Cardiotonic steroids attenuate ERK phosphorylation and generate cell cycle arrest to block human hepatoma cell growth

Cinobufagin: a promising therapeutic agent for cancer

OBJECTIVES

Cinobufagin is a natural active ingredient isolated from the traditional Chinese medicine Venenum Bufonis (Chinese: Chansu), which is the dried secretion of the postauricular gland or skin gland of the Bufo gargarizans Cantor or Bufo melanostictus Schneider. There is increasing evidence indicating that cinobufagin plays an important role in the treatment of cancer. This article is to review and discuss the antitumor pharmacological effects and mechanisms of cinobufagin, along with a description of its toxicity and pharmacokinetics.

METHODS

The public databases including PubMed, China National Knowledge Infrastructure and Elsevier were referenced, and 'cinobufagin', 'Chansu', 'Venenum Bufonis', 'anticancer', 'cancer', 'carcinoma', and 'apoptosis' were used as keywords to summarize the comprehensive research and applications of cinobufagin published up to date.

KEY FINDINGS

Cinobufagin can induce tumour cell apoptosis and cycle arrest, inhibit tumour cell proliferation, migration, invasion and autophagy, reduce angiogenesis and reverse tumour cell multidrug resistance, through triggering DNA damage and activating the mitochondrial pathway and the death receptor pathway.

CONCLUSIONS

Cinobufagin has the potential to be further developed as a new drug against cancer.

Dual targets of lethal apoptosis and protective autophagy in liver cancer with periplocymarin elicit a limited therapeutic effect

Cardiac glycosides (CGs) are candidate anticancer agents that function by increasing [Ca²⁺]i to induce apoptotic cell death in several types of cancer cells. However, new findings have shown that the anti‑cancer effects of CGs involve complex cell‑signal transduction mechanisms. Hence, exploring the potential mechanisms of action of CGs may provide insight into their anti‑cancer effects and thus aid in the selection of the appropriate CG. Periplocymarin (PPM), which is a cardiac glycoside, is an active ingredient extracted from Cortex periplocae. The role of PPM was evaluated in HepG2 cells and xenografted nude mice. Cell proliferation, real‑time ATP rate assays, western blotting, cell apoptosis assays, short interfering RNA transfection, the patch clamp technique, electron microscopy, JC‑1 staining, immunofluorescence staining and autophagic flux assays were performed to evaluate the function and regulatory mechanisms of PPM in vitro. The in vivo activity of the PPM was assessed using a mouse xenograft model. The present study demonstrated that PPM synchronously activated lethal apoptosis and protective autophagy in liver cancer, and the initiation of autophagy counteracted the inherent pro‑apoptotic capacity and impaired the anti‑cancer effects. Specifically, PPM exerted a pro‑-apoptotic effect in HepG2 cells and activated macroautophagy by initiation of the AMPK/ULK1 and mTOR signaling pathways. Activation of macroautophagy counteracted the pro‑apoptotic effects of PPM, but when it was combined with an autophagy inhibitor, the anti‑cancer effects of PPM in mice bearing HepG2 xenografts were observed. Collectively, these results indicated that a self‑limiting effect impaired the pro‑apoptotic effects of PPM in liver cancer, but when combined with an autophagy inhibitor, it may serve as a novel therapeutic option for the management of liver cancer.

Chemically synthesized cinobufagin suppresses nasopharyngeal carcinoma metastasis by inducing ENKUR to stabilize p53 expression

Clinically, the metastasis of tumor cells is the key factor of death in patients with cancer. In this study, we used a model of metastatic nasopharyngeal carcinoma (NPC) to explore the effects of a new chemical, cinobufagin (CB), combined with cisplatin (DDP). We observed that chemically synthesized CB strongly decreased the metastasis of NPC. Furthermore, a better therapeutic effect was shown when CB was combined with DDP. Molecular analysis revealed that CB induced ENKUR expression by deregulating the PI3K/AKT pathway and suppressing c-Jun, an oncogenic transcriptional factor that binds to the ENKUR promoter and negatively modulated its expression in NPC. ENKUR as a tumor suppressor binds to MYH9 and decreases its expression by recruiting β-catenin via its enkurin domain to prevent its nuclear accumulation, which therefore suppresses c-Jun-induced MYH9 expression. Subsequently, downregulated MYH9 reduces the enlistment of E3 ligase UBE3A and thus decreases the UBE3A-mediated ubiquitination degradation of p53, a key tumor suppressor that decreases epithelial-mesenchymal transition (EMT). Clinical sample analysis demonstrated that the ENKUR expression level was significantly reduced in NPC tissues. Its decreased expression substantially promoted clinical progression and reflected poor prognosis for patients with NPC. This study demonstrated that CB induced ENKUR to repress the β-catenin/c-Jun/MYH9 signal and thus decreased UBE3A-mediated p53 ubiquitination degradation. As a result, the EMT signal was inactivated to suppress NPC metastasis.

Cinobufagin-induced DNA damage response activates G2/M checkpoint and apoptosis to cause selective cytotoxicity in cancer cells

Background

Processed extracts from toad skin and parotoid gland have long been used to treat various illnesses including cancer in many Asian countries. Recent studies have uncovered a family of bufadienolides as the responsible pharmacological compounds, and the two major molecules, cinobufagin and bufalin, have been shown to possess robust antitumor activity; however, the underlying mechanisms remain poorly understood.

Methods

Intracellular reactive oxygen species (ROS) were measured by DCFH-DA staining and flow cytometry, and DNA damage was analyzed by immunofluorescent staining and the alkaline comet assay. Cytotoxicity was measured by MTT as well as colony formation assays, and cell cycle and apoptosis were analyzed by flow cytometry. In addition, apoptosis was further characterized by TUNEL and mitochondrial membrane potential assays.

Results

Here we showed that sublethal doses of cinobufagin suppressed the viability of many cancer but not noncancerous cell lines. This tumor-selective cytotoxicity was preceded by a rapid, cancer-specific increase in cellular ROS and was significantly reduced by the ROS inhibitor N-acetyl cysteine (NAC), indicating oxidative stress as the primary source of cinobufagin-induced cancer cell toxicity. Sublethal cinobufagin-induced ROS overload resulted in oxidative DNA damage and intense replication stress in cancer cells, leading to strong DNA damage response (DDR) signaling. Subsequent phosphorylation of CDC25C and stabilization of p53 downstream of DDR resulted in activation of the G₂/M checkpoint followed by induction of apoptosis. These data indicate that cinobufagin suppresses cancer cell viability via DDR-mediated G₂ arrest and apoptosis.

Conclusion

As elevated oxidative pressure is shared by most cancer cells that renders them sensitive to further oxidative insult, these studies suggest that nontoxic doses of cinobufagin can be used to exploit a cancer vulnerability for induction of cancer-specific cytotoxicity.

Supplementary Information

The online version contains supplementary materials available at 10.1186/s12935-021-02150-0.

Cytotoxicity of glucoevatromonoside alone and in combination with chemotherapy drugs and their effects on Na+,K+-ATPase and ion channels on lung cancer cells

Cardiac glycosides (CGs) are useful drugs to treat cardiac illnesses and have potent cytotoxic and anticancer effects in cultured cells and animal models. Their receptor is the Na⁺,K⁺ ATPase, but other plasma membrane proteins might bind CGs as well. Herein, we evaluated the short- and long-lasting cytotoxic effects of the natural cardenolide glucoevatromonoside (GEV) on non-small-cell lung cancer H460 cells. We also tested GEV effects on Na⁺,K⁺ -ATPase activity and membrane currents, alone or in combination with selected chemotherapy drugs. GEV reduced viability, migration, and invasion of H460 cells spheroids. It also induced cell cycle arrest and death and reduced the clonogenic survival and cumulative population doubling. GEV inhibited Na⁺,K⁺-ATPase activity on A549 and H460 cells and purified pig kidney cells membrane. However, it showed no activity on the human red blood cell plasma membrane. Additionally, GEV triggered a Cl-mediated conductance on H460 cells without affecting the transient voltage-gated sodium current. The administration of GEV in combination with the chemotherapeutic drugs paclitaxel (PAC), cisplatin (CIS), irinotecan (IRI), and etoposide (ETO) showed synergistic antiproliferative effects, especially when combined with GEV + CIS and GEV + PAC. Taken together, our results demonstrate that GEV is a potential drug for cancer therapy because it reduces lung cancer H460 cell viability, migration, and invasion. Our results also reveal a link between the Na⁺,K⁺-ATPase and Cl^- ion channels.

Antitumor Activity of the Cardiac Glycoside αlDiginoside by Modulating Mcl-1 in Human Oral Squamous Cell Carcinoma Cells

We recently isolated a cardiac glycoside (CG), αldiginoside, from an indigenous plant in Taiwan, which exhibits potent tumor-suppressive efficacy in oral squamous cell carcinoma (OSCC) cell lines (SCC2095 and SCC4, IC₅₀ < 0.2 µM; 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays). Here, we report that αldiginoside caused Sphase arrest and apoptosis, through the inhibition of a series of signaling pathways, including those mediated by cyclin E, phospho-CDC25C (p-CDC25C), and janus kinase/signal transducer and activator of transcription (JAK/STAT)3. αldiginoside induced apoptosis, as indicated by caspase activation and poly (ADP-ribose) polymerase (PARP) cleavage. Equally important, αldiginoside reduced Mcl-1 expression through protein degradation, and overexpression of Mcl-1 partially protected SCC2095 cells from αldiginoside’s cytotoxicity. Taken together, these data suggest the translational potential of αldiginoside to foster new therapeutic strategies for OSCC treatment.

Anticancer and Antiviral Properties of Cardiac Glycosides: A Review to Explore the Mechanism of Actions

Cardiac glycosides (CGs) have a long history of treating cardiac diseases. However, recent reports have suggested that CGs also possess anticancer and antiviral activities. The primary mechanism of action of these anticancer agents is by suppressing the Na+/k+-ATPase by decreasing the intracellular K+ and increasing the Na+ and Ca2+. Additionally, CGs were known to act as inhibitors of IL8 production, DNA topoisomerase I and II, anoikis prevention and suppression of several target genes responsible for the inhibition of cancer cell proliferation. Moreover, CGs were reported to be effective against several DNA and RNA viral species such as influenza, human cytomegalovirus, herpes simplex virus, coronavirus, tick-borne encephalitis (TBE) virus and Ebola virus. CGs were reported to suppress the HIV-1 gene expression, viral protein translation and alters viral pre-mRNA splicing to inhibit the viral replication. To date, four CGs (Anvirzel, UNBS1450, PBI05204 and digoxin) were in clinical trials for their anticancer activity. This review encapsulates the current knowledge about CGs as anticancer and antiviral drugs in isolation and in combination with some other drugs to enhance their efficiency. Further studies of this class of biomolecules are necessary to determine their possible inhibitory role in cancer and viral diseases.

Molecular mechanisms of bufadienolides and their novel strategies for cancer treatment

Bufadienolides are cardioactive C24 steroids with an α-pyrone ring at position C17. In the last ten years, accumulating studies have revealed the anticancer activities of bufadienolides and their underlying mechanisms, such as induction of autophagy and apoptosis, cell cycle disruption, inhibition of angiogenesis, epithelial-mesenchymal transition (EMT) and stemness, and multidrug resistance reversal. As Na⁺/K⁺-ATPase inhibitors, bufadienolides have inevitable cardiotoxicity. Short half-lives, poor stability, low plasma concentration and oral bioavailability in vivo are obstacles for their applications as drugs. To improve the drug potency of bufadienolides and reduce their side effects, prodrug strategies and drug delivery systems such as liposomes and nanoparticles have been applied. Therefore, systematic and recapitulated information about the antitumor activity of bufadienolides, with special emphasis on the molecular or cellular mechanisms, prodrug strategies and drug delivery systems, is of high interest. Here, we systematically review the anticancer effects of bufadienolides and the molecular or cellular mechanisms of action. Research advancements regarding bufadienolide prodrugs and their tumor-targeting delivery strategies are critically summarized. This work highlights recent scientific advances regarding bufadienolides as effective anticancer agents from 2011 to 2019, which will help researchers to understand the molecular pathways involving bufadienolides, resulting in a selective and safe new lead compound or therapeutic strategy with improved therapeutic applications of bufadienolides for cancer therapy.

Ouabain pre-treatment modulates B and T lymphocytes and improves survival of melanoma-bearing animals

Ouabain (OUA) is a glycoside shown to modulate B and T lymphocytes. Nevertheless, ouabain effects on B16F10 melanoma immune response, a mouse lineage that mimics human melanoma, are still unknown. Our aim was to study how OUA in vivo treatment modulates lymphocytes and if it improves the response against B16F10 cells. C57BL/6 mice were pre-treated with intraperitoneal (i.p) injection of OUA (0.56 mg/Kg) for three consecutive days. On the 4^th day, 10⁶ B16F10 cells or vehicle were i.p. injected. Animals were euthanized on days 4^th and 21^st for organs removal and subsequent lymphocyte analyses by flow cytometry. In vivo ouabain-treatment reduced regulatory T cells in the spleen in both melanoma and non-melanoma groups. Ouabain preserved the number and percentage of B lymphocytes in peripheral organs of melanoma-injected mice. Melanoma-injected mice pre-treated with OUA also survive longer. Our findings contribute to a better understanding of OUA immunological effects in a melanoma model.

Cinobufagin Triggers Defects in Spindle Formation and Cap-Dependent Translation in Liver Cancer Cells by Inhibiting the AURKA-mTOR-eIF4E Axis

Cinobufagin is a Na+/K+-ATPase (NKA) inhibitor with excellent anticancer effects to prolong the survival of patients. The purpose of the present study was to clarify the underlying mechanism of the anticancer effects of cinobufagin using overexpression or inhibition of aurora kinase A (AURKA) signaling. First, high expression of Na+/K+-ATPase alpha 1 subunit (ATP1A1) and AURAK resulted in increased malignant transformation in hepatocellular carcinoma (HCC) patients using the cancer genome atlas (TCGA) data and tissue samples. After treatment with cinobufagin, we successfully screened 202, 249, and 335 changing expression proteins in Huh-7 cells under normal, overexpression, and inhibition of AURKA using tandem mass tags (TMT)-labeled quantitative proteomics coupled to 2D liquid chromatography-tandem mass spectrometry (LC-MS/MS). Bioinformatics analysis revealed that these molecules were closely associated with chromosome segregation, DNA damage, and regulation of translation processes. We further confirmed that cinobufagin induced DNA damage and chromosome segregation disorders and suppresses translational processing in oncogenes by decreasing the expression of AURKA, mechanistic target of rapamycin kinase (mTOR), p-mTOR, p-extracellular regulated protein kinases (ERK), eukaryotic translation initiation factor 4E (eIF4E), and p-eIF4E, while increasing the expression of p-eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1) (S65, T37, T46, T45) and increasing the interaction between eIF4 and 4E-BP1. Our results suggested that cinobufagin performed an antitumor effects in liver cancer cells by inhibiting the AURKA-mTOR-eIF4E axis.

21-Benzylidene digoxin decreases proliferation by inhibiting the EGFR/ERK signaling pathway and induces apoptosis in HeLa cells

Cardiotonic steroids (CTS) are agents traditionally known for their capacity to bind to the Na,K-ATPase (NKA), affecting the ion transport and the contraction of the heart. Natural CTS have been shown to also have effects on cell signaling pathways. With the goal of developing a new CTS derivative, we synthesized a new digoxin derivative, 21-benzylidene digoxin (21-BD). Previously, we have shown that this compound binds to NKA and has cytotoxic actions on cancer, but not on normal cells. Here, we further studied the mechanisms of actions of 21-BD. Working with HeLa cells, we found that 21-BD decreases the basal, as well as the insulin stimulated proliferation. 21-BD reduces phosphorylation of the epidermal growth factor receptor (EGFR) and extracellular-regulated kinase (ERK), which are involved in pathways that stimulate cell proliferation. In addition, 21-BD promotes apoptosis, which is mediated by the translocation of Bax from the cytosol to mitochondria and the release of mitochondrial cytochrome c to the cytosol. 21-BD also activated caspases-8, -9 and -3, and induced the cleavage of poly (ADP-ribose) polymerase-1 (PARP-1). Altogether, these results show that the new compound that we have synthesized exerts cytotoxic actions on HeLa cells by inhibition of cell proliferation and the activation of both the extrinsic and intrinsic apoptotic pathways. These results support the relevance of the cardiotonic steroid scaffold as modulators of cell signaling pathways and potential agents for their use in cancer.

Tracing MYC Expression for Small Molecule Discovery

Our inability to effectively "drug" targets such as MYC for therapeutic purposes requires the development of new approaches. We report on the implementation of a phenotype-based assay for monitoring MYC expression in multiple myeloma cells. The open reading frame (ORF) encoding an unstable variant of GFP was engineered immediately downstream of the MYC ORF using CRISPR/Cas9, resulting in co-expression of both proteins from the endogenous MYC locus. Using fluorescence readout as a surrogate for MYC expression, we implemented a pilot screen in which ∼10,000 compounds were prosecuted. Among known MYC expression inhibitors, we identified cardiac glycosides and cytoskeletal disruptors to be quite potent. We demonstrate the power of CRISPR/Cas9 engineering in establishing phenotype-based assays to identify gene expression modulators.

Cardiac Glycoside Glucoevatromonoside Induces Cancer Type-Specific Cell Death

Cardiac glycosides (CGs) are natural compounds used traditionally to treat congestive heart diseases. Recent investigations repositioned CGs as potential anticancer agents. To discover novel cytotoxic CG scaffolds, we selected the cardenolide glucoevatromonoside (GEV) out of 46 CGs for its low nanomolar anti-lung cancer activity. GEV presented reduced toxicity toward non-cancerous cell types (lung MRC-5 and PBMC) and high-affinity binding to the Na⁺/K⁺-ATPase α subunit, assessed by computational docking. GEV-induced cell death was caspase-independent, as investigated by a multiparametric approach, and culminates in severe morphological alterations in A549 cells, monitored by transmission electron microscopy, live cell imaging and flow cytometry. This non-canonical cell death was not preceded or accompanied by exacerbation of autophagy. In the presence of GEV, markers of autophagic flux (e.g. LC3I-II conversion) were impacted, even in presence of bafilomycin A1. Cell death induction remained unaffected by calpain, cathepsin, parthanatos, or necroptosis inhibitors. Interestingly, GEV triggered caspase-dependent apoptosis in U937 acute myeloid leukemia cells, witnessing cancer-type specific cell death induction. Differential cell cycle modulation by this CG led to a G2/M arrest, cyclin B1 and p53 downregulation in A549, but not in U937 cells. We further extended the anti-cancer potential of GEV to 3D cell culture using clonogenic and spheroid formation assays and validated our findings in vivo by zebrafish xenografts. Altogether, GEV shows an interesting anticancer profile with the ability to exert cytotoxic effects via induction of different cell death modalities.

Cinobufagin inhibits tumor growth by inducing intrinsic apoptosis through AKT signaling pathway in human nonsmall cell lung cancer cells

The cinobufagin (CB) has a broad spectrum of cytotoxicity to inhibit cell proliferation of various human cancer cell lines, but the molecular mechanisms still remain elusive. Here we observed that CB inhibited the cell proliferation and tumor growth, but induced cell cycle arrest and apoptosis in a dose-dependent manner in non-small cell lung cancer (NSCLC) cells. Treatment with CB significantly increased the reactive oxygen species but decreased the mitochondrial membrane potential in NSCLC cells. These effects were markedly blocked when the cells were pretreated with N-acetylcysteine, a specific reactive oxygen species inhibitor. Furthermore, treatment with CB induced the expression of BAX but reduced that of BCL-2, BCL-XL and MCL-1, leading to an activation of caspase-3, chromatin condensation and DNA degradation in order to induce programmed cell death in NSCLC cells. In addition, treatment with CB reduced the expressions of p-AKT^T308 and p-AKT^S473 and inhibited the AKT/mTOR signaling pathway in NSCLC cells in a time-dependent manner. Our results suggest that CB inhibits tumor growth by inducing intrinsic apoptosis through the AKT signaling pathway in NSCLC cells.

Anticancer and Immunogenic Properties of Cardiac Glycosides

Cardiac glycosides (CGs) are natural compounds widely used in the treatment of several cardiac conditions and more recently have been recognized as potential antitumor compounds. They are known to be ligands for Na/K-ATPase, which is a promising drug target in cancer. More recently, in addition to their antitumor effects, it has been suggested that CGs activate tumor-specific immune responses. This review summarizes the anticancer aspects of CGs as new strategies for immunotherapy and drug repositioning (new horizons for old players), and the possible new targets for CGs in cancer cells.

Ouabain targets the Na+/K+-ATPase α3 isoform to inhibit cancer cell proliferation and induce apoptosis

Ouabain has been used for the treatment of heart failure and atrial fibrillation. Its potential anticancer effect has also attracted great interest. The aim of the present study was to evaluate the anticancer effect of ouabain and investigate its molecular target. The effects of ouabain on the viability of and induction of cellular death on OS-RC-2 renal cancer cells were examined using the MTT assay and acridine orange/ethidium bromide staining. The levels of Ca²⁺ and reactive oxygen species were determined using Fura-3-acetoxymethyl ester and dichloro-dihydro-fluorescein diacetate probes, respectively. Apoptosis was examined using annexin V-fluorescein isothiocyanate/propidium iodide staining and western blotting. The expression profile of the different Na⁺/K⁺-ATPase (NKA) isoforms in NCI-H446 small cell lung cancer cells was determined using immunocytochemistry and reverse transcription polymerase chain reaction analysis. In the present study, it was demonstrated that ouabain inhibited cancer cell proliferation and induced apoptosis while no significant difference in the expression of NKA α₁ and α₃ isoforms was detected following 48 h of ouabain treatment. Furthermore, expression of NKA α₃ but not the α₁ isoform was associated with ouabain sensitivity. The results of the present study indicated that ouabain targets the NKA α₃ isoform, inhibits cancer cell proliferation and induces apoptosis.

Contrasting effects of cardiac glycosides on cisplatin- and etoposide-induced cell death

Cardiac glycosides (CGs) or cardiotonic steroids, which constitute a group of naturally occurring compounds with a steroid-like structure, can act on Na+/K+-ATPase as a receptor and activate intracellular signaling messengers leading to a variety of cellular responses. Epidemiological studies have revealed that CGs, used for the treatment of cardiac disorders, may also be beneficial as anti-cancer agents. CGs, acting in combination with other chemotherapeutic agents, may significantly alter their efficiency in relation to cancer cell elimination, causing both sensitization and an increase in cancer cell death, and in some cases resistance to chemotherapy. Here we show the ability of CGs to modulate apoptotic response to conventionally used anti-cancer drugs. In combination with etoposide, CGs digoxin may enhance cytotoxic potential, thereby allowing the chemotherapeutic dose to be decreased and minimizing toxicity and adverse reactions. Mechanisms behind this event are discussed.

SILAC-based proteomic analysis reveals that salidroside antagonizes cobalt chloride-induced hypoxic effects by restoring the tricarboxylic acid cycle in cardiomyocytes

Hypoxic status alters the energy metabolism and induces cell injury in cardiomyocytes, and it further triggers the occurrence and development of cardiovascular diseases. Our previous studies have shown that salidroside (SAL) exhibits anti-hypoxic activity. However, the mechanisms remain obscure. In the present study, we successfully screened 92 different expression proteins in CoCl2-induced hypoxic conditions, 106 different expression proteins in the SAL-mediated anti-hypoxic group were compared with the hypoxic group using quantitative proteomics strategy, respectively. We confirmed that SAL showed a positive protective function involving the acetyl-CoA metabolic, tricarboxylic acid (TCA) cycle using bioinformatics analysis. We also demonstrated that SAL plays a critical role in restoring the TCA cycle and in protecting cardiomyocytes from oxidative injury via up-regulation expressions of PDHE1-B, ACO2, SUCLG1, SUCLG2 and down-regulation of MDH2. SAL also inhibited H9c2 cell apoptosis by inhibiting the activation of pro-apoptotic molecules caspase 3 and caspase 9 as well as activation of the anti-apoptotic molecular Bcl-2. Additionally, SAL also improved mitochondrial membrane potential (ΔΨm), reduced reactive oxygen species (ROS) and intercellular Ca(2+) concentration ([Ca(2+)]i) accumulation and inhibited the excessive consumption of ATP in H9c2 cells.

Cardenolide glycosides from the seeds of Digitalis purpurea exhibit carcinoma-specific cytotoxicity toward renal adenocarcinoma and hepatocellular carcinoma cells

Four cardenolide glycosides, glucodigifucoside (2), 3'-O-acetylglucoevatromonoside (9), digitoxigenin 3-O-β-D-glucopyranosyl-(1 → 4)-β-D-glucopyranosyl-(1 → 4)-3-O-acetyl-β-D-digitoxopyranoside (11), and purpureaglycoside A (12), isolated from the seeds of Digitalis purpurea, exhibited potent cytotoxicity against human renal adenocarcinoma cell line ACHN. These compounds exhibited significantly lower IC50 values against ACHN than that against normal human renal proximal tubule-derived cell line HK-2. In particular, 2 exhibited the most potent and carcinoma-specific cytotoxicity, with a sixfold lower IC50 value against ACHN than that against HK-2. Measurement of cyclin-dependent kinase inhibitor levels revealed that upregulation of p21/Cip1 expression was involved in the carcinoma-specific cytotoxicity of 2. Further, compound 2 also exhibited the carcinoma-specific cytotoxicity toward hepatocellular carcinoma cell line.

Ouabain elicits human glioblastoma cells apoptosis by generating reactive oxygen species in ERK-p66SHC-dependent pathway

Excessive reactive oxygen species (ROS) generation has been implicated as one of main agents in ouabain-induced anticancer effect. Unfortunately, the signaling pathways under it are not very clarified. In the present study, we investigated the molecular mechanism involved in ouabain-induced ROS generation and cell apoptosis on human U373MG and U87MG glioma cells. Ouabain-induced glioblastoma cells apoptosis and increased ROS generation. Clearance ROS by three different ROS scavenger partly, but not totally, reversed ouabain's effect on cell apoptosis. Ouabain-induced ROS generation was not regulated by calcium overload, reduced nicotinamide adenine dinucleotide phosphate oxidation, but by p66Shc phosphorylation. Ouabain treatment increased p66Shc Ser36 phosphorylation. Knockdown of p66Shc by siRNA significantly inhibited ROS generations in response to ouabain. Ouabain-induced p66Shc phosphorylation through Src/Ras/extracellular signal-regulated kinase signal pathway. Our results uncovered a novel signaling pathway with p66Shc, ouabain-induced ROS generation, and glioblastoma cell apoptosis.

The antiproliferative effects of ouabain and everolimus on adrenocortical tumor cells

Ouabain is a cardiotonic steroid obtained from Strophanthus. Recently its role as antiproliferative agent has been investigated in tumor cells. Everolimus is a derivative of rapamycin and acts as a signal transduction inhibitor. Adrenocortical carcinoma is a rare cancer, with poor prognosis. This research focuses on antineoplastic properties of ouabain and its association with everolimus. We analyzed the effects of drugs on cells by MTT assay, by [(3)H] thymidine assay, by Wright's staining, by homogeneous caspases assay, by flow cytometry analysis and by Western blot analysis on H295R and SW13 cells and on primary adrenocortical tumor cells. Ouabain induced cell viability reduction in SW13, H295R and 5 primary adrenocortical tumor cells. Combination of ouabain with everolimus produced a stronger cytotoxic effect on cell proliferation and viability. Marked morphological changes were observed in both SW13 and H295R cell lines after ouabain treatment, with an increase in necrosis. Cell cycle distribution was altered by ouabain in SW13. Analysis of apoptosis demonstrated an increase in caspase activity, clearly evident for SW13 at 72h. FACS analysis by Annexin V-FITC kit and propidium iodide confirmed an increased level of necrosis at higher concentrations. Western blot analysis showed that PI3k/Akt signaling pathway was modified after ouabain treatments in SW13. Ouabain exerts antiproliferative effects on SW13 and H295R cell lines and on primary adrenocortical tumor cells. These data suggest that ouabain or ouabain derivatives may be potential anticancer agents.

Current status on development of steroids as anticancer agents

Steroids are important biodynamic agents. Their affinities for various nuclear receptors have been an interesting feature to utilize them for drug development particularly for receptor mediated diseases. Steroid biochemistry and its crucial role in human physiology, has attained importance among the researchers. Recent years have seen an extensive focus on modification of steroids. The rational modifications of perhydrocyclopentanophenanthrene nucleus of steroids have yielded several important anticancer lead molecules. Exemestane, SR16157, fulvestrant and 2-methoxyestradiol are some of the successful leads emerged on steroidal pharmacophores. The present review is an update on some of the steroidal leads obtained during past 25 years. Various steroid based enzyme inhibitors, antiestrogens, cytotoxic conjugates and steroidal cytotoxic molecules of natural as well as synthetic origin have been highlighted. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Cardiac glycosides block cancer growth through HIF-1α- and NF-κB-mediated Plk1

Cardiac glycosides as inhibitors of the sodium/potassium adenosine triphosphatase (sodium pump) have been reported to block cancer growth by inducing G2/M phase arrest in many cancer cells. However, no detailed studies have been performed to distinguish between these two phases of cardiac glycoside-arrested cells. Furthermore, the underlying mechanisms involved in this cell cycle arrest process are still not known. Here, we report that bufalin and other cardiac glycosides potently induce mitotic arrest by the downregulation of polo-like kinase 1 (Plk1) expression. Live-cell imaging results demonstrate that bufalin-treated cells exhibit a marked delay in entering prophase at an early stage and are then arrested at prometaphase or induced entry into apoptosis. This phenotypic change is attributed to the downregulation of Plk1. We also show that bufalin and the knockdown of sodium pump reduce Plk1, at least in part, through downregulation of the nuclear transcription factors, hypoxia-inducible factor-1α (HIF-1α) and nuclear factor-kappa B (NF-κB). These findings suggest that cardiac glycosides induce mitotic arrest and apoptosis through HIF-1α- and NF-κB-mediated downregulation of Plk1 expression, demonstrating that HIF-1α and NF-κB are critical targets of cardiac glycosides in exerting their anticancer action.

Inhibition of cell migration by ouabain in the A549 human lung cancer cell line

The Na⁺/K⁺-ATPase α subunit is highly expressed in malignant cells. Ouabain, a cardioactive glycoside, binds to the Na⁺/K⁺-ATPase α subunit and inhibits the activity of Na⁺/K⁺-ATPase. In the present study, the effect of ouabain on the migration of A549 cells was analyzed using the wound healing and transwell chamber migration assays. The impact of ouabain on the expression of E-cadherin, N-cadherin, vimentin, matrix metalloprotease (MMP)-2 and MMP-9 was also evaluated. Ouabain treatment not only inhibited the epidermal growth factor (EGF)-enhanced migration of A549 cells, but also inhibited the basal migration of A549 cells in the absence of EGF. Ouabain decreased the overexpression of N-cadherin and vimentin induced by EGF, and decreased the expression of MMP-2 and -9 in the presence or absence of EGF. Na⁺/K⁺-ATPase is a potent therapeutic target in lung cancer and these observations indicated that the Na⁺/K⁺-ATPase inhibitor, ouabain, retards the invasion of lung cancer cells.

The mechanisms of chansu in inducing efficient apoptosis in colon cancer cells

Chansu is one of the most widely used traditional Chinese medicines in China, Japan, and other Southeast Asian countries primarily for antipain, anti-inflammation, and recently anticancer. Over 10 recipes and remedies contained Chansu, which are easily available in pharmacies and hospitals, but the mechanisms of action were not clearly articulated. In the present study, Cinobufagin (CBF), the major compound of Chansu, was employed as a surrogate marker to determine its ability in inducing cancer cell death. As expected, CBF has significant cancer-killing capacity for a range of cancers, but such ability differs markedly. Colon and prostate cancers are more sensitive than skin and lung cancers. Interestingly, cancer cells die through apoptotic pathway either being biphasic caspase-3-dependent (HCT116) or independent (HT29). Multipathway analysis reveals that CBF-induced apoptosis is likely modulated by the hypoxia-inducing factor-1 alpha subunit (HIF-1α) as its inhibition was evident in vitro and in vivo. Taken together, these results demonstrate that CBF is a potent apoptotic inducer with potential for further development as a novel and effective anticancer agent for a range of cancers, especially colon cancer.

A signaling cascade mediated by ceramide, src and PDGFRβ coordinates the activation of the redox-sensitive neutral sphingomyelinase-2 and sphingosine kinase-1

Stress-inducing agents, including oxidative stress, generate the sphingolipid mediators ceramide (Cer) and sphingosine-1-phosphate (S1P) that are involved in stress-induced cellular responses. The two redox-sensitive neutral sphingomyelinase-2 (nSMase2) and sphingosine kinase-1 (SK1) participate in transducing stress signaling to ceramide and S1P, respectively; however, whether these key enzymes are coordinately regulated is not known. We investigated whether a signaling link coordinates nSMase2 and SK1 activation by H2O2. In mesenchymal cells, H2O2 elicits a dose-dependent biphasic effect, mitogenic at low concentration (5μM), and anti-proliferative and toxic at high concentration (100μM). Low H2O2 concentration triggered activation of nSMase2 and SK1 through a nSMase2/Cer-dependent signaling pathway that acted upstream of activation of SK1. Further results implicated src and the trans-activation of PDGFRβ, as supported by the blocking effect of specific siRNAs, pharmacological inhibitors, and genetically deficient cells for nSMase2, src and SK1. The H2O2-induced src/PDGFRβ/SK1 signaling cascade was impaired in nSMase2-deficient fro/fro cells and was rescued by exogenous C2Cer that activated src/PDGFRβ/SK1. Thus, the results define a nSMase2/SK1 signaling pathway implicated in the mitogenic response to low oxidative stress. On the other hand, high oxidative stress induced inhibition of SK1. The results also showed that the toxicity of high H2O2 concentration was comparable in control and nSMase2-deficient cells. Taken together the results identify a tightly coordinated nSMase2/SK1 pathway that mediates the mitogenic effects of H2O2 and may sense the degree of oxidative stress.

The effect of ouabain on mitochondrial DNA damage in HepG2 cell lines

Our purpose in this study is to analyze mitochondrial DNA (mtDNA) lesion frequencies and mtDNA(4977) deletion in HepG2 cells to examine the effects of ouabain on mtDNA. HepG2 cells were treated with 0.75, 7.5, 75, and 750 nM of ouabain for 24 h in the presence and absence of 10 mM 2-deoxyglucose (2-DG). The frequency of mtDNA(4977) deletions and mitochondrial lesions were determined by real-time polymerase chain reaction. A ≥ 1.2-fold change or greater was considered significant. Ouabain doses of 750, 75, and 7.5 nM alone increased the frequency of mtDNA(4977) deletions 1.39, 1.92, and 1.44 times, respectively. The 750 and 75 nM ouabain doses combined with 2-DG increased the mtDNA(4977) deletion frequency 4.94 and 1.57 times, respectively. The 750 and 75 nM ouabain doses alone increased the mtDNA lesion frequency 2.5 and 1.5 times, respectively. The 750 nM ouabain dose combined with 2-DG increased the mtDNA lesion frequency 2.28 times. The 7.5 nM ouabain dose alone and combined with 2-DG decreased the mtDNA lesion frequency 0.67 and 0.45 times, respectively. Ouabain alone and when combined with 2-DG increases mtDNA lesion and mtDNA(4977) deletion frequencies. This supports the thesis that ouabain creates oxidative stress and induces DNA damage and apoptosis.

Characterization of the anticancer properties of monoglycosidic cardenolides isolated from Nerium oleander and Streptocaulon tomentosum

AIM OF THE STUDY

For identification of the active constituents we investigated the anticancer activity of cardenolides from Streptocaulon tomentosum Wight & Arn. (Asclepiadaceae) and from Nerium oleander L. (Apocynaceae) which are both used against cancer in the traditional medicine in their region of origin.

MATERIAL, METHODS AND RESULTS

The antiproliferative activity of cardenolides isolated from roots of Streptocaulon tomentosum (IC(50)<1-15.3 μM after 2 days in MCF7) and of cardenolide containing fractions from the cold aqueous extract of Nerium oleander leaves ("Breastin", mean IC(50) 0.85 μg/ml in a panel of 36 human tumor cell lines), their influence on the cellular viability and on the cell cycle (block at the G2/M-phase or at the S-phase in tumor cells, respectively) were determined using different cell lines. The murine cell line L929 and normal non-tumor cells were not affected. Bioactivity guided fractionation of Breastin resulted in the isolation of the monoglycosidic cardenolides oleandrine, oleandrigeninsarmentoside, neritaloside, odoroside H, and odoroside A (IC(50)-values between 0.010 and 0.071 μg/ml).

CONCLUSIONS

The observed anticancer activities of extracts and isolated cardenolides are in agreement with the ethnomedicinal use of Streptocaulon tomentosum and Nerium oleander. The most active anticancer compounds from both species are monoglycosidic cardenolides possessing the 3β,14β-dihydroxy-5β-card-20(22)-enolide structure with or without an acetoxy group at C-16. The results indicate that the cytotoxic effects are induced by the inhibition of the plasma membrane bound Na(+)/K(+)-ATPase.