Salinomycin exerts anticancer effects on human breast carcinoma MCF-7 cancer stem cells via modulation of Hedgehog signaling

Therapeutic vulnerabilities in triple negative breast cancer: Stem-like traits explored within molecular classification

Triple Negative Breast Cancer (TNBC) is the most aggressive type of breast cancer (BC). Despite advances in the clinical management of TNBC, recurrence-related mortality remains a challenge. The stem-like phenotype of TNBC plays a significant role in the persistence of minimal disease residue after therapy. Individuals exhibiting stem-like characteristics are particularly prone to inducing malignant relapse accompanied by strong resistance. Therefore, stem-like traits have been broadly proposed as therapeutic vulnerabilities to treat TNBC and reduce recurrence. However, heterogeneity within TNBC often generally restricts the stability of the therapeutic efficacy. To understand the heterogeneity and manage TNBC more precisely, multiple TNBC subtyping categories have been reported, providing the basis for profile-according therapeutic regimens. To provide more insight into targeting stem-like traits to ablate TNBC and reduce recurrence in the context of heterogeneity, this paper reviewed the molecular subtyping of TNBC, identified the consensus subtypes with distinct stem-like phenotypes, characterized the stemness hierarchy of TNBC, outlined the biological models for stem-like TNBC subtypes, summarized the therapeutic vulnerabilities in stem-like traits of the subtypes, and proposed potential therapeutic regimens targeting stem-like characteristics to improve TNBC prognosis.

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.

The tale of antibiotics beyond antimicrobials: Expanding horizons

Antibiotics had proved to be a godsend for mankind since their discovery. They were once the magical solution to the vexing problem of infection-related deaths. German scientist Paul Ehrlich had termed salvarsan as the silver bullet to treatsyphilis.As time passed, the magic of newly discovered silver bullets got tarnished with raging antibiotic resistance among bacteria and associated side-effects. Still, antibiotics remain the primary line of treatment for bacterial infections. Our understanding of their chemical and biological activities has increased immensely with advancement in the research field. Non-antibacterial effects of antibiotics are studied extensively to optimise their safer, broad-range use. These non-antibacterial effects could be both useful and harmful to us. Various researchers across the globe including our lab are studying the direct/indirect effects and molecular mechanisms behind these non-antibacterial effects of antibiotics. So, it is interesting for us to sum up the available literature. In this review, we have briefed the possible reason behind the non-antibacterial effects of antibiotics, owing to the endosymbiotic origin of host mitochondria. We further discuss the physiological and immunomodulatory effects of antibiotics. We then extend the review to discuss molecular mechanisms behind the plausible use of antibiotics as anticancer agents.

Nanoarchitectonics of doxycycline-loaded vitamin E-D-α-tocopheryl polyethylene glycol 1000 succinate micelles for ovarian cancer stem cell treatment

Aim: This study aim to develop doxycycline within the D-α-tocopheryl polyethylene glycol 1000 succinate micelle platform as an anticancer stem cell agent. Materials & methods: The optimized nanomicelle formulation was prepared using the solvent casting method and evaluated through physicochemical and biological characterization. Results: Nanomicelles exhibited mean particle sizes of 14.48 nm (polydispersity index: 0.22) using dynamic light scattering and 18.22 nm using transmission electron micrography. Drug loading and encapsulation efficiency were 2% and 66.73%, respectively. Doxycycline-loaded micelles exhibited sustained release, with 98.5% released in 24 h. IC50 values were 20 μg/ml for free drug and 5 μg/ml for micelles after 48 h of cell exposure. A significant 74% reduction in CD44 biomarker and 100% colony formation inhibition were observed. Conclusion: Doxycycline in hemo/biocompatible nanomicelles holds potential for ovarian cancer stem cell therapy.

Signaling pathways governing the maintenance of breast cancer stem cells and their therapeutic implications

Breast cancer stem cells (BCSCs) represent a distinct subpopulation of cells with the ability to self-renewal and differentiate into phenotypically diverse tumor cells. The involvement of CSC in treatment resistance and cancer recurrence has been well established. Numerous studies have provided compelling evidence that the self-renewal ability of cancer stem cells is tightly regulated by specific signaling pathways, which exert critical roles to maintain an undifferentiated phenotype and prevent the differentiation of CSCs. Signaling pathways such as Wnt/β-catenin, NF-κB, Notch, Hedgehog, TGF-β, and Hippo have been implicated in the promotion of self-renewal of many normal and cancer stem cells. Given the pivotal role of BCSCs in driving breast cancer aggressiveness, targeting self-renewal signaling pathways holds promise as a viable therapeutic strategy for combating this disease. In this review, we will discuss the main signaling pathways involved in the maintenance of the self-renewal ability of BCSC, while also highlighting current strategies employed to disrupt the signaling molecules associated with stemness.

Overcoming apoptotic resistance afforded by Bcl-2 in lymphoid tumor cells: a critical role for dexamethasone

Bcl-2 is an anti-apoptotic protein that promotes cell survival and resistance to cell death. Predictably, Bcl-2 as well as other anti-apoptotic Bcl-2 family members have been found to be overexpressed in a variety of human cancers. Approaches to overcome apoptotic resistance afforded by Bcl-2 in cells include anti-sense oligonucleotides, drugs that inhibit Bcl-2 function, and BH3 mimics have not been universally effective; thus, the need to understand the underlying mechanism of this resistance is vital. Glucocorticoids are stress hormones that act through their cognate receptors to control the transcription of numerous target genes, and in turn regulate a diverse array of biological processes. Synthetic glucocorticoids, such as dexamethasone, are prescribed in many chemotherapy protocols for neoplasms of lymphoid origin based on their ability to inhibit lymphocyte proliferation and promote apoptosis. However, lymphoid cells expressing Bcl-2 are resistant to glucocorticoid-induced cell death. We observed both pro- and anti-apoptotic characteristics in lymphoid cells expressing Bcl-2 following glucocorticoid treatment. These cells exhibited a profound change in their intracellular ionic composition, but a limited apoptotic ion flux and the absence of cell death. Provocatively, mimicking the loss of intracellular potassium using a low dose of a microbial toxin that acts as a potassium ionophore in combination with dexamethasone overcame the resistance afforded by Bcl-2 and killed the cells. Extending our study using other potassium ionophores revealed that direct depolarization of the mitochondria membrane potential coupled with prior treatment with glucocorticoids is the key mechanism for activating the cell death program and bypassing the resistance afforded by Bcl-2 in lymphoid cells. Finally, we show that the duration of dexamethasone pre-treatment is critical for regulating distinct genes and signaling pathways that sensitize the cells to die.

Evidence of Metallic and Polyether Ionophores as Potent Therapeutic Drug Candidate in Cancer Management

Cancer remains one of the most crucial human malignancies with a higher mortality rate globally, and is predicted to escalate soon. Dysregulated ion homeostasis in cancerous cells prompted the researchers to investigate further ion homeostasis impeding agents as potent anticancerous agents. Reutilization of FDA-approved non-cancerous drugs has emerged as a practical approach to developing potent, cost-effective drugs for cancer treatment. Across the globe, most nations are incapable of fulfilling the medical demands of cancer patients due to costlier cancerous drugs. Therefore, we have inclined our review towards emphasizing recent advancements in cancer therapies involving ionophores utilization in exploring potent anticancer drugs. Numerous research reports have established the significant anticancerous potential of ionophores in several pre-clinical reports via modulating aberrant cell signaling pathways and enhancing antitumor immunity in immune cells. This review has mainly summarized the most significant ion homeostasis impeding agents, including copper, zinc, calcium, and polyether, that presented remarkable potential in cancer therapeutics via enhanced antitumor immunity and apoptosis induction. Altogether, this study could provide a robust future perspective for developing cost-effective anticancerous drugs rapidly and cost-effectively, thereby combating the limitations of currently available drugs used in cancer treatment.

Rapid Access to Ironomycin Derivatives by Click Chemistry

Salinomycin, a natural carboxylic polyether ionophore, shows a very interesting spectrum of biological activities, including selective toxicity toward cancer stem cells (CSCs). Recently, we have developed a C20-propargylamine derivative of salinomycin (ironomycin) that exhibits more potent activity in vivo and greater selectivity against breast CSCs compared to the parent natural product. Since ironomycin contains a terminal alkyne motif, it stands out as being an ideal candidate for further functionalization. Using copper-catalyzed azide-alkyne cycloaddition (CuAAC), we synthesized a series of 1,2,3-triazole analogs of ironomycin in good overall yields. The in vitro screening of these derivatives against a well-established model of breast CSCs (HMLER CD24^low/CD44^high) and its corresponding epithelial counterpart (HMLER CD24^high/CD44^low) revealed four new products characterized by higher potency and improved selectivity toward CSCs compared to the reference compound ironomycin. The present study highlights the therapeutic potential of a new class of semisynthetic salinomycin derivatives for targeting selectively the CSC niche and highlights ironomycin as a promising starting material for the development of new anticancer drug candidates.

Role, molecular mechanism and the potential target of breast cancer stem cells in breast cancer development

Breast cancer (BC) is one of the most common malignant tumors in women globally, and its occurrence has surpassed lung cancer and become the biggest threat for women. At present, breast cancer treatment includes surgical resection or postoperative chemotherapy and radiotherapy. However, tumor relapse and metastasis usually lead to current therapy failure thanks to breast cancer stem cells (BCSCs)-mediated tumorigenicity and drug resistance. Drug resistance is mainly due to the long-term quiescent G0 phase, strong DNA repairability, and high expression of ABC transporter, and the tumorigenicity is reflected in the activation of various proliferation pathways related to BCSCs. Therefore, understanding the characteristics of BCSCs and their intracellular and extracellular molecular mechanisms is crucial for the development of targeted drugs for BCSCs. To this end, we discussed the latest developments in BCSCs research, focusing on the analysis of specific markers, critical signaling pathways that maintain the stemness of BCSCs，such as NOTCH, Wnt/β-catenin, STAT3, Hedgehog, and Hippo-YAP signaling, immunomicroenviroment and summarizes targeting therapy strategies for stemness maintenance and differentiation, which provides a theoretical basis for further exploration of treating breast cancer and preventing relapse derived from BCSCs.

Anticancer Mechanisms of Salinomycin in Breast Cancer and Its Clinical Applications

Breast cancer (BC) is the most frequent cancer among women worldwide and is the leading cause of cancer-related deaths in women. Cancer cells with stem cell-like features and tumor-initiating potential contribute to drug resistance, tumor recurrence, and metastasis. To achieve better clinical outcomes, it is crucial to eradicate both bulk BC cells and breast cancer stem cells (BCSCs). Salinomycin, a monocarboxylic polyether antibiotic isolated from Streptomyces albus, can precisely kill cancer stem cells (CSCs), particularly BCSCs, by various mechanisms, including apoptosis, autophagy, and necrosis. There is increasing evidence that salinomycin can inhibit cell proliferation, invasion, and migration in BC and reverse the immune-inhibitory microenvironment to prevent tumor growth and metastasis. Therefore, salinomycin is a promising therapeutic drug for BC. In this review, we summarize established mechanisms by which salinomycin protects against BC and discuss its future clinical applications.

Epithelial-mesenchymal transition sensitizes breast cancer cells to cell death via the fungus-derived sesterterpenoid ophiobolin A

The epithelial-mesenchymal transition (EMT) imparts properties of cancer stem-like cells, including resistance to frequently used chemotherapies, necessitating the identification of molecules that induce cell death specifically in stem-like cells with EMT properties. Herein, we demonstrate that breast cancer cells enriched for EMT features are more sensitive to cytotoxicity induced by ophiobolin A (OpA), a sesterterpenoid natural product. Using a model of experimentally induced EMT in human mammary epithelial (HMLE) cells, we show that EMT is both necessary and sufficient for OpA sensitivity. Moreover prolonged, sub-cytotoxic exposure to OpA is sufficient to suppress EMT-imparted CSC features including sphere formation and resistance to doxorubicin. In vivo growth of CSC-rich mammary cell tumors, is suppressed by OpA treatment. These data identify a driver of EMT-driven cytotoxicity with significant potential for use either in combination with standard chemotherapy or for tumors enriched for EMT features.

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.

Regulating Stem Cell-Related Genes Induces the Plastic Differentiation of Cancer Stem Cells to Treat Breast Cancer

Relapse of cancer is associated with multidirectional differentiation and unrestricted proliferative replication potential of cancer stem cells. Herein, we propose the plastic differentiation strategy for irreversible differentiation of cancer stem cells; further, salinomycin and its newly constructed functional liposomes are used to implement this strategy. Whole gene, cancer stem cell-related RNA, and protein expression analyses reveal that salinomycin induces the cancer stem cells into normal cells, dormant cells, and mature cancer cells. Besides, the results indicate that the gatekeeper is related to the inhibition of the protein kinase C (PKC) α signaling pathway. The differentiated normal or dormant cells are incorporated into normal tissue, whereas the rest are killed by chemotherapy. The findings would offer the evidence for plastic differentiation of cancer stem cells and propose a novel strategy for cancer therapy.

Salinomycin inhibits epigenetic modulator EZH2 to enhance death receptors in colon cancer stem cells

Drug resistance is one of the trademark features of Cancer Stem Cells (CSCs). We and others have recently shown that paucity of functional death receptors (DR4/5) on the cell surface of tumour cells is one of the major reasons for drug resistance, but their involvement in the context of in CSCs is poorly understood. By harnessing CSC specific cytotoxic function of salinomycin, we discovered a critical role of epigenetic modulator EZH2 in regulating the expression of DRs in colon CSCs. Our unbiased proteome profiler array approach followed by ChIP analysis of salinomycin treated cells indicated that the expression of DRs, especially DR4 is epigenetically repressed in colon CSCs. Concurrently, EZH2 knockdown demonstrated increased expression of DR4/DR5, significant reduction of CSC phenotypes such as spheroid formation in-vitro and tumorigenic potential in-vivo in colon cancer. TCGA data analysis of human colon cancer clinical samples shows strong inverse correlation between EZH2 and DR4. Taken together, this study provides an insight about epigenetic regulation of DR4 in colon CSCs and advocates that drug-resistant colon cancer can be therapeutically targeted by combining TRAIL and small molecule EZH2 inhibitors.

Synthesis and Anticancer Activity of Tertiary Amides of Salinomycin and Their C20-oxo Analogues

The polyether ionophore salinomycin (SAL) has captured much interest because of its potent activity against cancer cells and cancer stem cells. Our previous studies have indicated that C1/C20 double-modification of SAL is a useful strategy to generate diverse agents with promising biological activity profiles. Thus, herein we describe the synthesis of a new class of SAL analogues that combine key modifications at the C1 and C20 positions. The activity of the obtained SAL derivatives was evaluated using primary acute lymphoblastic leukemia, human breast adenocarcinoma and normal mammary epithelial cells. One single- [N,N-dipropyl amide of salinomycin (5 a)] and two novel double-modified analogues [N,N-dipropyl amide of C20-oxosalinomycin (5 b) and piperazine amide of C20-oxosalinomycin (13 b)] were found to be more potent toward the MDA-MB-231 cell line than SAL or its C20-oxo analogue 2. When select analogues were tested against the NCI-60 human tumor cell line panel, 4 a [N,N-diethyl amide of salinomycin] showed particular activity toward the ovarian cancer cell line SK-OV-3. Additionally, both SAL and 2 were found to be potent ex vivo against human ER/PR⁺ , Her2^- invasive mammary carcinoma, with 2 showing minimal toxicity toward normal epithelial cells. The present findings highlight the therapeutic potential of SAL derivatives for select targeting of different cancer types.

Tocopherols inhibit estrogen-induced cancer stemness and OCT4 signaling in breast cancer

Estrogen plays an important role in breast cancer development. While the mechanism of the estrogen effects is not fully elucidated, one possible route is by increasing the stem cell-like properties in the tumors. Tocopherols are known to reduce breast cancer development and progression. The aim of the present study is to investigate the effects of tocopherols on the regulation of breast cancer stemness mediated by estrogen. To determine the effects of tocopherols on estrogen-influenced breast cancer stem cells, the MCF-7 tumorsphere culture system, which enriches for mammary progenitor cells and putative breast cancer stem cells, was utilized. Treatment with estrogen resulted in an increase in the CD44+/CD24- subpopulation and aldehyde dehydrogenase activity in tumorspheres as well as the number and size of tumorspheres. Tocopherols inhibited the estrogen-induced expansion of the breast cancer stem population. Tocopherols decreased the levels of stem cell markers, including octamer-binding transcription factor 4 (OCT4), CD44 and SOX-2, as well as estrogen-related markers, such as trefoil factor (TFF)/pS2, cathepsin D, progesterone receptor and SERPINA1, in estrogen-stimulated tumorspheres. Overexpression of OCT4 increased CD44 and sex-determining region Y-box-2 levels and significantly increased cell invasion and expression of the invasion markers, matrix metalloproteinases, tissue inhibitors of metalloproteinase and urokinase plasminogen activator, and tocopherols inhibited these OCT4-mediated effects. These results suggest a potential inhibitory mechanism of tocopherols in estrogen-induced stemness and cell invasion in breast cancer.

Salinomycin exerts anti-colorectal cancer activity by targeting the β-catenin/T-cell factor complex

Background and Purpose

Salinomycin is a well‐known inhibitor of human cancer stem cells (CSCs). However, the molecular mechanism(s) by which salinomycin targets colorectal CSCs is poorly understood. Here, we have investigated underlying antitumour mechanisms of salinomycin in colorectal cancer cells and three tumour models.

Experimental Approach

The inhibitory effect of salinomycin on the Wnt/β‐catenin pathway was analysed with the SuperTopFlash reporter system. The mRNA expression of Wnt target genes was evaluated with real‐time PCR. Effects of salinomycin on β‐catenin/TCF4E interaction were examined using co‐immunoprecipitation and an in vitro GST pull‐down assay. Cell proliferation was determined by BrdU incorporation and soft agar colony formation assay. The stemness of the cells was assessed by sphere formation assay. Antitumour effects of salinomycin on colorectal cancers was evaluated with colorectal CSC xenografts, APC^min/+ transgenic mice, and patient‐derived colorectal tumour xenografts.

Key Results

Salinomycin blocked β‐catenin/TCF4E complex formation in colorectal cancer cells and in an in vitro GST pull‐down assay, thus decreasing expression of Wnt target genes. Salinomycin also suppressed the transcriptional activity mediated by β‐catenin/LEF1 or β‐catenin/TCF4E complex and exhibited an inhibitory effect on the sphere formation, proliferation, and anchorage‐independent growth of colorectal cancer cells. In colorectal tumour xenografts and APC^min/+ transgenic mice, administration of salinomycin significantly reduced tumour growth and the expression of CSC‐related Wnt target genes including LGR5.

Conclusions and Implications

Our study suggested that salinomycin could suppress the growth of colorectal cancer by disrupting the β‐catenin/TCF complex and thus may be a promising agent for colorectal cancer treatment.

Salinomycin and its derivatives - A new class of multiple-targeted "magic bullets"

The history of drug development clearly shows the scale of painstaking effort leading to a finished product - a highly biologically active agent that would be at the same time no or little toxic to human organism. Moreover, the aim of modern drug discovery can move from "one-molecule one-target" concept to more promising "one-molecule multiple-targets" one, particularly in the context of effective fight against cancer and other complex diseases. Gratifyingly, natural compounds are excellent source of potential drug leads. One of such promising naturally-occurring drug candidates is a polyether ionophore - salinomycin (SAL). This compound should be identified as multi-target agent for two reasons. Firstly, SAL combines a broad spectrum of bioactivity, including antibacterial, antifungal, antiviral, antiparasitic and anticancer activity, with high selectivity of action, proving its significant therapeutic potential. Secondly, the multimodal mechanism of action of SAL has been shown to be related to its interactions with multiple molecular targets and signalling pathways that are synergistic for achieving a therapeutic anticancer effect. On the other hand, according to the Paul Ehrlich's "magic bullet" concept, invariably inspiring the scientists working on design of novel target-selective molecules, a very interesting direction of research is rational chemical modification of SAL. Importantly, many of SAL derivatives have been found to be more promising as chemotherapeutics than the native structure. This concise review article is focused both on the possible role of SAL and its selected analogues in future antimicrobial and/or cancer therapy, and on the potential use of SAL as a new class of multiple-targeted "magic bullet" because of its multimodal mechanism of action.

Prevention of breast cancer by dietary polyphenols-role of cancer stem cells

Breast cancer is a common malignancy with poor prognosis. Cancer cells are heterogeneous and cancer stem cells (CSCs) are primarily responsible for tumor relapse, treatment-resistance and metastasis, so for breast cancer stem cells (BCSCs). Diets are known to be associated with carcinogenesis. Food-derived polyphenols are able to attenuate the formation and virulence of BCSCs, implying that these compounds and their analogs might be promising agents for preventing breast cancer. In the present review, we summarized the origin and surface markers of BCSCs and possible mechanisms responsible for the inhibitory effects of polyphenols on BCSCs. The suppressive effects of common dietary polyphenols against BCSCs, such as curcumin, epigallocatechin gallate (EGCG) and related polyphenolic compounds were further discussed.

Aglycone Polyether Nanchangmycin and Its Homologues Exhibit Apoptotic and Antiproliferative Activities against Cancer Stem Cells

The potential of the polyether salinomycin as an inhibitory agent against cancer stem cells has attracted interest in this family of compounds. In this study, we found that the aglycone polyether nanchangmycin and its homologues show promising activities against breast cancer stem cells as well as 38 other different types of cancer cells by in vitro assays. We found that aglycone polyethers caused elevations in calcium levels, an accumulation of reactive oxygen species and mitochondrial inner membrane permeability to H⁺ and K⁺, resulting in the release of cytochrome c and apoptosis-inducing factor and the triggering of caspase-dependent apoptosis. Our analyses also indicate that aglycone polyethers are potent Wnt/β-catenin signaling inhibitors, blocking the Wnt pathway and resulting in reduced cell survival. Notably, the key autophagy-related proteins LC3A/B were also activated by aglycone polyether treatment. Furthermore, nanchangmycin showed inhibitory effects toward somatic tumors developed from MCF-7 paclitaxel-resistant breast cancer cells injected into BALB/c mice. Our study not only provides promising candidates for therapy against cancer stem cells but also provides the groundwork for identifying stronger therapeutic agents among the natural polyether compounds.

Ionophores: Potential Use as Anticancer Drugs and Chemosensitizers

Ion homeostasis is extremely important for the survival of both normal as well as neoplastic cells. The altered ion homeostasis found in cancer cells prompted the investigation of several ionophores as potential anticancer agents. Few ionophores, such as Salinomycin, Nigericin and Obatoclax, have demonstrated potent anticancer activities against cancer stem-like cells that are considered highly resistant to chemotherapy and responsible for tumor relapse. The preclinical success of these compounds in in vitro and in vivo models have not been translated into clinical trials. At present, phase I/II clinical trials demonstrated limited benefit of Obatoclax alone or in combination with other anticancer drugs. However, future development in targeted drug delivery may be useful to improve the efficacy of these compounds. Alternatively, these compounds may be used as leading molecules for the development of less toxic derivatives.

The Molecular Basis for Inhibition of Stemlike Cancer Cells by Salinomycin

Tumors are phenotypically heterogeneous and include subpopulations of cancer cells with stemlike properties. The natural product salinomycin, a K⁺-selective ionophore, was recently found to exert selectivity against such cancer stem cells. This selective effect is thought to be due to inhibition of the Wnt signaling pathway, but the mechanistic basis remains unclear. Here, we develop a functionally competent fluorescent conjugate of salinomycin to investigate the molecular mechanism of this compound. By subcellular imaging, we demonstrate a rapid cellular uptake of the conjugate and accumulation in the endoplasmic reticulum (ER). This localization is connected to induction of Ca²⁺ release from the ER into the cytosol. Depletion of Ca²⁺ from the ER induces the unfolded protein response as shown by global mRNA analysis and Western blot analysis of proteins in the pathway. In particular, salinomycin-induced ER Ca²⁺ depletion up-regulates C/EBP homologous protein (CHOP), which inhibits Wnt signaling by down-regulating β-catenin. The increased cytosolic Ca²⁺ also activates protein kinase C, which has been shown to inhibit Wnt signaling. These results reveal that salinomycin acts in the ER membrane of breast cancer cells to cause enhanced Ca²⁺ release into the cytosol, presumably by mediating a counter-flux of K⁺ ions. The clarified mechanistic picture highlights the importance of ion fluxes in the ER as an entry to inducing phenotypic effects and should facilitate rational development of cancer treatments.

Salinomycin, as an autophagy modulator-- a new avenue to anticancer: a review

Since Salinomycin (Sal) emerged its ability to target breast cancer stem cells in 2009, numerous experiments have been carried out to test Sal’s anticancer effects. What deserve to be mentioned is that Sal can efficiently induce proliferation inhibition, cell death and metastasis suppression against human cancers from different origins both in vivo and in vitro without causing serious side effects as the conventional chemotherapeutical drugs on the body. There may be novel cell death pathways involving the anticancer effects of Sal except the conventional pathways, such as autophagic pathway. This review is focused on how autophagy involves the effects of Sal, trying to describe clearly and systematically why autophagy plays a vital role in predominant anticancer effects of Sal, including its distinctive characteristic. Based on recent advances, we present evidence that a dual role of Sal involving in autophagy may account for its unique anticancer effects - the preference for cancer cells. Further researches are required to confirm the authenticity of this suppose in order to develop an ideal anticancer drug.

Synergistic induction of apoptosis by salinomycin and gefitinib through lysosomal and mitochondrial dependent pathway overcomes gefitinib resistance in colorectal cancer

Here, we showed the antibiotic salinomycin (SAL) combined with GEF exerted synergistic cytotoxicity effects in colorectal cancer cells irrespective of their EGFR and KRAS status, with a relatively low toxicity to normal cells. Additionally, combination of the two drugs overcame Ras-induced resistance and the acquired resistance to GEF. Further, we identified a new potential mechanism of this cooperative interaction by showing that GEF and SAL acted together to enhance production of reactive oxygen species (ROS), loss of mitochondrial membrane potential (MMP) and lysosomal membrane potential (LMP). And the ROS contributed the loss of MMP and LMP. We also found that GEF and SAL acted in concert to induce apoptosis via a mitochondrial-lysosomal cross-talk and caspase-independent pathway triggered by cathepsin B and D. Lastly, SAL in combination with GEF sensitized GEF-resistant cells to GEF in a nude mouse xenograft model. This novel combination treatment might provide a potential clinical application to overcome GEF resistance in colorectal cancer.

Salinomycin overcomes acquired tamoxifen resistance through AIB1 and inhibits cancer cell invasion in endocrine resistant breast cancer

Salinomycin is a monocarboxylic polyether ionophore isolated from Streptomyces albus. It has been widely used as an antibiotic in veterinary medicine in poultry. A recent study demonstrated that salinomycin selectively inhibits human breast cancer stem cells; one possible mechanism of tamoxifen resistance. Our results show that salinomycin is effective in inhibiting MCF-7/LCC2 and MCF-7/LCC9 cell lines which are well-established endocrine resistant cells and has a synergistic effect in combination with tamoxifen using MTT proliferation assay. The inhibitory effect of salinomycin on the reduction of critical ER co-activator; amplified breast 1 (AIB1) mRNA and protein expression is overcoming tamoxifen resistance. Moreover, salinomycin significantly inhibits cell invasion in Matrigel invasion assay. The effect was mediated at least in part by the decrease of matrix metalopeptidase 9 (MMP-9) which is one critical enzyme facilitated in the cell invasion process. In conclusion, salinomycin should be developed as a novel agent used alone or in combination for endocrine-resistant breast cancer.

Breast cancer stem cells as last soldiers eluding therapeutic burn: A hard nut to crack

Cancer stem cells (CSCs) are found in many cancer types, including breast carcinoma. Breast cancer stem cells (BCSCs) are considered as seed of cancer formation and they are associated with metastasis and genotoxic drug resistance. Several studies highlighted the presence of BCSCs in tumor microenvironment and they are accentuated with several carcinoma events including metastasis and resistance to genotoxic drugs and they also rebound after genotoxic burn. Stemness properties of a small population of cells in carcinoma have provided clues regarding the role of tumor microenvironment in tumor pathophysiology. Hence, insights in cancer stem cell biology with respect to molecular signaling, genetics and epigenetic behavior of CSCs have been used to modulate tumor drug resistance due to genotoxic drugs and signaling protein inhibitors. This review summarizes major scientific breakthroughs in understanding the contribution of BCSCs towards tumor's capability to endure destruction inflicted by molecular as well as genotoxic drugs.

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.

HER2 in Breast Cancer Stemness: A Negative Feedback Loop towards Trastuzumab Resistance

HER2 receptor tyrosine kinase that is overexpressed in approximately 20% of all breast cancers (BCs) is a poor prognosis factor and a precious target for BC therapy. Trastuzumab is approved by FDA to specifically target HER2 for treating HER2+ BC. However, about 60% of patients with HER2+ breast tumor develop de novo resistance to trastuzumab, partially due to the loss of expression of HER2 extracellular domain on their tumor cells. This is due to shedding/cleavage of HER2 by metalloproteinases (ADAMs and MMPs). HER2 shedding results in the accumulation of intracellular carboxyl-terminal HER2 (p95HER2), which is a common phenomenon in trastuzumab-resistant tumors and is suggested as a predictive marker for trastuzumab resistance. Up-regulation of the metalloproteinases is a poor prognosis factor and is commonly seen in mesenchymal-like cancer stem cells that are risen during epithelial to mesenchymal transition (EMT) of tumor cells. HER2 cleavage during EMT can explain why secondary metastatic tumors with high percentage of mesenchymal-like cancer stem cells are mostly resistant to trastuzumab but still sensitive to lapatinib. Importantly, many studies report HER2 interaction with oncogenic/stemness signaling pathways including TGF-β/Smad, Wnt/β-catenin, Notch, JAK/STAT and Hedgehog. HER2 overexpression promotes EMT and the emergence of cancer stem cell properties in BC. Increased expression and activation of metalloproteinases during EMT leads to proteolytic cleavage and shedding of HER2 receptor, which downregulates HER2 extracellular domain and eventually increases trastuzumab resistance. Here, we review the hypothesis that a negative feedback loop between HER2 and stemness signaling drives resistance of BC to trastuzumab.

Salinomycin: A new paradigm in cancer therapy

The primary hurdle in the treatment of cancer is acquisition of resistance by the tumor cells toward multiple drugs and selectively targeting the cancer stem cells. This problem was overcome by the chemotherapeutic property of recently discovered drug salinomycin. Exact mechanism of action of salinomycin is not yet known, but there are multiple pathways by which salinomycin inhibits tumor growth. Salinomycin decreases the expression of adenosine triphosphate-binding cassette transporter in multidrug resistance cells and interferes with Akt signaling pathway, Wnt/β-catenin, Hedgehog, and Notch pathways of cancer progression. Salinomycin selectively targets cancer stem cells. The potential of salinomycin to eliminate both cancer stem cells and therapy-resistant cancer cells may characterize the compound as a novel and an efficient chemotherapeutic drug.

Anti-Cancer Stem-like Cell Compounds in Clinical Development - An Overview and Critical Appraisal

Cancer stem-like cells (CSC) represent a subpopulation of tumor cells with elevated tumor-initiating potential. Upon differentiation, they replenish the bulk of the tumor cell population. Enhanced tumor-forming capacity, resistance to antitumor drugs, and metastasis-forming potential are the hallmark traits of CSCs. Given these properties, it is not surprising that CSCs have become a therapeutic target of prime interest in drug discovery. In fact, over the last few years, an enormous number of articles describing compounds endowed with anti-CSC activities have been published. In the meanwhile, several of these compounds and also approaches that are not based on the use of pharmacologically active compounds (e.g., vaccination, radiotherapy) have progressed into clinical studies. This article gives an overview of these compounds, proposes a tentative classification, and describes their biological properties and their developmental stage. Eventually, we discuss the optimal clinical setting for these compounds, the need for biomarkers allowing patient selection, the redundancy of CSC signaling pathways and the utility of employing combinations of anti-CSC compounds and the therapeutic limitations posed by the plasticity of CSCs.

Breast cancer stem cell selectivity of synthetic nanomolar-active salinomycin analogs

Background

Cancer stem cells (CSCs) have been invoked in resistance, recurrence and metastasis of cancer. Consequently, curative cancer treatments may be contingent on CSC selective approaches. Of particular interest in this respect is the ionophore salinomycin, a natural product shown to be 100-fold more active against CSCs than clinically used paclitaxel. We have previously reported that synthetic salinomycin derivatives display increased activity against breast cancer cell lines. Herein we specifically investigate the CSC selectivity of the most active member in each class of C20-O-acylated analogs as well as a C1-methyl ester analog incapable of charge-neutral metal ion transport.

Methods

JIMT-1 breast cancer cells were treated with three C20-O-acylated analogs, the C1-methyl ester of salinomycin, and salinomycin. The effects of treatment on the CSC-related CD44⁺/CD24⁻ and the aldehyde dehydrogenase positive (ALDH⁺) populations were determined using flow cytometry. The survival ability of CSCs after treatment was investigated with a colony formation assay under serum free conditions. The effect of the compounds on cell migration was evaluated using wound-healing and Boyden chamber assays. The expression of vimentin, related to mesenchymal traits and expression of E-cadherin and β-catenin, related to the epithelial traits, were investigated using immunofluorescence microscopy.

Results

Treatment with each of the three C20-acylated analogs efficiently decreased the putative CSC population as reflected by reduction of the CD44⁺/CD24⁻ and ALDH⁺ populations already at a 50 nM concentration. In addition, colony forming efficiency and cell migration were reduced, and the expression of the epithelial markers E-cadherin and β-catenin at the cell surface were increased. In contrast, salinomycin used at the same concentration did not significantly influence the CSC population and the C1-methyl ester was inactive even at a 20 μM concentration.

Conclusions

Synthetic structural analogs of salinomycin, previously shown to exhibit increased activity against cancer cells, also exhibited improved activity against CSCs across several assays even at nanomolar concentrations where salinomycin was found inactive. The methyl ester analog of salinomycin, incapable of charge-neutral metal ion transport, did not show activity in CSC assays, lending experimental support to ionophoric stress as the molecular initiating event for the CSC effects of salinomycin and related structures.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2142-3) contains supplementary material, which is available to authorized users.

Salinomycin suppresses TGF-β1-induced epithelial-to-mesenchymal transition in MCF-7 human breast cancer cells

Epithelial-to-mesenchymal transition (EMT) is the major cause of breast cancer to initiate invasion and metastasis. Salinomycin (Sal) has been found as an effective chemical compound to kill breast cancer stem cells. However, the effect of Sal on invasion and metastasis of breast cancer is unclear. In the present study, we showed that Sal reversed transforming growth factor-β1 (TGF-β1) induced invasion and metastasis accompanied with down-regulation of MMP-2 by experiments on human breast cancer cell line MCF-7. Sal was able to inhibit TGF-β1-induced EMT phenotypic transition and the activation of key signaling molecules involved in Smad (p-Smad2/3,Snail1) and non-Smad (β-catenin, p-p38 MAPK) signals which cooperatively regulate the induction of EMT. Importantly, in a series of breast cancer specimens, we found strong correlation among E-cadherin expression, β-catenin expression, and the lymph node metastatic potential of breast cancer. Our research suggests that Sal is promised to be a chemotherapeutic drug by suppressing the metastasis of breast cancer.

Cancer Stem and Progenitor-Like Cells as Pharmacological Targets in Breast Cancer Treatment

The present review is focused on the current role of neoplastic stem and progenitor-like cells as primary targets in the pharmacotherapy of cancer as well as in the development of new anticancer drugs. We begin by summarizing the main characteristics of these tumor-initiating cells and key concepts that support their participation in therapeutic failure. In particular, we discuss the differences between the major carcinogenesis models (ie, clonal evolution vs cancer stem cell (CSC) model) with emphasis on breast cancer (given its importance to the study of CSCs) and their implications for the development of new treatment strategies. In addition, we describe the main ways to target these cells, including the main signaling pathways that are more activated or altered in CSCs. Finally, we provide a comprehensive compilation of the most recently tested drugs.

TMPRSS4 induces cancer stem cell-like properties in lung cancer cells and correlates with ALDH expression in NSCLC patients

Metastasis involves a series of changes in cancer cells that promote their escape from the primary tumor and colonization to a new organ. This process is related to the transition from an epithelial to a mesenchymal phenotype (EMT). Recently, some authors have shown that migratory cells with an EMT phenotype share properties of cancer stem cells (CSCs), which allow them to form a new tumor mass. The type II transmembrane serine protease TMPRSS4 is highly expressed in some solid tumors, promotes metastasis and confers EMT features to cancer cells. We hypothesized that TMPRSS4 could also provide CSC properties. Overexpression of TMPRSS4 reduces E-cadherin and induces N-cadherin and vimentin in A549 lung cancer cells, supporting an EMT phenotype. These changes are accompanied by enhanced migration, invasion and tumorigenicity in vivo. TMPRSS4 expression was highly increased in a panel of lung cancer cells cultured as tumorspheres (a typical assay to enrich for CSCs). H358 and H441 cells with knocked-down TMPRSS4 levels were significantly less able to form primary and secondary tumorspheres than control cells. Moreover, they showed a lower proportion of ALDH+ cells (examined by FACS analysis) and lower expression of some CSC markers than controls. A549 cells overexpressing TMPRSS4 conferred the opposite phenotype and were also more sensitive to the CSC-targeted drug salinomycin than control cells, but were more resistant to regular chemotherapeutic drugs (cisplatin, gemcitabine and 5-fluorouracil). Analysis of 70 NSCLC samples from patients revealed a very significant correlation between TMPRSS4 expression and CSC markers ALDH (p = 0.0018) and OCT4 (p = 0.0004), suggesting that TMPRSS4 is associated with a CSC phenotype in patients' tumors. These results show that TMPRSS4, in addition to inducing EMT, can also promote CSC features in lung cancer; therefore, CSC-targeting drugs could be an appropriate treatment for TMPRSS4+ tumors.