostate cancer progression, metastatic colonization and growth, tumor recurrence, and resistance to treatment.

Cancer Cell Plasticity: Rapid Reversal of Chemosensitivity and Expression of Stemness Markers in Lung and Breast Cancer Tumorspheres

In cancer cells, the reversible nature of the stemness status in terms of chemoresistance has been poorly characterized. In this study, we have simulated one cycle of environmental conditions to study such reversibility by first generating floating tumorspheres (FTs) from lung and breast cancer cells by culturing them in serum-free media without the addition of any external mitogenic stimulation, and subsequently (after 2 weeks) re-incubating them back in serum-containing media to simulate routine culture conditions (RCCs). We found that cancer cells are extremely plastic: cells grown under RCCs become multidrug-resistant when grown as FTs, but upon re-incubation under RCCs quickly re-attach and lose the acquired resistance. These phenotypic changes are accompanied by concomitant changes in the expression of key proteins associated with multiple pathways important for chemoresistance, survival, and stemness maintenance. Therefore, our strategy provides an excellent experimental model to study environmental factors that modulate the plasticity of cancer cells. J. Cell. Physiol. 232: 2280-2286, 2017. © 2016 Wiley Periodicals, Inc.

MicroRNA-320a and microRNA-4496 attenuate Helicobacter pylori cytotoxin-associated gene A (CagA)-induced cancer-initiating potential and chemoresistance by targeting β-catenin and ATP-binding cassette, subfamily G, member 2

Infection with Helicobacter pylori is closely linked to an increased risk of gastric cancer. Although cytotoxin-associated gene A (CagA), a major virulence factor of H. pylori, is known to be a causal factor for gastric carcinogenesis, the molecular link between CagA and gastric cancer-initiating cell (CIC)-like properties remains elusive. Here, we demonstrate that CagA is required for increased expression of β-catenin and its target CIC markers via downregulation of microRNA (miR)-320a and miR-4496. CagA promoted gastric CIC properties and was responsible for chemoresistance. miR-320a and miR-4496 attenuated the in vitro self-renewal and tumour-initiating capacity of CagA-expressing CICs by targeting β-catenin. Moreover, miR-320a and miR-4496 decreased CagA-induced chemoresistance by targeting ATP-binding cassette, subfamily G, member 2 (ABCG2) at the transcriptional and post-transcriptional levels, respectively. Combination therapy with 5-fluorouracil and miR-320a/miR-4496 suppressed gastric tumourigenesis and metastatic potential in an orthotopic mouse model, probably via suppression of CagA-induced CIC properties and chemoresistance. Our results provide novel evidence that CIC properties, chemoresistance and tumourigenesis associated with H. pylori are linked to CagA-induced upregulation of β-catenin and ABCG2. These data provide novel insights into the molecular mechanisms of CagA-induced carcinogenisis and the therapeutic potential of of miR-320a and miR-4496. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Relevance of mortalin to cancer cell stemness and cancer therapy

Mortalin/mtHsp70 is a member of Hsp70 family of proteins. Enriched in a large variety of cancers, it has been shown to contribute to the process of carcinogenesis by multiple ways including inactivation of tumor suppressor p53 protein, deregulation of apoptosis and activation of EMT signaling. In this study, we report that upregulation of mortalin contributes to cancer cell stemness. Several cancer cell stemness markers, such as ABCG2, OCT-4, CD133, ALDH1, CD9, MRP1 and connexin were upregulated in mortalin-overexpressing cells that showed higher ability to form spheroids. These cells also showed higher migration, and were less responsive to a variety of cancer chemotherapeutic drugs. Of note, knockdown of mortalin by specific shRNA sensitized these cells to all the drugs used in this study. We report that low doses of anti-mortalin molecules, MKT-077 and CAPE, also caused similar sensitization of cancer cells to chemotherapeutic drugs and hence are potential candidates for effective cancer chemotherapy.

A-803467, a tetrodotoxin-resistant sodium channel blocker, modulates ABCG2-mediated MDR in vitro and in vivo

ATP-binding cassette subfamily G member 2 (ABCG2) is a member of the ABC transporter superfamily proteins, which has been implicated in the development of multidrug resistance (MDR) in cancer, apart from its physiological role to remove toxic substances out of the cells. The diverse range of substrates of ABCG2 includes many antineoplastic agents such as topotecan, doxorubicin and mitoxantrone. ABCG2 expression has been reported to be significantly increased in some solid tumors and hematologic malignancies, correlated to poor clinical outcomes. In addition, ABCG2 expression is a distinguishing feature of cancer stem cells, whereby this membrane transporter facilitates resistance to the chemotherapeutic drugs. To enhance the chemosensitivity of cancer cells, attention has been focused on MDR modulators. In this study, we investigated the effect of a tetrodotoxin-resistant sodium channel blocker, A-803467 on ABCG2-overexpressing drug selected and transfected cell lines. We found that at non-toxic concentrations, A-803467 could significantly increase the cellular sensitivity to ABCG2 substrates in drug-resistant cells overexpressing either wild-type or mutant ABCG2. Mechanistic studies demonstrated that A-803467 (7.5 μM) significantly increased the intracellular accumulation of [³H]-mitoxantrone by inhibiting the transport activity of ABCG2, without altering its expression levels. In addition, A-803467 stimulated the ATPase activity in membranes overexpressed with ABCG2. In a murine model system, combination treatment of A-803467 (35 mg/kg) and topotecan (3 mg/kg) significantly inhibited the tumor growth in mice xenografted with ABCG2-overexpressing cancer cells. Our findings indicate that a combination of A-803467 and ABCG2 substrates may potentially be a novel therapeutic treatment in ABCG2-positive drug resistant cancers.

Aptamers for CD Antigens: From Cell Profiling to Activity Modulation

Nucleic acid-based aptamers are considered to be a promising alternative to antibodies because of their strong and specific binding to diverse targets, fast and inexpensive chemical synthesis, and easy labeling with a fluorescent dye or therapeutic agent. Cluster of differentiation (CD) proteins are among the most popular antigens for aptamers on the cell surface. These anti-CD aptamers could be used in cell biology and biomedicine, from simple cell phenotyping by flow cytometry or fluorescent microscopy to diagnosis and treatment of HIV/AIDS to cancer and immune therapies. The unique feature of aptamers is that they can act simultaneously as an agonist and antagonist of CD receptors depending on a degree of aptamer oligomerization. Aptamers can also deliver small interfering RNA to silence vital genes in CD-positive cells. In this review, we summarize nucleic acid sequences of anti-CD aptamers and their use, which have been validated in multiple studies.

Chemotherapy curable malignancies and cancer stem cells: a biological review and hypothesis

Background

Cytotoxic chemotherapy brings routine cures to only a small select group of metastatic malignancies comprising gestational trophoblast tumours, germ cell tumours, acute leukemia, Hodgkin’s disease, high grade lymphomas and some of the rare childhood malignancies.

We have previously postulated that the extreme sensitivity to chemotherapy for these malignancies is linked to the on-going high levels of apoptotic sensitivity that is naturally linked with the unique genetic events of nuclear fusion, meiosis, VDJ recombination, somatic hypermutation, and gastrulation that have occurred within the cells of origin of these malignancies.

In this review we will examine the cancer stem cell/cancer cell relationship of each of the chemotherapy curable malignancies and how this relationship impacts on the resultant biology and pro-apoptotic sensitivity of the varying cancer cell types.

Discussion

In contrast to the common epithelial cancers, in each of the chemotherapy curable malignancies there are no conventional hierarchical cancer stem cells. However cells with cancer stem like qualities can arise stochastically from within the general tumour cell population. These stochastic stem cells acquire a degree of resistance to DNA damaging agents but also retain much of the key characteristics of the cancer cells from which they develop. We would argue that the balance between the acquired resistance of the stochastic cancer stem cell and the inherent chemotherapy sensitivity of parent tumour cell determines the overall chemotherapy curability of each diagnosis.

Summary

The cancer stem cells in the chemotherapy curable malignancies appear to have two key biological differences from those of the more common chemotherapy incurable malignancies. The first difference is that the conventional hierarchical pattern of cancer stem cells is absent in each of the chemotherapy curable malignancies.

The other key difference, we suggest, is that the stochastic stem cells in the chemotherapy curable malignancies take on a significant aspect of the biological characteristics of their parent cancer cells. This action includes for the chemotherapy curable malignancies the heightened pro-apoptotic sensitivity linked to their respective associated unique genetic events.

For the chemotherapy curable malignancies the combination of the relationship of their cancer stem cells combined with the extreme inherent sensitivity to induction of apoptosis from DNA damaging agents plays a key role in determining their overall curability with chemotherapy.

MiRNAs and miRNA Polymorphisms Modify Drug Response

Differences in expression of drug response-related genes contribute to inter-individual variation in drugs’ biological effects. MicroRNAs (miRNAs) are small noncoding RNAs emerging as new players in epigenetic regulation of gene expression at post-transcriptional level. MiRNAs regulate the expression of genes involved in drug metabolism, drug transportation, drug targets and downstream signal molecules directly or indirectly. MiRNA polymorphisms, the genetic variations affecting miRNA expression and/or miRNA-mRNA interaction, provide a new insight into the understanding of inter-individual difference in drug response. Here, we provide an overview of the recent progress in miRNAs mediated regulation of biotransformation enzymes, drug transporters, and nuclear receptors. We also describe the implications of miRNA polymorphisms in cancer chemotherapy response.

Kinetic Modeling of ABCG2 Transporter Heterogeneity: A Quantitative, Single-Cell Analysis of the Side Population Assay

The side population (SP) assay, a technique used in cancer and stem cell research, assesses the activity of ABC transporters on Hoechst staining in the presence and absence of transporter inhibition, identifying SP and non-SP cell (NSP) subpopulations by differential staining intensity. The interpretation of the assay is complicated because the transporter-mediated mechanisms fail to account for cell-to-cell variability within a population or adequately control the direct role of transporter activity on staining intensity. We hypothesized that differences in dye kinetics at the single-cell level, such as ABCG2 transporter-mediated efflux and DNA binding, are responsible for the differential cell staining that demarcates SP/NSP identity. We report changes in A549 phenotype during time in culture and with TGFβ treatment that correlate with SP size. Clonal expansion of individually sorted cells re-established both SP and NSPs, indicating that SP membership is dynamic. To assess the validity of a purely kinetics-based interpretation of SP/NSP identity, we developed a computational approach that simulated cell staining within a heterogeneous cell population; this exercise allowed for the direct inference of the role of transporter activity and inhibition on cell staining. Our simulated SP assay yielded appropriate SP responses for kinetic scenarios in which high transporter activity existed in a portion of the cells and little differential staining occurred in the majority of the population. With our approach for single-cell analysis, we observed SP and NSP cells at both ends of a transporter activity continuum, demonstrating that features of transporter activity as well as DNA content are determinants of SP/NSP identity.

Implications of cancer stem cells in developing therapeutic resistance in oral cancer

Conventional therapeutics are often frequented with recurrences, refraction and regimen resistance in oral cavity cancers which are predominantly manifested by cancer stem cells (CSCs). During oncoevolution, cancer cells may undergo structural and functional reprogramming wherein they evolve as highly tolerant CSC phenotypes with greater survival advantages. The CSCs possess inherent and exclusive properties including self-renewal, hierarchical differentiation, and tumorigenicity that serve as the basis of chemo-radio-resistance in oral cancer. However, the key mechanisms underlying the CSC-mediated therapy resistance need to be further elucidated. A spectrum of dysfunctional cellular pathways including the developmental signaling, apoptosis, autophagy, cell cycle regulation, DNA damage responses and epigenetic regulations protect the CSCs from conventional therapies. Moreover, tumor niche shelters CSCs and creates an immunosuppressive environment favoring the survival of CSCs. Maintenance of lower redox status, epithelial-to-mesenchymal transition (EMT), metabolic reprogramming and altered drug responses are the accessory features that aid in the process of chemo-radio-resistance in oral CSCs. This review deals with the functional and molecular basis of cancer cell pluripotency-associated resistance highlighting the abrupt fundamental cellular processes; targeting these events may hold a great promise in the successful treatment of oral cancer.