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Abstract

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.

Keywords

Salinomycin cancer cancer stem cells multidrug resistance

Introduction

Salinomycin was isolated from the Streptomyces albus species of bacterium and produced by tank fermentation technology.¹ This monocarboxylic polyether ionophore is a 751-Da antibiotic (Figure 1) and is weakly acidic in nature. Salinomycin is a membrane ionophore, which shows affinity for alkali ions, preferentially for potassium ions.² Salinomycin eases the ion flux through cytoplasmic membrane and the mitochondrial membrane. It works as a mobile carrier and discharges K⁺ rapidly.^1,3 For a long time, salinomycin has been used as an anticoccidial agent in the poultry industries. Coccidiosis is a single-cell parasitic disease of the intestinal tract, which leads to huge economic loss. Salinomycin improves mortality, weight gain, and lesion score at different concentrations. Low dose of salinomycin was as effective as higher dose of monensin, another polyether ionophore, in controlling coccidiosis.⁴ Salinomycin also has antimalarial property. Plasmodium berghei–infected rats showed complete destruction of parasite when 20 mg/kg of salinomycin was given subcutaneously or 80 mg/kg of salinomycin was given orally. In fact, low dose of salinomycin was able to destroy asexual stage of P. berghei.⁵ In sheep, rumen fungal count was diminished after salinomycin treatment, but restored when the treatment was halted.⁶

Figure 1.

Structure of salinomycin.

Recently, Gupta et al. unveiled that salinomycin selectively kills breast cancer stem cells (CSCs). Salinomycin activity toward CSCs is 100 folds higher than the comparatively conventional chemotherapeutic drug paclitaxel.⁷ Previously, Zhou et al.⁸ and Antoszczak and Huczynski⁹ discussed about anticancer aspects of salinomycin and their selective targeting to cancer stem cells. P-glycoprotein (P-gp) is a class of adenosine triphosphate (ATP)-binding cassette (ABC) transporter that facilitates movement of drug out of cells making them multidrug resistant. Salinomycin is a potent inhibitor of P-gp, which makes cells sensitive to chemotherapeutic drugs.¹⁰ Salinomycin exhibits antibacterial activity, especially against Gram-positive bacteria, including various antibiotic-resistant species of Streptomyces. Antifungal, antiparasitic, antiviral, and anti-inflammatory activities have also been reported for this molecule.¹¹ Various studies demonstrated the anticancer potential of salinomycin in different cancers. In this review, we have discussed the challenges in cancer treatment and the mechanisms by which salinomycin overcomes them.

Challenges in cancer treatment

Multidrug resistance

Multidrug resistance (MDR) is a phenomenon by which cancer cells escape the cytotoxic effect of various chemotherapeutic agents.¹² MDR in cancer cells is a major problem and sensitizing them toward chemotherapeutic drugs is a challenging task.¹³ MDR is a strategy exhibited by diverse type of cancers involving cellular and non-cellular mechanisms for resistance to survive.¹⁴ Major mechanisms of MDR in cancer cells are energy-dependent efflux of drug, driven by P-gp pump.^12,13 In humans, P-gp is encoded by MDR1 gene and is one of the members of ABC family which is an ATP-dependent transporters.¹⁵ According to Goldstein, MDR1 RNA level was elevated in more than 400 human cancers like colon, kidney, liver, pancreas, and adrenal gland, as well as in chronic myelogenous leukemia and non-small-cell lung carcinoma with neuroendocrine properties. However, MDR1 expression was occasionally elevated in other untreated cancers, like neuroblastoma, acute lymphocytic leukemia in adults, acute non-lymphocytic leukemia in adults, and indolent non-Hodgkin’s lymphoma.¹⁶ There are other subfamilies of ABC transporters which confer drug resistance, such as breast cancer resistance protein (BCRP) and MDR proteins (MRPs). BCRP is a G-subfamily member of ABC transporter that plays an important role in efflux of anticancer drugs like methotrexate, mitoxantrone, indolocarbazole topoisomerase I inhibitors, camptothecin, flavopiridol, and quinazoline ErbB1 inhibitors, whereas MRP transports anionic drugs across membrane.^17,18

Cells acquire resistance by the following three major mechanisms: (1) water-soluble drugs like cisplatin and folate antagonists are less absorbed by cells. (2) Variety of changes in cells like increased DNA repair, changes in the cell-cycle protein, and reduced level of apoptosis. (3) Efflux of hydrophobic drug via ABC transporter. In humans, ABC transporter is encoded by 48 genes.¹⁹ In breast cancer, ABC transporters play vital role in making it multidrug resistant. High-level expression of ABC transporter decreases chemotherapeutic drug response in cancer cell lines.²⁰ Kickhoefer et al.¹³ found involvement of vault protein in MDR of cancer cells. According to Nygren et al.,²¹ MDR can be generated because of the elevated intracellular calcium ion level in small-cell lung cancer cells. In some MDR cell lines, transporter associated with antigen processing (TAP) is more expressed. It was found that TAP is overexpressed in parallel with major histocompatibility complex class I (MHC I), which helps in antigen presentation on endoplasmic reticulum membrane. Increased expression levels of TAP and MHC I confer resistance to cells.²²

Cancer stem cells and its role in cancer

Cancer stem cells are “stem-like” cells within a tumor that “possess the capacity of self-renewal and can produce the heterogeneous lineages of cancer cells that complete the tumor.”²³ These cells facilitate tumor maintenance and are also referred to as tumorigenic cells or tumor-initiating cells.²⁴ First, these cells develop resistance to chemotherapy and radiation due to their ability to regenerate, accumulate mutations, and differentiate into chemoresistant cells and second because of their quiescent behavior which protects them from cytotoxic therapy that targets rapidly dividing cells. Therefore, CSCs are primarily responsible for relapse and poor survival in various cancers. CSCs can evade the chemopreventive strategies for cancer treatment, and hence, it is necessary to identify drugs that selectively target this population^25,26 (Figure 2).

Figure 2.

Schematic representation of CSC targeting in chemotherapeutic approach and its importance.

Cell surface markers (immunophenotype) help in phenotypically distinguishing stem cells from their differentiated progeny. Different patterns of cell surface markers are expressed in CSCs of different tumor types. Various putative surface biomarkers include CD44, CD24, LGR5, and CD44v6. ALDH1A1 and ALDH1A3 are functional biomarkers. These cells show elevated levels of stem cell–associated transcription factors like Nanog, Oct4, Sox2, Hes1, and Klf4.²⁷ CD34⁺ and CD38⁻ leukemia cells, which overexpress ALDH1 (aldehyde dehydrogenase 1), show resistance toward cyclophosphamide.²⁸ Colorectal cancer stem cells express CD133 and CD44 cell surface biomarkers; high expression of these biomarkers is associated with metastasis and worse prognosis.²⁹ Pancreatic cancer stem cells expressing CD44⁺/CD24⁺/ESA⁺, CD133⁺ and ALDH1 are the most common cancer stem cell markers.³⁰

Similar signaling pathways that occur in normal stem cells regulate the self-renewal in CSCs. Wnt, Hedgehog, NF-κB, Notch, and PI3K/AKT/mTOR are the major pathways that play a pivotal role in CSC self-renewal and lineage fate regulation.^23,31 Suppression of GSK3β activity was shown to be critical for maintenance of murine pluripotent stem cells.³² Wnt signaling inhibition by Axin decreases hematopoietic stem cell (HSC) proliferation. Stimulation of sonic hedgehog (Shh) pathway in human HSC in vitro culture (CD34⁺Lin⁻CD38⁻) exhibits increased self-renewal response. Role of Notch activation in self-renewal of HSCs was confirmed by Reya et al.³³ through Jagged-1 ligand culture that constantly expresses increased amount of primitive progenitor activity in vitro and in vivo. Shh is also linked to transcription factor NF-κB signaling. NF-κB activates overexpression of Shh in pancreatic cancer and accelerates pancreatic cancer cell proliferation.³⁴ Loss of tumor-suppressor gene phosphatase and tensin homolog (PTEN), which regulates PI3K signaling, has been shown to mediate AKT activation and increase stemness properties of CSC populations in prostate cancer.³⁵ Furthermore, crosstalk between PI3K/AKT and other pro-survival as well as mitogenic pathways has been shown to drive cancer growth.³⁶

Salinomycin as an anticancer agent

Antitumor property of salinomycin was revealed by Gupta et al. after high-throughput screening of 16,000 compound libraries. Salinomycin selectively targets breast CSCs and inhibits tumor proliferation.⁷ Salinomycin has shown anticancer property, specifically targets the CSCs of multiple types of cancer, and sensitizes MDR human cancer cells.⁸

Exciting effects of salinomycin on cancer

Gupta et al. showed that salinomycin inhibits growth of human breast cancer and decreased metastasis in non-obese diabetic (NOD)/severe combined immunodeficient (SCID) mice.⁷ Salinomycin caused apoptosis in human CD4⁺ T-cell of acute T-cell leukemia, MDR human MES-SA/Dx5 uterine sarcoma cells which express P-gp.²

Salinomycin mediates its anticancer effect by various mechanisms in different cancer types. It reduced the activity of ABC transporters in leukemia stem cells, inhibited Wnt signaling cascade in colorectal cancer cells,¹¹ inhibited growth and migration of prostate cancer cells,³⁷ and led to reactive oxygen species (ROS) accumulation in androgen-dependent and independent prostate cancer cells.³⁸ It reduced cisplatin-resistant mesenchymal-like subpopulation of squamous cell carcinoma cells expressing high or low levels of E-cadherin in vitro;³⁹ expression of apoptotic and anti-apoptotic genes is dysregulated in cisplatin-resistant SW620 colorectal cancer cells.²⁷ It also induced apoptosis in human hepatocellular carcinoma cells⁴⁰ and autophagy response in prostate and breast cancer cells.⁴¹

Salinomycin caused apoptosis in human nasopharyngeal carcinoma cells CNE-1, CNE-2, and CNE-2/DDP via caspase activation and destabilization of mitochondrial membrane potential.⁴² Time- and concentration-dependent cytotoxic effect of salinomycin was exhibited in LNM35 and A549 lung cancer cell lines⁴³ and human cholangiocarcinoma cells in vitro.⁴⁴ Lower salinomycin concentration reduced proliferation in high-density cell populations of Hs578T and MCF-7 breast cancer cells.⁴⁵ Salinomycin also acted on OVCAR-3 human ovarian cancer cells through caspase-mediated apoptosis without harming normal cells.⁴⁶ Recently, salinomycin demonstrated anti-angiogenic activity by VEGFR2 signaling pathway in gastric cancer xenografts.⁴⁷

How salinomycin works?

Mechanism of action of salinomycin as chemotherapeutic drug is not well understood. Various mechanisms of action and molecular targets of salinomycin have been summarized in this review (Figure 3; Table 1).

Figure 3.

Salinomycin interferes with cell-cycle regulation in cancer cells by inhibiting Skp-2 activity.

Table 1.

In vitro and in vivo studies elucidating molecular targets of salinomycin.

S. no.	Cancer type	Molecular targets of salinomycin	References
1	Breast cancer	Decreases metastasis in SCID mice. Killing of CSCs. Cell death by autophagy response. Damage cellular DNA by increasing DNA damage protein pH2AX and p53BP1. Inhibits CSCs in mammosphere of breast cancer patient and tumor growth in MCF-7 mammosphere tumor xenograft in nude mice by modulating hedgehog signaling. STAT3-mediated CSC targeting in triple-negative breast cancer.	Gupta et al.,⁷ Naujokat and Steinhart,²³ Jangamreddy et al.,⁴¹ He et al.,⁴⁸ An et al.⁴⁹ and Kim et al.⁵⁰
2	Prostate cancer	Decreases NF-κB-mediated CD44⁺ CSCs population and inhibition of growth and migration. Growth inhibition in LNCaP, DU-145, and PC-3 prostate cancer cells. ROS-mediated apoptosis and PARP cleavage in PC-3 cells. Cell death occurs through autophagy and mitophagy responses in PC-3 cells.	Ketola et al.,³⁷ Choi et al.,³⁸ and Jangamreddy et al.⁴¹
3	Colorectal cancer	Targets Wnt signaling in human colon adenocarcinoma cell line LoVo. ROS-mediated apoptosis in cisplatin-resistant SW620 cells.	Huczynski et al.¹¹ and Zhou et al.²⁷
4	Lung cancer	Caspase-mediated cell death and inhibition of cell migration and invasion in LNM35 and A549 human lung cancer cells. ER stress mediated autophagic response in A549 cells.	Arafat et al.⁴³ and Li et al.⁵¹
5	Hepatocellular carcinoma	Induces apoptosis by modulating intracellular Ca²⁺ and Wnt/β-catenin pathway in HepG2, SMMC-7721, and BEL-7402 cells in vitro and HepG2 orthotopic tumor in nude mice.	Wang et al.⁴⁰
6	Osteosarcoma	Inhibits osteosarcoma stem cells of U2OS, MG63, and SAOS2 in vitro and tumor regression of human osteosarcoma cells ZOS in nude mice.	Tang et al.⁵²
7	Head and neck cancer	Induces cell death and decreases CD44⁺ population in JLO-1 and UMSSC-10B in head and neck squamous cell carcinoma in vitro.	Kuo et al.⁵³
8	Ovarian cancer	Apoptosis induction in human ovarian epithelial carcinoma cell line A2780 through inhibition of multidrug resistance protein gp170. Caspase-mediated cell death in OVCAR-8 cell line in vitro. Stat3 inactivation and downregulates Skp-2 in OVCAR-8 ovarian cancer cell line and its multidrug-resistant derivative cells	Riccioni et al.,¹⁰ Kaplan and Teksen,⁴⁶ and Koo et al.⁵⁴
9	Burkitt’s lymphoma	Induces apoptosis in human CD4+ T-cell leukemia cells from human patient	Fuchs et al.²
10	Human acute lymphoblastic leukemia	Salinomycin enhances apoptosis in Jurkat cells when used in combination with vincristine.	Liu et al.⁵⁵
11	Human promyeloblastic leukemia	Inhibits expression of ABC transporters such as P-gp, BCRP, and MRP8 in human leukemia stem cell–like KG-1a cells	Fuchs et al.⁵⁶
12	Chronic lymphocytic leukemia	Targets Wnt signaling in cells isolated from patients of chronic lymphocytic leukemia.	Lu et al.⁵⁷
13	Squamous cell carcinomas	Salinomycin inhibits EMT phenomenon in OCTT2 and SCC9 squamous cell carcinoma cell lines. Reduction in E-cadherin low-cell population of OCTT2 and SCC9 tumor xenograft. Direct tumor xenograft from patients of human tumor originated from OCTT2 cells implanted in NSG mice results in tumor shrinkage and depletion in vimentin-positive cells after salinomycin treatment.	Basu et al.³⁹
14	Human cholangiocarcinoma	Salinomycin induces apoptosis and inhibits cell proliferation in human cholangiocarcinoma cells Mz-ChA-1, TFK-1, and EGI-1 in vitro.	Lieke et al.⁴⁴
15	Uterine sarcoma	Decrease P-gp expression in MDR human uterine sarcoma cells MES-SA/Dx5 expressing P-gp in vitro.	Fuchs et al.²
16	Human nasopharyngeal carcinoma (NPC)	Salinomycin causes cell death via caspase activation and destabilizing MMP in highly differentiated NPC cells CNE-1, in poorly differentiated NPC cells CNE-2, and in drug-resistant NPC cells CNE-2/DDP in vitro. Salinomycin suppresses CNE-2 tumor xenograft growth in nude mice (BALB/c-nu) in vivo.	Wu et al.⁴²
17	Gastric cancer	Inhibits angiogenesis by impairing VEGFR2 signaling in HUVEC cells in vitro. Reduces neovascularization in mice Matrigel plug assay in vivo. Inhibits STAT3 signaling and induces apoptosis in human gastric cancer SGC-7901 cells in vitro. Salinomycin reduces CD-31-positive endothelial cells and VEGF, p-VEGFR2, and p-STAT3 expression in SGC-7901 tumor xenograft growth in BALB/cA nude mice in vivo. Inhibits CD44⁺ Oct4⁺ gastric CSCs population in Wnt1-transfected AGS gastric cancer cells in nude mice	Li et al.⁴⁷ and Mao et al.⁵⁸
18	Human endometrial cancer	Inhibit proliferation, migration, invasion, and Wnt signaling in rat endometrial cells RENT4 and human endometrial cancer stem-like cells of Hec-1.	Kusunoki et al.⁵⁹
19	Tongue carcinoma	Induce cell death and autophagy response in LK0412 cells in vitro.	Jangamreddy et al.⁶⁰
20	Larynx cancer	Inhibit cell proliferation, specifically target CD44⁺ cells in vitro.	Jangamreddy et al.⁶⁰

Ionophoric action of salinomycin

Salinomycin shows cytotoxic effect by disrupting the balance of sodium and potassium ions across the cell membrane and the mitochondrial membrane ¹¹ (Figure 4). Rapid efflux of K⁺ from cell membrane and mitochondrial membrane led to apoptosis of cells.⁶¹ In rat liver, salinomycin caused efflux of alkali cations, especially K⁺ from mitochondrial membrane. Salinomycin inhibited mitochondrial oxidative phosphorylation without affecting the substrate-level phosphorylation.³

Figure 4.

Overview of multiple targets of salinomycin and its mechanism of action through acting on cancer cells and CSCs.

Selective targeting of CSCs by salinomycin

In a study, a chemical library screened by Gupta et al.,⁷ salinomycin was shown to decrease the percentage of CD44^high/CD24^low breast cancer stem cells by 20 folds. Salinomycin has been shown to kill CSCs in different types of human cancers. The breast CSC population was reduced by eliminating ALDH1⁺ and SOX2-positive cells. Salinomycin inhibited Oct4, Nanog, Sox2 expressing lung CSCs, and CD44⁺ expressing VCaP- and LNCaP-derived prostate CSCs. Salinomycin also killed CD133⁺ colorectal and pancreatic CSCs. Lower expression of E-cadherin makes cells drug resistant and promotes epithelial-to-mesenchymal transition (EMT). Salinomycin eliminated squamous cell carcinoma CSCs population by inhibiting low E-cadherin expressing cell proliferation. Interestingly, salinomycin inhibited tumorosphere formation in breast, lung, gastric, osteosarcoma, colorectal, and pancreatic CSCs.²³

Sensitization of MDR cells by salinomycin

Salinomycin inhibited P-gp, type of ABC transporter which is expressed in most of the cancers and involved in efflux of chemotherapeutic drugs.⁶² Human promyeloblastic leukemia KG-1a cells, which express ABC transporters, such as P-gp, BCRP, and MRP8, made them resistant toward many chemotherapeutic drugs. Polyether ionophore antibiotic salinomycin sensitized KG-1a cells for apoptosis.⁵⁶ Riccioni et al.¹⁰ reported that salinomycin also inhibited MRP gp170. Fuchs showed that salinomycin triggers apoptotic cascade, independent of the cell-cycle arrest, p53, and proteasome. Salinomycin induced apoptosis even in those cells which acquire resistance to the apoptosis by higher expression of ABC transporters, anti-apoptotic protein Bcl-2, or 26S proteasomes.²

Molecular pathways targeted by salinomycin

Salinomycin targeted prostate cancer cells by reducing activities of NF-κB pathway and ALDH. It induced oxidative stress and finally reduced CD44 cell population.³⁷ Salinomycin induced oxidative stress in PC-3 prostate cancer and declined pro-caspase 3 activity. However, it increased Bax/Bcl-2 ratio and PARP-1 cleavage.³⁸ Salinomycin also activated mitogen-activated protein (MAP) kinase pathway through ROS generation by involvement of Jun N-terminal kinase (JNK) pathway.⁶³ Salinomycin killed cisplatin-resistant colorectal cancer cells via ROS-mediated apoptosis.²⁷ It induced apoptosis in NCI/ADR-RES, DXR, and OVCAR-8 ovarian cancer cells through downregulation of S-phase kinase-associated protein-2 (Skp-2) and Stat3 inactivation. Skp-2 overexpressed in most of the cancer stem cells that target tumor-suppressor genes. Skp, cullin, and F-box together form SCF complex; the SCF complex and anaphase-promoting complex (APC) are major E3 ubiquitin ligases that regulate cyclin and cdk levels. However, negative cell-cycle regulators Cip and Kip inhibit cdk, leading to cell-cycle arrest. Skp-2 ubiquitinized Kip that induced cell-cycle progression. Salinomycin downregulated Skp-2, ultimately cell proliferation was stopped and apoptosis was induced⁵⁴ (Figure 5). Salinomycin inhibited proliferation of CD44⁺ Oct4⁺ gastric CSC population growth by targeting Wnt signaling proteins.⁵⁸ Salinomycin also decreased CD44(+)/CD24(−) breast CSC population through modulation of hedgehog signaling pathway in MCF-7 cells and MCF-7 mammosphere cells.^48,64 However, in MDA-MB-231 triple-negative breast cancer cells, CSC population was inhibited by STAT3 down-regulation.⁴⁹

Figure 5.

Schematic diagram representing salinomycin-induced ROS generation inside the cell which blocked oxidative phosphorylation in mitochondria; salinomycin acts as ionophore and causes mitochondrial and cytoplasmic K⁺ ions efflux which led to cell death.

DNA damage and autophagy induction by salinomycin

Salinomycin increased tumor-suppressor protein p53 and DNA damaging protein pH2AX and decreased cyclin D1 level, which led to cell-cycle arrest and high DNA damage. Comet assay and immunocytochemical staining of pH2AX protein confer salinomycin-mediated DNA damage. In Hs578T and MDA-MB231 cells, salinomycin reduced cyclin D1 expression, consequently leading to cell death.⁵⁰

In salinomycin-treated MCF-7 cells, autophagy pathways of cell death via vacuole formation have been observed. Salinomycin treatment to MCF-7 cells generates multiple vacuoles, which is confirmed in other cell lines like SW480, SW620, and RKO to check monodansyl cadaverine uptake, which is a specific autophagic marker. Salinomycin also led to ROS generation in above-mentioned cells. ROS like O₂N and H₂O₂ are responsible for autophagic response. Autophagy was also dependent on endoplasmic reticulum via the elevation in the level of DNA-damage-inducible transcript 3 (DDIT3) and activating transcription factor 4 (ATF4) after salinomycin treatment in non-small-cell lung carcinoma. Endoplasmic reticulum regulates the autophagy through ATF4, DDIT3, AKT1, and mechanistic target of rapamycin (MTOR) pathways which are modulated by salinomycin.⁵¹ Salinomycin worked effectively in glucose starvation and serum starvation conditions; since there is nutrient and oxygen deprivation in tumor, it worked efficiently to kill autophagic cancer cells.⁶⁰

Anti-angiogenic activity of salinomycin

Fibronectin promoted elongation of micro-vessels during angiogenesis in vitro.⁶⁵ Francis et al.⁶⁶ showed that fibronectin is required in early blood vessel development. Salinomycin reduced the level of fibronectin expression in human endometrial cancer (Hec-1-SP) cells.⁵⁹ In chronic lymphocytic leukemia cells with constitutive Wnt activation, nanomolar concentrations of salinomycin downregulated the expression of Wnt target gene, such as fibronectin.⁵⁷ VEGFR2 signaling plays crucial role in angiogenesis and tumor progression. Salinomycin inhibited VEGFR2 phosphorylation and STAT3 activation in Human Umbilical Vein Endothelial Cells (HUVEC). Therefore, salinomycin hampered VEGF-induced angiogenesis in gastric cancer.⁴⁷

Effect of salinomycin on EMT phenomenon

EMT allows cells to detach from the tissue and gain motility by losing its basoapical polarity. A series of transcriptional and post-translational events cause epithelial cells to acquire mesenchymal characteristics.⁶⁷ This process enables them to invade and form macroscopic metastases in different organs. Re-expression of E-cadherin causes cells to regain epithelial phenotype, a phenomenon called mesenchymal-to-epithelial transition (MET). The EMT-MET-related differentiation status is identified by the differential expression of markers of extracellular (fibronectin and matrix metalloproteinases) and cellular (vimentin and E-cadherin) localization.⁶⁸ Several studies conducted in vitro as well as in vivo suggest that salinomycin inhibits migratory and invasive potential of cancer cells.^59,69–73

Salinomycin was able to significantly reduce the metastasis and invasion abilities of bladder cancer cell line T24. Tumor tissues from rats inoculated with T24 cells showed high expression of E-cadherin and lower vimentin expression level in the salinomycin-treated group.⁶⁹ Salinomycin inhibited migratory and invasive potential of side population cells in human and rat endometrial cells Hec-1 and RK12V, respectively.⁵⁹ Another study reported that salinomycin was able to inhibit transforming growth factor-β1 (TGF-β1)-induced EMT phenotypic transition in human breast cancer cell line MCF-7 and activated key signaling molecules involved in Smad (p-Smad2/3 and Snail1) and non-Smad (β-catenin and p-p38 MAPK) signals which cooperatively regulate the induction of EMT.⁷³

Salinomycin works synergistically with conventional chemotherapeutic drugs to inhibit invasion and migration of cancer cells. Transcription factor ZEB1, known for promoting metastasis in cancers, is highly expressed in primary mantle cell lymphoma (MCL) orchestrated by active Wnt signaling. Salinomycin blocked Wnt signaling and downregulated ZEB1, thereby increasing the sensitivity of MCL cells to the cytotoxic effect of gemcitabine, cytarabine, and doxorubicin.⁷⁰ In combination with metformin, salinomycin was able to block TGFβ-induced EMT and inhibit EMT-induced cell migration in the two non-small-cell lung cancer cell lines A549 and HCC4006.⁷¹ Salinomycin inhibited doxorubicin-induced EMT by activation of FOXO3a which disrupted the interaction of β-catenin and TCF and suppressed its downstream targets like ZEB1, CyclinD1, and c-Myc, involved in doxorubicin-induced EMT in hepatocellular carcinoma cells.⁷²

Epigenetic modifications induced by salinomycin

Epigenetics governs normal cell maintenance and gene expression in mammalian cells. It is the study of alterations in gene expression without affecting prime DNA sequence. Deleterious epigenetic changes lead to malignant cellular transformation. Epigenetic regulation in cancer progression includes DNA methylation, covalent histone modifications, nucleosome positioning, and noncoding microRNAs which post-transcriptionally regulate gene expression.^74,75 High degree of histone hyperacetylation is associated with increased cell death and senescence.⁷⁶ Acetylation profiling of histone H3 and H4 in MDA-MB-231 cells showed a gradual increase in their acetylation status upon salinomycin treatment.⁷⁷

CSC-associated phenotypes including drug resistance are modulated by polycomb group (PcG) protein-mediated dynamic epigenetic alternations in histone methylation landscapes. TRAIL (ligand for DR4/DR5) resistance in colon CSCs may be attributed to the catalytic component of polycomb repressive complex 2, enhancer of zeste homolog 2 (EZH2)-mediated H3K27me3 in promoter region, thereby epigenetically repressing the expression of DR4/DR5. Salinomycin treatment increased the DR4/DR5 expression with a simultaneous decrease in EZH2 localization in the nucleus and hence reduced EZH2 recruitment and histone methylation near promoters, thereby restoring their TRAIL sensitivity.⁷⁸

Dysregulation of microRNAs is involved in many aspects of cancer progression including EMT, invasion, and stem cell properties. Microarray analysis of global microRNA expression in a putative head and neck squamous cell carcinoma stem cell culture JLO-1 treated with increasing doses of salinomycin revealed a set of microRNAs that were consistently upregulated or downregulated, among these microRNAs were miR-328 and miR-199a-3p, both with known roles in promoting drug sensitivity.^79–81 Expression of miR-203, associated with the inhibition of EMT, was downregulated.⁸² This suggests that the changes in microRNA expression may be mediated by salinomycin.

Effects of salinomycin derivative

Salinomycin reacts with many divalent metal ions to form new derivative which shows enhanced bioactivities such as antifungal, antibacterial, herbicidal, and anti-inflammatory. Sodium salinomycin (SalNa) formed novel derivative with Co(II), Cu(II), Ni(II), or Zn(II). Antibacterial activity improved especially against Gram-positive bacteria Bacillus cereus. These derivatives showed dose-dependent cell toxicity in human leukemia cell lines. Complexes with Co(II) and Cu(II) showed more prominent cytotoxic effect.⁸³

Huczynski et al. screened the anticancer activity of different salinomycin amide substitutes like n-butylamine, phenylamine, 4′-aminobenzo-15-crown-5, 3-morpholine-propylamine, benzylamine, tryptamine, and 3,6,9-trioxadecylamine, benzotriazole ester of salinomycin. Antiproliferative activities of all these compounds were evaluated against MDR phenotype of colon cancer cells LoVo and leukemia cells HL-60. Many derivatives of salinomycin showed less toxicity and stronger anticancer activity against doxorubicin-resistant colon cancer subline LoVo/DX than unmodified salinomycin.¹¹

The cell count of MDA-MB-231 breast cancer cells with CD44 expression was decreased when treated with hyaluronic acid–coated salinomycin nanoparticles indicating their effectiveness toward cancer stem cells.⁸⁴ The glioblastoma polysorbate 80–coated poly(lactic-co-glycolic acid)-encapsulated salinomycin nanoparticle worked effectively with enhanced cell death.⁸⁵

Advantage of salinomycin over other anticancer agents

A chemotherapeutic drug should show cytotoxic effect on cancer cells, but at the same time, growth of normal cells must remain unaffected. There are recent reports published on salinomycin preferentially acting on tumor cells over non-tumorigenic cells. Salinomycin induced cell-cycle arrest and decreased expression of anti-apoptotic genes in MCF-7 cells, thereby causing apoptotic cell death of tumor cells, whereas non-tumor cells were able to evade this toxic effect.⁸⁶

Salinomycin induced apoptosis in human T-cells of leukemia, but Fuchs et al.² reported that normal human T-lymphocytes were not harmed. Salinomycin is more potent in inhibiting P-gp than verapamil, which is a known drug for inhibiting ABC transporter.⁶² Recently, Huczynski et al.¹¹ studied salinomycin-mediated induction of apoptosis in tumor cells of different tissue origin and its ability to overcome MDR phenomenon.

High-density cancer cell populations were reduced by salinomycin at lower dose upon long time exposure. This dose helped in detachment of cells, and these detached cells undergo apoptosis in breast cancer cells like MCF-7 and Hs578T. The advantage of using lower dose with longer exposure is that salinomycin toxicity can be reduced and multidrug-resistant tumors can be conquered.⁴⁵

Salinomycin toxicity

Although salinomycin has great chemotherapeutic effect over large number of cancer cells, it has shown to be toxic in vivo. Some researches and case studies have been presented here.

Weight of mice in the salinomycin-treated group greatly reduced as compared to control group. Salinomycin toxicity to male reproductive system was revealed by Ojo et al. Weight of testis, seminal vesicle, and epididymis was reduced irreversibly. Lot of structural changes occurred in interstitium of the testis and seminiferous tubules. Oxidative stress was generated in testicular and epididymal tissues, and significant decrease in enzymes lactate dehydrogenase (LDH), superoxide dismutase (SOD), catalase (CAT), and reduced glutathione (GSH) was noticed.⁸⁷

Neuronal cells damaged by salinomycin treatment came in light by histopathologic analysis. Severe vacuolation occurred in dorsal root ganglia of neurons in horses. Horses developed ataxia, muscular atrophy, and persistent weakness.⁸⁸ Cardiac arrhythmia, tachycardia, recumbency, sudoresis, groaning, incoordination, and paddling movements of limbs were observed in horse after monensin poisoning; along with myoglobinuria, elevated serum creatine phosphokinase level was observed.⁸⁹ Once hybrid turkey hens accidently ingested sodium salinomycin in feed, which resulted in death of 34% of turkey hens.⁹⁰ Dorsal root ganglia and Schwann cells were also affected by salinomycin. Toxicity is due to elevated cytosolic Na⁺ concentrations, ultimately leading to increase in cytosolic Ca²⁺ level by means of Na⁺/Ca²⁺ exchangers in the both plasma membrane and mitochondria. Higher Ca²⁺ level induced caspase-dependent apoptosis.⁹¹

In pig, toxicity was seen after consumption of food containing 720 and 441 ppm of salinomycin. Pigs behaved abnormally, some were unwilling to stand, and elevated rectal temperatures were recorded, which finally led to death.⁹² Shortness of breath, dizziness, nausea, leg weakness, photophobia, and increased blood pressure are the symptoms of salinomycin toxicity in humans.

Amelioration of toxicity and synergistic effects

Cancer stem cells express certain markers specific to them. HER2_high and HER2_low mammosphere reveal markers like OCT4, SOX2, and NANOG. Salinomycin selectively targeted these CSCs by decreasing SOX2 expressing cell population. However, combinatorial treatment of salinomycin with anti-HER2 monoclonal antibody trastuzumab showed enhanced cell death, which could not be achieved by salinomycin or trastuzumab alone. This combination treatment was also effective on MCF-7, MDA-MB-231, and BT-474 cell lines.⁹³

Salinomycin combination with doxorubicin enhanced caspase activation and pause the cells in sub-G1 phase. Greater transcription level of p21, p53, PUMA, and NF-κB activity was achieved in co-treatment. Chemosensitivity of sarcoma cell line increased upon combined treatment of salinomycin and doxorubicin even at minute concentrations, so the toxic effect of salinomycin is reduced and relative cell growth rate decreased from 1.5 to 3 fold in the sarcoma cell lines SW872, A204 and HT-1080.⁹⁴ Combinatorial treatment of salinomycin and doxorubicin shows variability in their cytotoxic effect in different cell lines. This treatment showed significant reduction of cell number in MES-SA/Dx5 and MCF-7 cell lines, while HepG2 and MDA-MB231 showed a lower inhibition rate.⁵⁰

Salinomycin and gemcitabine eradicated pancreatic cancer cells. However, poly(ethylene glycol)-b-poly(lactic acid) micelles mediated salinomycin delivery in gemcitabine-resistant pancreatic cancer cells with increased efficacy.^95,96 Salinomycin significantly inhibited cell proliferation of imatinib-resistant Kit^lowCd44⁺ Cd34⁺ interstitial cells of Cajal stem cells. Treatment of salinomycin along with imatinib retarded cell division effectively than when these drugs were used alone.⁹⁷ Trace amount of salinomycin sensitized tumor cells when treated with antimitotic drugs like docetaxel, paclitaxel, colchicine, and vinblastine. Combination reduced salinomycin toxicity and increased apoptosis.⁴⁵ Salinomycin with cisplatin or paclitaxel revealed strong synergism for antitumor activity in head and neck squamous cell carcinoma stem cells.⁵³ MCF-7 xenografts and salinomycin-loaded PEG-b-PCL polymeric micelles combined with octreotide-modified paclitaxel-loaded PEG-b-PCL polymeric micelles showed stronger antitumor activity.⁹⁸

Lapatinib is an anticancer drug used in the treatment of breast cancer. Lapatinib or salinomycin combination therapy with adriamycin for breast cancer was capable of eliminating breast tumor–initiating cells by 50 fold. These two drugs significantly reduced mammosphere formation and proportion of ALDH1⁺ cells by 10–100 folds in CD44⁺/CD24⁻ breast tumor–initiating cells isolated from MCF-7 cells, primary cells, and epirubicin-resistant SKBR3 cells. Adriamycin and salinomycin together increased cell death by 32%–77% in breast tumor–initiating cells.⁹⁹ Activation of Akt led to cancer progression by the PI3K-mediated pathway, which can be impeded by the PI3K inhibitors like LY294002 and wortmannin. Combined treatment of salinomycin with PI3K inhibitors led to increased apoptosis as compared to single treatment.¹⁰⁰ Aydin developed a novel approach to deliver salinomycin in tumor cells. He synthesized Herceptin (HER)-immobilized nanoparticle made up of poly(lactic-co-glycolic acid) encapsulated with salinomycin to target breast cancer cells. Its small size and Herceptin immobilization efficiency made it suitable for uptake by MCF-7 cells.¹⁰¹

Tumor requires vascularization (angiogenesis) for productive growth and metastasis. Overexpression of HER2 in human tumor cells is closely associated with increased expression of vascular endothelial growth factor (VEGF) and angiogenesis.¹⁰² HER2 signaling upregulated angiogenesis at different levels and different mechanisms.¹⁰³ Combinatorial treatment of mammospheres with trastuzumab and salinomycin efficiently targets HER2-positive cancer cells and CSCs.⁹³ In gefitinib-resistant colorectal cancer, salinomycin synergies induce apoptosis through caspase-dependent and independent pathways and are involved in loss of membrane potential of lysosome and mitochondria.¹⁰⁴ Salinomycin was also found useful in treatment of T-cell acute lymphoblastic leukemia Jurkat cells by treating along with vincristine.⁵⁵ Histone deacetylase inhibitor and salinomycin together worked on CSCs of triple-negative breast cancer in NOD/severe combined immunodeficiency gamma mice model. This combination induced apoptosis, cell-cycle arrest, and modulation of EMT in breast CSCs.¹⁰⁵

Conclusion

Polyether ionophore antibiotic salinomycin has been used as a coccidiostat agent in the poultry industry for a long period, but now it has emerged as an effective anti-proliferative agent. There are very few drugs available in market which target CSCs. Salinomycin is known for selective killing CSCs and sensitizing tumor cells at a very low concentration. It has exhibited toxicity over a broad range of cancers. Salinomycin sensitized tumor cells for many chemotherapeutic drugs by inhibiting many MDR genes. Selective targeting and evasion of MDR phenomenon make salinomycin a promising anticancer drug. Clinical pilot studies conducted by Naujokat and Steinhart²³ revealed that salinomycin induced partial clinical regression of heavily pretreated and therapy-resistant cancers, particularly in combination with novel tumor-targeted drugs. Further clinical studies need to be conducted for evaluating the safety and effectiveness of this drug. Approaches for drug delivery, dose optimization, and amelioration of its systemic toxicity should be deciphered.

Footnotes

Acknowledgements

CSIR-CDRI communication Number for this article is 9452.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Miyazaki

Shibuya

Sugawara

. Salinomycin, a new polyether antibiotic. J Antibiot 1974; 27: 814–821.

Fuchs

Heinold

Opelz

. Salinomycin induces apoptosis and overcomes apoptosis resistance in human cancer cells. Biochem Biophys Res Commun 2009; 390: 743–749.

Mitani

Yamanishi

Miyazaki

. Salinomycin effects on mitochondrial ion translocation and respiration. Antimicrob Agents Chemother 1976; 9: 655–660.

Danforth

Ruff

Reid

. Anticoccidial activity of salinomycin in battery raised broiler chickens. Poult Sci 1977; 56: 926–932.

Mehlhorn

Ganster

Raether

. Effect of salinomycin-Na on malaria parasites (Plasmodium falciparum and P. berghei). Zentralbl Bakteriol Mikrobiol Hyg A 1984; 256: 305–313.

Cann

Kobayashi

Onoda

. Effects of some ionophore antibiotics and polyoxins on the growth of anaerobic rumen fungi. J Appl Bacteriol 1993; 74: 127–133.

Gupta

Onder

Jiang

. Identification of selective inhibitors of cancer stem cells by high-throughput screening. Cell 2009; 138: 645–659.

Zhou

Wang

Wong

. Salinomycin: a novel anti-cancer agent with known anti-coccidial activities. Curr Med Chem 2013; 20: 4095–4101.

Antoszczak

Huczynski

Anticancer activity of polyether ionophore-salinomycin. Anticancer Agents Med Chem 2015; 15: 575–591.

10.

Riccioni

Dupuis

Bernabei

. The cancer stem cell selective inhibitor salinomycin is a p-glycoprotein inhibitor. Blood Cells Mol Dis 2010; 45: 86–92.

11.

Huczynski

Janczak

Antoszczak

. Antiproliferative activity of salinomycin and its derivatives. Bioorg Med Chem Lett 2012; 22: 7146–7150.

12.

Zhang

Multidrug resistance and breast cancer. In: Aft R (ed.) Targeting new pathways and cell death in breast cancer, 2012, pp. 131–156.

13.

Kickhoefer

Rajavel

Scheffer

. Vaults are up-regulated in multidrug-resistant cancer cell lines. J Biol Chem 1998; 273: 8971–8974.

14.

Ullah

MF.

Cancer multidrug resistance (MDR): a major impediment to effective chemotherapy. Asian Pac J Cancer Prev 2008; 9: 1–6.

15.

Gottesman

MM.

Mechanisms of cancer drug resistance. Annu Rev Med 2002; 53: 615–627.

16.

Goldstein

Galski

Fojo

. Expression of a multidrug resistance gene in human cancers. J Natl Cancer Inst 1989; 81: 116–124.

17.

Doyle

Ross

DD.

Multidrug resistance mediated by the breast cancer resistance protein BCRP (ABCG2). Oncogene 2003; 22: 7340–7358.

18.

Borst

Evers

Kool

. A family of drug transporters: the multidrug resistance-associated proteins. J Natl Cancer Inst 2000; 92: 1295–1302.

19.

Szakacs

Paterson

Ludwig

. Targeting multidrug resistance in cancer. Nat Rev Drug Discov 2006; 5: 219–234.

20.

Wind

Holen

Multidrug resistance in breast cancer: from in vitro models to clinical studies. Int J Breast Cancer 2011; 2011: 967419.

21.

Nygren

Larsson

Gruber

. Doxorubicin selected multidrug-resistant small cell lung cancer cell lines characterised by elevated cytoplasmic Ca2+ and resistance modulation by verapamil in absence of P-glycoprotein overexpression. Br J Cancer 1991; 64: 1011–1018.

22.

Izquierdo

Neefjes

Mathari

. Overexpression of the ABC transporter TAP in multidrug-resistant human cancer cell lines. Br J Cancer 1996; 74: 1961–1967.

23.

Naujokat

Steinhart

Salinomycin as a drug for targeting human cancer stem cells. J Biomed Biotechnol 2012; 2012: 950658.

24.

Liu

Wang

Chen

. The prognostic role of a gene signature from tumorigenic breast-cancer cells. N Engl J Med 2007; 356: 217–226.

25.

Singh

Settleman

Emt, cancer stem cells and drug resistance: an emerging axis of evil in the war on cancer. Oncogene 2010; 29: 4741–4751.

26.

Karsten

Goletz

What makes cancer stem cell markers different?

SpringerPlus 2013; 2: 301.

27.

Zhou

Xue

. Salinomycin induces apoptosis in cisplatin-resistant colorectal cancer cells by accumulation of reactive oxygen species. Toxicol Lett 2013; 222: 139–145.

28.

Bao

Polk

Nordberg

. Comparative gene expression analysis of a chronic myelogenous leukemia cell line resistant to cyclophosphamide using oligonucleotide arrays and response to tyrosine kinase inhibitors. Leuk Res 2007; 31: 1511–1520.

29.

Sahlberg

Spiegelberg

Glimelius

. Evaluation of cancer stem cell markers CD133, CD44, CD24: association with AKT isoforms and radiation resistance in colon cancer cells. PLoS ONE 2014; 9: e94621.

30.

Fredebohm

Boettcher

Eisen

. Establishment and characterization of a highly tumourigenic and cancer stem cell enriched pancreatic cancer cell line as a well defined model system. PLoS ONE 2012; 7: e48503.

31.

Taipale

Beachy

PA.

The Hedgehog and Wnt signalling pathways in cancer. Nature 2001; 411: 349–354.

32.

Yeung

Esposito

Gandillet

. β-Catenin mediates the establishment and drug resistance of MLL leukemic stem cells. Cancer Cell 2010; 18: 606–618.

33.

Reya

Morrison

Clarke

. Stem cells, cancer, and cancer stem cells. Nature 2001; 414: 105–111.

34.

Nakashima

Nakamura

Yamaguchi

. Nuclear factor-kappaB contributes to hedgehog signaling pathway activation through sonic hedgehog induction in pancreatic cancer. Cancer Res 2006; 66: 7041–7049.

35.

Kim

Bae

Hong

. PTEN loss-mediated Akt activation increases the properties of cancer stem-like cell populations in prostate cancer. Oncology 2014; 87: 270–279.

36.

Aksamitiene

Kiyatkin

Kholodenko

BN.

Cross-talk between mitogenic Ras/MAPK and survival PI3K/Akt pathways: a fine balance. Biochem Soc Trans 2012; 40: 139–146.

37.

Ketola

Hilvo

Hyotylainen

. Salinomycin inhibits prostate cancer growth and migration via induction of oxidative stress. Br J Cancer 2012; 106: 99–106.

38.

Choi

H-J

Kim

K-Y

S-N

. Salinomycin-induced apoptosis in human prostate cancer cells, 2013, http://cdn.intechopen.com/pdfs/41811/InTech-Salinomycin_induced_apoptosis_in_human_prostate_cancer_cells.pdf

39.

Basu

Montone

Wang

. Detecting and targeting mesenchymal-like subpopulations within squamous cell carcinomas. Cell Cycle 2011; 10: 2008–2016.

40.

Wang

Dai

. Salinomycin inhibits proliferation and induces apoptosis of human hepatocellular carcinoma cells in vitro and in vivo. PLoS ONE 2012; 7: e50638.

41.

Jangamreddy

Ghavami

Grabarek

. Salinomycin induces activation of autophagy, mitophagy and affects mitochondrial polarity: differences between primary and cancer cells. Biochim Biophys Acta 1833; 2013: 2057–2069.

42.

Zhang

Huang

. Salinomycin inhibits proliferation and induces apoptosis of human nasopharyngeal carcinoma cell in vitro and suppresses tumor growth in vivo. Biochem Biophys Res Commun 2014; 443: 712–717.

43.

Arafat

Iratni

Takahashi

. Inhibitory effects of salinomycin on cell survival, colony growth, migration, and invasion of human non-small cell lung cancer a549 and lnm35: involvement of nag-1. PLoS ONE 2013; 8: e66931.

44.

Lieke

Ramackers

Bergmann

. Impact of salinomycin on human cholangiocarcinoma: induction of apoptosis and impairment of tumor cell proliferation in vitro. BMC Cancer 2012; 12: 466.

45.

Kim

. Lower salinomycin concentration increases apoptotic detachment in high-density cancer cells. Int J Mol Sci 2012; 13: 13169–13182.

46.

Kaplan

Teksen

Apoptotic effects of salinomycin on human ovarian cancer cell line (ovcar-3). Tumour Biol 2016; 37: 3897–3903.

47.

Liu

Shen

. Salinomycin exerts anti-angiogenic and anti-tumorigenic activities by inhibiting vascular endothelial growth factor receptor 2-mediated angiogenesis. Oncotarget 2016; 7: 26580–26592.

48.

Yan

. The Hedgehog signalling pathway mediates drug response of MCF-7 mammosphere cells in breast cancer patients. Clin Sci 2015; 129: 809–822.

49.

Kim

. Salinomycin promotes anoikis and decreases the CD44+/CD24− stem-like population via inhibition of STAT3 activation in MDA-MB-231 cells. PLoS ONE 2015; 10: e0141919.

50.

Kim

Chae

Kim

. Salinomycin sensitizes cancer cells to the effects of doxorubicin and etoposide treatment by increasing DNA damage and reducing p21 protein. Br J Pharmacol 2011; 162: 773–784.

51.

Zhong

. Salinomycin induces cell death with autophagy through activation of endoplasmic reticulum stress in human cancer cells. Autophagy 2013; 9: 1057–1068.

52.

Tang

Zhao

. Salinomycin inhibits osteosarcoma by targeting its tumor stem cells. Cancer Lett 2011; 311: 113–121.

53.

Kuo

Blair

Rahimy

. Salinomycin induces cell death and differentiation in head and neck squamous cell carcinoma stem cells despite activation of epithelial-mesenchymal transition and Akt. BMC Cancer 2012; 12: 556.

54.

Koo

Kim

Bae

. Salinomycin induces cell death via inactivation of Stat3 and downregulation of Skp2. Cell Death Dis 2013; 4: e693.

55.

Liu

Zhu

Liu

. Salinomycin enhances the apoptosis of t-cell acute lymphoblastic leukemia cell line Jurkat cells induced by vincristine. Zhongguo Shi Yan Xue Te Xue Za Zhi 2015; 23: 653–657.

56.

Fuchs

Daniel

Sadeghi

. Salinomycin overcomes ABC transporter-mediated multidrug and apoptosis resistance in human leukemia stem cell-like KG-1a cells. Biochem Biophys Res Commun 2010; 394: 1098–1104.

57.

Choi

. Salinomycin inhibits Wnt signaling and selectively induces apoptosis in chronic lymphocytic leukemia cells. Proc Natl Acad Sci U S A 2011; 108: 13253–13257.

58.

Mao

Fan

. Roles of Wnt/β-catenin signaling in the gastric cancer stem cells proliferation and salinomycin treatment. Cell Death Dis 2014; 5: e1039.

59.

Kusunoki

Kato

Tabu

. The inhibitory effect of salinomycin on the proliferation, migration and invasion of human endometrial cancer stem-like cells. Gynecol Oncol 2013; 129: 598–605.

60.

Jangamreddy

Jain

Hallbeck

. Glucose starvation-mediated inhibition of salinomycin induced autophagy amplifies cancer cell specific cell death. Oncotarget 2015; 6: 10134–10145.

61.

McCarthy

Cotter

TG.

Cell shrinkage and apoptosis: a role for potassium and sodium ion efflux. Cell Death Differ 1997; 4: 756–770.

62.

Kim

Yoon

. Salinomycin, a p-glycoprotein inhibitor, sensitizes radiation-treated cancer cells by increasing DNA damage and inducing G2 arrest. Invest New Drugs 2012; 30: 1311–1318.

63.

Verdoodt

Vogt

Schmitz

. Salinomycin induces autophagy in colon and breast cancer cells with concomitant generation of reactive oxygen species. PLoS ONE 2012; 7: e44132.

64.

Mao

. Salinomycin exerts anticancer effects on human breast carcinoma MCF-7 cancer stem cells via modulation of hedgehog signaling. Chem Biol Interact 2015; 228: 100–107.

65.

Nicosia

Bonanno

Smith

Fibronectin promotes the elongation of microvessels during angiogenesis in vitro. J Cell Physiol 1993; 154: 654–661.

66.

Francis

Goh

Hodivala-Dilke

. Central roles of alpha5beta1 integrin and fibronectin in vascular development in mouse embryos and embryoid bodies. Arterioscler Thromb Vasc Biol 2002; 22: 927–933.

67.

Kalluri

Weinberg

RA.

The basics of epithelial-mesenchymal transition. J Clin Invest 2009; 119: 1420–1428.

68.

Voulgari

Pintzas

Epithelial-mesenchymal transition in cancer metastasis: mechanisms, markers and strategies to overcome drug resistance in the clinic. Biochim Biophys Acta 2009; 1796: 75–90.

69.

Yuan

. Effect of salinomycin on metastasis and invasion of bladder cancer cell line T24. Asian Pac J Trop Med 2015; 8: 578–582.

70.

Sanchez-Tillo

Fanlo

Siles

. The EMT activator ZEB1 promotes tumor growth and determines differential response to chemotherapy in mantle cell lymphoma. Cell Death Differ 2014; 21: 247–257.

71.

Koeck

Amann

Huber

. The impact of metformin and salinomycin on transforming growth factor β-induced epithelial-to-mesenchymal transition in non-small cell lung cancer cell lines. Oncol Lett 2016; 11: 2946–2952.

72.

Zhou

Liang

Xue

. Salinomycin decreases doxorubicin resistance in hepatocellular carcinoma cells by inhibiting the β-catenin/TCF complex association via FOXO3a activation. Oncotarget 2015; 6: 10350–10365.

73.

Zhang

. Salinomycin suppresses TGF-β1-induced epithelial-to-mesenchymal transition in MCF-7 human breast cancer cells. Chem Biol Interact 2016; 248: 74–81.

74.

Sharma

Kelly

Jones

PA.

Epigenetics in cancer. Carcinogenesis 2010; 31: 27–36.

75.

Kanwal

Gupta

Epigenetic modifications in cancer. Clin Genet 2012; 81: 303–311.

76.

Gao

. Oridonin induces apoptosis and senescence in colorectal cancer cells by increasing histone hyperacetylation and regulation of p16, p21, p27 and c-myc. BMC Cancer 2010; 10: 610.

77.

Al Dhaheri

Attoub

Arafat

. Salinomycin induces apoptosis and senescence in breast cancer: upregulation of p21, downregulation of survivin and histone H3 and H4 hyperacetylation. Biochim Biophys Acta 2013; 1830: 3121–3135.

78.

Singh

Maheshwari

Arya

. Salinomycin targets EZH2 driven epigenetic repression of death receptors in colon cancer stem cells. In: NCRI cancer conferences, 2015, http://abstracts.ncri.org.uk/abstract/salinomycin-targets-ezh2-driven-epigenetic-repression-of-death-receptors-in-colon-cancer-stem-cells-2/

79.

Henry

Park

Jiang

. miR-199a-3p targets CD44 and reduces proliferation of CD44 positive hepatocellular carcinoma cell lines. Biochem Biophys Res Commun 2010; 403: 120–125.

80.

Fornari

Milazzo

Chieco

. MiR-199a-3p regulates mTOR and c-Met to influence the doxorubicin sensitivity of human hepatocarcinoma cells. Cancer Res 2010; 70: 5184–5193.

81.

Pan

Morris

AM.

MicroRNA-328 negatively regulates the expression of breast cancer resistance protein (BCRP/ABCG2) in human cancer cells. Mol Pharmacol 2009; 75: 1374–1379.

82.

Saini

Majid

Yamamura

. Regulatory role of mir-203 in prostate cancer progression and metastasis. Clin Cancer Res 2011; 17: 5287–5298.

83.

Ivanova

Pantcheva

Zhorova

. Synthesis, spectral properties, antibacterial and antitumor activity of salinomycin complexes with the transition metal ions Co(II), Ni(II), Cu(II) and Zn(II). J Chem Chem Eng 2012; 6: 551–562.

84.

Muntimadugu

Kumar

Saladi

. CD44 targeted chemotherapy for co-eradication of breast cancer stem cells and cancer cells using polymeric nanoparticles of salinomycin and paclitaxel. Colloids Surf B: Biointerfaces 2016; 143: 532–546.

85.

Tigli Aydin

Kaynak

Gumusderelioglu

Salinomycin encapsulated nanoparticles as a targeting vehicle for glioblastoma cells. J Biomed Mater Res A 2016; 104: 455–464.

86.

Niwa

D Epiro

Marques

. Salinomycin efficiency assessment in non-tumor (HB4a) and tumor (MCF-7) human breast cells. Naunyn Schmiedebergs Arch Pharmacol 2016; 389: 557–571.

87.

Ojo

Bhadauria

Rath

SK.

Dose-dependent adverse effects of salinomycin on male reproductive organs and fertility in mice. PLoS ONE 2013; 8: e69086.

88.

Aleman

Magdesian

Peterson

. Salinomycin toxicosis in horses. J Am Vet Med Assoc 2007; 230: 1822–1826.

89.

Bezerra

Driemeier

Loretti

. Monensin poisoning in Brazilian horses. Vet Hum Toxicol 1999; 41: 383–385.

90.

Van Assen

. A case of salinomycin intoxication in turkeys. Can Vet J 2006; 47: 256–258.

91.

Boehmerle

Endres

Salinomycin induces calpain and cytochrome c-mediated neuronal cell death. Cell Death Dis 2011; 2: e168.

92.

Plumlee

Johnson

Galey

FD.

Acute salinomycin toxicosis of pigs. J Vet Diagn Invest 1995; 7: 419–420.

93.

Oak

Kopp

Thakur

. Combinatorial treatment of mammospheres with trastuzumab and salinomycin efficiently targets her2-positive cancer cells and cancer stem cells. Int J Cancer 2012; 131: 2808–2819.

94.

Liffers

Tilkorn

Stricker

. Salinomycin increases chemosensitivity to the effects of doxorubicin in soft tissue sarcomas. BMC Cancer 2013; 13: 490.

95.

Zhang

Liang

Zhou

. Combination of salinomycin and gemcitabine eliminates pancreatic cancer cells. Cancer Lett 2011; 313: 137–144.

96.

Daman

Montazeri

Azizi

. Polymeric micelles of PEG-PLA copolymer as a carrier for salinomycin against gemcitabine-resistant pancreatic cancer. Pharm Res 2015; 32: 3756–3767.

97.

Bardsley

Horvath

Asuzu

. Kit(low) stem cells cause resistance to Kit/platelet-derived growth factor alpha inhibitors in murine gastrointestinal stromal tumors. Gastroenterology 2010; 139: 942–952.

98.

Zhang

Wang

. The eradication of breast cancer and cancer stem cells using octreotide modified paclitaxel active targeting micelles and salinomycin passive targeting micelles. Biomaterials 2012; 33: 679–691.

99.

Gong

Yao

Liu

. Markers of tumor-initiating cells predict chemoresistance in breast cancer. PLoS ONE 2010; 5: e15630.

100.

Kim

Choi

Kim

. Low amount of salinomycin greatly increases Akt activation, but reduces activated p70S6K levels. Int J Mol Sci 2013; 14: 17304–17318.

101.

Aydin

RS.

Herceptin-decorated salinomycin-loaded nanoparticles for breast tumor targeting. J Biomed Mater Res A 2013; 101: 1405–1415.

102.

Kumar

Yarmand-Bagheri

The role of HER2 in angiogenesis. Semin Oncol 2001; 28: 27–32.

103.

Alameddine

Otrock

Awada

. Crosstalk between HER2 signaling and angiogenesis in breast cancer: molecular basis, clinical applications and challenges. Curr Opin Oncol 2013; 25: 313–324.

104.

Zou

Nie

. Synergistic induction of apoptosis by salinomycin and gefitinib through lysosomal and mitochondrial dependent pathway overcomes gefitinib resistance in colorectal cancer. Oncotarget. Epub ahead of print 7 October 2015. DOI: 10.18632/oncotarget.5628.

105.

Kai

Kanaya

. Targeting breast cancer stem cells in triple-negative breast cancer using a combination of LBH589 and salinomycin. Breast Cancer Res Treat 2015; 151: 281–294.

Salinomycin: A new paradigm in cancer therapy

Abstract

Keywords

Introduction

Challenges in cancer treatment

Multidrug resistance

Cancer stem cells and its role in cancer

Salinomycin as an anticancer agent

Exciting effects of salinomycin on cancer

How salinomycin works?

Ionophoric action of salinomycin

Selective targeting of CSCs by salinomycin

Sensitization of MDR cells by salinomycin

Molecular pathways targeted by salinomycin

DNA damage and autophagy induction by salinomycin

Anti-angiogenic activity of salinomycin

Effect of salinomycin on EMT phenomenon

Epigenetic modifications induced by salinomycin

Effects of salinomycin derivative

Advantage of salinomycin over other anticancer agents

Salinomycin toxicity

Amelioration of toxicity and synergistic effects

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References