Monensin-mediated growth inhibition in human lymphoma cells through cell cycle arrest and apoptosis

Ultrastructural changes in cardiac and skeletal myoblasts following in vitro exposure to monensin, salinomycin, and lasalocid

Carboxylic ionophores are polyether antibiotics used in production animals as feed additives, with a wide range of benefits. However, ionophore toxicosis often occurs as a result of food mixing errors or extra-label use and primarily targets the cardiac and skeletal muscles of livestock. The ultrastructural changes induced by 48 hours of exposure to 0.1 μM monensin, salinomycin, and lasalocid in cardiac (H9c2) and skeletal (L6) myoblasts in vitro were investigated using transmission electron microscopy and scanning electron microscopy. Ionophore exposure resulted in condensed mitochondria, dilated Golgi apparatus, and cytoplasmic vacuolization which appeared as indentations on the myoblast surface. Ultrastructurally, it appears that both apoptotic and necrotic myoblasts were present after exposure to the ionophores. Apoptotic myoblasts contained condensed chromatin and apoptotic bodies budding from their surface. Necrotic myoblasts had disrupted plasma membranes and damaged cytoplasmic organelles. Of the three ionophores, monensin induced the most alterations in myoblasts of both cell lines.

Golgi apparatus targeted therapy in cancer: Are we there yet?

Membrane trafficking within the Golgi apparatus plays a pivotal role in the intracellular transportation of lipids and proteins. Dysregulation of this process can give rise to various pathological manifestations, including cancer. Exploiting Golgi defects, cancer cells capitalise on aberrant membrane trafficking to facilitate signal transduction, proliferation, invasion, immune modulation, angiogenesis, and metastasis. Despite the identification of several molecular signalling pathways associated with Golgi abnormalities, there remains a lack of approved drugs specifically targeting cancer cells through the manipulation of the Golgi apparatus. In the initial section of this comprehensive review, the focus is directed towards delineating the abnormal Golgi genes and proteins implicated in carcinogenesis. Subsequently, a thorough examination is conducted on the impact of these variations on Golgi function, encompassing aspects such as vesicular trafficking, glycosylation, autophagy, oxidative mechanisms, and pH alterations. Lastly, the review provides a current update on promising Golgi apparatus-targeted inhibitors undergoing preclinical and/or clinical trials, offering insights into their potential as therapeutic interventions. Significantly more effort is required to advance these potential inhibitors to benefit patients in clinical settings.

Effect of antibiotic monensin on cell proliferation and IGF1R signaling pathway in human colorectal cancer cells

BACKGROUND/AIMS

Colorectal cancer is the third leading cause of death in patients with cancers in America. Monensin has represented anti-cancer effect on various human cancer cells. We seek to investigate the effect of monensin on proliferation of human colorectal cancer cells and explore whether IGF1R signaling pathway is involved in anti-cancer mechanism of monensin.

METHODS

Cell proliferation and migration were assessed by crystal violet staining and cell wounding assay respectively. Cell apoptosis was analyzed by Hoechst 33258 staining and flow cytometry. Cell cycle progression was detected with the use of flow cytometry. Cancer-associated pathways were assessed with the use of pathway-specific reporters. Gene expression was detected by touchdown-quantitative real-time PCR. Inhibition of IGF1R was tested by immunofluorescence staining. Inhibition of IGF1R signaling was accomplished by adenovirus-mediated expression of IGF1.

RESULTS

We found that monensin not only effectively inhibited cell proliferation, cell migration as well as cell cycle progression, but also induced apoptosis and G1 arrest in human colorectal cancer cells. Monensin was shown to target multiple cancer-related signaling pathways such as Elk1, AP1, as well as Myc/max, and suppressed IGF1R expression via increasing IGF1 in colorectal cancer cells.

CONCLUSION

Monensin could suppressed IGF1R expression via increasing IGF1 in colorectal cancer cells. It has the potential to be repurposed as an anti-colorectal cancer agent, but further studies are still required to investigate the detailed mechanisms of monensin underlying its anti-cancer motion.Key MessagesMonensin inhibits the cell proliferation and the migration, induces apoptosis and inhibits cell cycle progression in human colorectal cancer cells.Monensin may exert anti-cancer activity by targeting multiple signaling pathways, including the IGF1R signaling pathway.Monensin has the potential to be repurposed as an anti-colorectal cancer agent.

Investigation of the anticancer mechanism of monensin via apoptosis-related factors in SH-SY5Y neuroblastoma cells

Monensin is an ionophore antibiotic that inhibits the growth of cancer cells. The aim of this study was to investigate the apoptosis-mediated anticarcinogenic effects of monensin in SH-SY5Y neuroblastoma cells. The effects of monensin on cell viability, invasion, migration, and colony formation were determined by XTT, matrigel-chamber, wound healing, and colony formation tests, respectively. The effects of monensin on apoptosis were determined by real-time polymerase chain reaction, TUNEL, Western blot, and Annexin V assay. We have shown that monensin suppresses neuroblastoma cell viability, invasion, migration, and colony formation. Moreover, we reported that monensin inhibits cell viability by triggering apoptosis of neuroblastoma cells. Monensin caused apoptosis by increasing caspase-3, 7, 8, and 9 expressions and decreasing Bax and Bcl-2 expressions in neuroblastoma cells. In Annexin V results, the rates of apoptotic cells were found to be 9.66 ± 0.01% (p < 0.001), 29.28 ± 0.88% (p < 0.01), and 62.55 ± 2.36% (p < 0.01) in the 8, 16, and 32 μM monensin groups, respectively. In TUNEL results, these values were, respectively; 35 ± 2% (p < 0.001), 34 ± 0.57% (p < 0.001), and 75 ± 2.51% (p < 0.001). Our results suggest that monensin may be a safe and effective therapeutic candidate for treating pediatric neuroblastoma.

Drug Repurposing Strategies for Non-Cancer to Cancer Therapeutics

Global efforts invested for the prevention and treatment of cancer need to be repositioned to develop safe, effective, and economic anticancer therapeutics by adopting rational approaches of drug discovery. Drug repurposing is one of the established approaches to reposition old, clinically approved off patent noncancer drugs with known targets into newer indications. The literature review suggests key role of drug repurposing in the development of drugs intended for cancer as well as noncancer therapeutics. A wide category of noncancer drugs namely, drugs acting on CNS, anthelmintics, cardiovascular drugs, antimalarial drugs, anti-inflammatory drugs have come out with interesting outcomes during preclinical and clinical phases. In the present article a comprehensive overview of the current scenario of drug repurposing for the treatment of cancer has been focused. The details of some successful studies along with examples have been included followed by associated challenges.

Monensin inhibits anaplastic thyroid cancer via disrupting mitochondrial respiration and AMPK/mTOR signaling

OBJECTIVE

The clinical management of anaplastic thyroid cancer (ATC) remains challenging and novel treatment methods are needed. Monensin is a carboxyl polyether ionophore that potently inhibits the growth of various cancer types. Our current work investigates whether monensin has selective anti-ATC activity and systematically explores its underlying mechanisms.

METHODS

Proliferation and apoptosis assays were performed using a panel of thyroid cancer cell lines. Mitochondrial biogenesis profiles, ATP levels, oxidative stress, AMPK and mTOR were examined in these cells after monensin treatment.

RESULTS

Monensin is effective to inhibit proliferation and induce apoptosis in a number of thyroid cancer cell lines. The results are consistent across cell lines of varying cellular origins and genetic mutations. Compared to other thyroid cancer cell types, ATC cell lines are the most sensitive to monensin. Of note, monensin used at our experimental concentration affects less of normal cells. Mechanistic studies reveal that monensin acts on ATC cells through disrupting mitochondrial function, inducing oxidative stress and damage, and AMPK activation-induced mTOR inhibition. We further show mitochondrial respiration is a critical target for monensin in ATC cells.

CONCLUSIONS

Our pre-clinical findings demonstrate the selective anti-ATC activities of monensin. This is supported by increasing evidence monensin can to be repurposed as a potential anti-cancer drug.

Maduramicin arrests myocardial cells at G0/G1 phase of the cell cycle through inhibiting AKT-Cyclin D1 signaling

Maduramicin, a polyether ionophore antibiotic used as an anticoccidial feed additive in poultry, is toxic to animals and humans and can cause heart failure. The present study was initiated to explore the underlying mechanism of toxicity in H9c2 myocardial cells. We observed using cell imaging and counting methods that maduramicin inhibited cell growth in a concentration-dependent manner. Furthermore, MTT assays showed that maduramicin inhibited cell proliferation in a concentration- and time-dependent manner, and was also confirmed by the finding that maduramicin time dependently blocked the incorporation of BrdU into DNA in H9c2 myocardial cells. Further studies revealed that maduramicin induced accumulation of the cells at G₀/G₁ phase of the cell cycle and concurrently, there was down regulation of expression of Cyclin D1. In addition, exposure to maduramicin pruned phosphorylation of AKT at both T308 and S473 sites. Finally, we found that pre-treatment of H9c2 myocardial cells with AKT activator SC79, attenuated the inhibitory effects of maduramicin on Cyclin D1 expression and cell proliferation. Collectively, our results suggest that maduramicin-suppressed AKT-Cyclin D1 signaling which results in G₀/G₁ phase cell cycle arrest, leading to the inhibition of myocardial cell proliferation.

Synthesis and Anticancer Activity of Dimeric Polyether Ionophores

Polyether ionophores represent a group of natural lipid-soluble biomolecules with a broad spectrum of bioactivity, ranging from antibacterial to anticancer activity. Three seem to be particularly interesting in this context, namely lasalocid acid, monensin, and salinomycin, as they are able to selectively target cancer cells of various origin including cancer stem cells. Due to their potent biological activity and abundant availability, some research groups around the world have successfully followed semi-synthetic approaches to generate original derivatives of ionophores. However, a definitely less explored avenue is the synthesis and functional evaluation of their multivalent structures. Thus, in this paper, we describe the synthetic access to a series of original homo- and heterodimers of polyether ionophores, in which (i) two salinomycin molecules are joined through triazole linkers, or (ii) salinomycin is combined with lasalocid acid, monensin, or betulinic acid partners to form 'mixed' dimeric structures. Of note, all 11 products were tested in vitro for their antiproliferative activity against a panel of six cancer cell lines including the doxorubicin resistant colon adenocarcinoma LoVo/DX cell line; five dimers (14-15, 17-18 and 22) were identified to be more potent than the reference agents (i.e., both parent compound(s) and commonly used cytostatic drugs) in selective targeting of various types of cancer. Dimers 16 and 21 were also found to effectively overcome the resistance of the LoVo/DX cancer cell line.

Monensin inhibits proliferation, migration, and promotes apoptosis of breast cancer cells via downregulating UBA2

Breast cancer is a malignant tumor that occurs in the epithelial tissue of the breast gland, the morbidity, and mortality of which continue to increase. Therefore, it is crucial to find new drugs to treat breast cancer. Monensin is a carrier antibiotic that has been reported to inhibit the growth of cancer cells; however, its impacts on breast cancer cells have not been reported. In this article, the cell survival rate was measured by CCK-8. Colony formation assay was utilized to detect the level of cell proliferation. Transwell was used to measure the ability of cell invasion, and wound healing was used to measure the ability of cell migration. RT-qPCR and western blot were, respectively, used to detect the expression of related genes and proteins. The level of apoptosis was detected by flow cytometry. Cell transfection technique was used for overexpressing UBA2. We found that Monensin inhibited the proliferation and migration of breast cancer cells and inhibited the expression of MMP-2 and MMP-9. In addition, Monensin promoted the apoptosis accompanied by the increase of Bax, caspase3, caspase7, and caspase9 and the decreased of bcl-2 of breast cancer cells. Monensin was also found to inhibit UBA2 expression in breast cancer cells. Subsequently, after overexpression of UBA2, the impacts of Monensin on proliferation, migration, and apoptosis of breast cancer cells was inhibited. In conclusion, Monensin can inhibit the proliferation and migration and activate apoptosis of breast cancer cells via downregulating the expression of UBA2.

Antiproliferative activity of ester derivatives of monensin A at the C-1 and C-26 positions

Monensin A (MON) is a polyether ionophore antibiotic, which shows a wide spectrum of biological activity, including anticancer activity. A series of structurally diverse monensin esters including its C-1 esters (1-9), C-26-O-acetylated derivatives (10-15), and lactone (16) was synthesized and for the first time evaluated for their antiproliferative activity against four human cancer cell lines with different drug-sensitivity level. All of the MON derivatives exhibited in vitro antiproliferative activity against cancer cells at micromolar concentrations. The majority of the compounds was able to overcome the drug resistance of LoVo/DX and MES-SA/DX5 cell lines. The most active compounds proved to be MON C-26-O-acetylated derivatives (10-15) which exhibited very good resistance index and high selectivity index.

Doxycycline, salinomycin, monensin and ivermectin repositioned as cancer drugs

Chemotherapy is one of the standard methods for the treatment of malignant tumors. It aims to cause lethal damage to cellular structures, mainly DNA. Noteworthy, in recent years discoveries of novel anticancer agents from well-known antibiotics have opened up new treatment pathways for several cancer diseases. The aim of this review article is to describe new applications for the following antibiotics: doxycycline (DOX), salinomycin (SAL), monensin (MON) and ivermectin (IVR) as they are known to show anti-tumor activity, but have not yet been introduced into standard oncological therapy. To date, these agents have been used for the treatment of a broad-spectrum of bacterial and parasitic infectious diseases and are widely available, which is why they were selected. The data presented here clearly show that the antibiotics mentioned above should be recognised in the near future as novel agents able to eradicate cancer cells and cancer stem cells (CSCs) across several cancer types.

Monensin inhibits cell proliferation and tumor growth of chemo-resistant pancreatic cancer cells by targeting the EGFR signaling pathway

Pancreatic ductal adenocarcinoma (PDAC) is one of the most deadly malignancies with <5% five-year survival rate due to late diagnosis, limited treatment options and chemoresistance. There is thus an urgent unmet clinical need to develop effective anticancer drugs to treat pancreatic cancer. Here, we study the potential of repurposing monensin as an anticancer drug for chemo-resistant pancreatic cancer. Using the two commonly-used chemo-resistant pancreatic cancer cell lines PANC-1 and MiaPaCa-2, we show that monensin suppresses cell proliferation and migration, and cell cycle progression, while solicits apoptosis in pancreatic cancer lines at a low micromole range. Moreover, monensin functions synergistically with gemcitabine or EGFR inhibitor erlotinib in suppressing cell growth and inducing cell death of pancreatic cancer cells. Mechanistically, monensin suppresses numerous cancer-associated pathways, such as E2F/DP1, STAT1/2, NFkB, AP-1, Elk-1/SRF, and represses EGFR expression in pancreatic cancer lines. Furthermore, the in vivo study shows that monensin blunts PDAC xenograft tumor growth by suppressing cell proliferation via targeting EGFR pathway. Therefore, our findings demonstrate that monensin can be repurposed as an effective anti-pancreatic cancer drug even though more investigations are needed to validate its safety and anticancer efficacy in pre-clinical and clinical models.

One-pot synthesis and antiproliferative activity of novel double-modified derivatives of the polyether ionophore monensin A

Monensin A (MON) is a polyether ionophore antibiotic, which shows a wide spectrum of biological activity. New MON derivatives such as double-modified ester-carbonates and double-modified amide-carbonates were obtained by a new and efficient one-pot synthesis with triphosgene as the activating reagent and the respective alcohol or amine. All new derivatives were tested for their antiproliferative activity against two drug-sensitive (MES-SA, LoVo) and two drug-resistant (MES-SA/DX5, LoVo/DX) cancer cell lines, and were also studied for their antimicrobial activity against different Staphylococcus aureus and Staphylococcus epidermidis bacterial strains. For the first time, the activity of MON and its derivatives against MES-SA and MES-SA/DX5 were evaluated.

Characterization of three pathway-specific regulators for high production of monensin in Streptomyces cinnamonensis

Monensin, a polyether ionophore antibiotic, is produced by Streptomyces cinnamonensis and worldwide used as a coccidiostat and growth-promoting agent in the field of animal feeding. The monensin biosynthetic gene cluster (mon) has been reported. In this study, the potential functions of three putatively pathway-specific regulators (MonH, MonRI, and MonRII) were clarified. The results from gene inactivation, complementation, and overexpression showed that MonH, MonRI, and MonRII positively regulate monensin production. Both MonH and MonRI are essential for monensin biosynthesis, while MonRII is non-essential and could be completely replaced by additional expression of monRI. Transcriptional analysis of the mon cluster by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and electrophoresis mobility shift assays (EMSAs) revealed a co-regulatory cascade process. MonH upregulates the transcription of monRII, and MonRII in turn enhances the transcription of monRI. MonRII is an autorepressor, while MonRI is an autoactivator. MonH activates the transcription of monCII-monE, and upregulates the transcription of monT that is repressed by MonRII. monAX and monD are activated by MonRI, and upregulated by MonRII. Co-regulation of those post-polyketide synthase (post-PKS) genes by MonH, MonRI, and MonRII would contribute to high production of monensin. These results shed new light on the transcriptional regulatory cascades of antibiotic biosynthesis in Streptomyces.

Ca2+ Selective Host Rotaxane Is Highly Toxic Against Prostate Cancer Cells

New therapies are needed to eradicate androgen resistant, prostate cancer. Prostate cancer usually metastasizes to bone where the concentration of calcium is high, making Ca²⁺ a promising toxin. Ionophores can deliver metal cations into cells, but are currently too toxic for human use. We synthesized a new rotaxane (CEHR2) that contains a benzyl 15-crown-5 ether as a blocking group to efficiently bind Ca²⁺. CEHR2 transfers Ca²⁺ from an aqueous solution into CHCl₃ to greater extent than alkali metal cations and Mg²⁺. It also transfers Ca²⁺ to a greater extent than CEHR1, which is a rotaxane with an 18-crown-6 ether as a blocking group. CEHR2 was more toxic against the prostate cancer cell lines PC-3, 22Rv1, and C4-2 than CEHR1. This project demonstrates that crown ether rotaxanes can be designed to bind a targeted metal cation, and this selective cation association can result in enhanced toxicity.

Antibiotic monensin synergizes with EGFR inhibitors and oxaliplatin to suppress the proliferation of human ovarian cancer cells

Ovarian cancer is the most lethal gynecologic malignancy with an overall cure rate of merely 30%. Most patients experience recurrence within 12–24 months of cure and die of progressively chemotherapy-resistant disease. Thus, more effective anti-ovarian cancer therapies are needed. Here, we investigate the possibility of repurposing antibiotic monensin as an anti-ovarian cancer agent. We demonstrate that monensin effectively inhibits cell proliferation, migration and cell cycle progression, and induces apoptosis of human ovarian cancer cells. Monensin suppresses multiple cancer-related pathways including Elk1/SRF, AP1, NFκB and STAT, and reduces EGFR expression in ovarian cancer cells. Monensin acts synergistically with EGFR inhibitors and oxaliplatin to inhibit cell proliferation and induce apoptosis of ovarian cancer cells. Xenograft studies confirm that monensin effectively inhibits tumor growth by suppressing cell proliferation through targeting EGFR signaling. Our results suggest monensin may be repurposed as an anti-ovarian cancer agent although further preclinical and clinical studies are needed.

Early effects of the antineoplastic agent salinomycin on mitochondrial function

Salinomycin, isolated from Streptomyces albus, displays antimicrobial activity. Recently, a large-scale screening approach identified salinomycin and nigericin as selective apoptosis inducers of cancer stem cells. Growing evidence suggests that salinomycin is able to kill different types of non-stem tumor cells that usually display resistance to common therapeutic approaches, but the mechanism of action of this molecule is still poorly understood. Since salinomycin has been suggested to act as a K(+) ionophore, we explored its impact on mitochondrial bioenergetic performance at an early time point following drug application. In contrast to the K(+) ionophore valinomycin, salinomycin induced a rapid hyperpolarization. In addition, mitochondrial matrix acidification and a significant decrease of respiration were observed in intact mouse embryonic fibroblasts (MEFs) and in cancer stem cell-like HMLE cells within tens of minutes, while increased production of reactive oxygen species was not detected. By comparing the chemical structures and cellular effects of this drug with those of valinomycin (K(+) ionophore) and nigericin (K(+)/H(+) exchanger), we conclude that salinomycin mediates K(+)/H(+) exchange across the inner mitochondrial membrane. Compatible with its direct modulation of mitochondrial function, salinomycin was able to induce cell death also in Bax/Bak-less double-knockout MEF cells. Since at the concentration range used in most studies (around 10 μM) salinomycin exerts its effect at the level of mitochondria and alters bioenergetic performance, the specificity of its action on pathologic B cells isolated from patients with chronic lymphocytic leukemia (CLL) versus B cells from healthy subjects was investigated. Mesenchymal stromal cells (MSCs), proposed to mimic the tumor environment, attenuated the apoptotic effect of salinomycin on B-CLL cells. Apoptosis occurred to a significant extent in healthy B cells as well as in MSCs and human primary fibroblasts. The results indicate that salinomycin, when used above μM concentrations, exerts direct, mitochondrial effects, thus compromising cell survival.

Synthesis and Antiproliferative Activity of Silybin Conjugates with Salinomycin and Monensin

Aiming at development of multitarget drugs for the anticancer treatment, new silybin (SIL) conjugates with salinomycin (SAL) and monensin (MON) were synthesized, in mild esterification conditions, and their antiproliferative activity was studied. The conjugates obtained exhibit anticancer activity against HepG2, LoVo and LoVo/DX cancer cell lines. Moreover, MON-SIL conjugate exhibits higher anticancer potential and better selectivity than the corresponding SAL-SIL conjugate.

Maduramicin inhibits proliferation and induces apoptosis in myoblast cells

Maduramicin, a polyether ionophore antibiotic derived from the bacterium Actinomadura yumaensis, is currently used as a feed additive against coccidiosis in poultry worldwide. It has been clinically observed that maduramicin can cause skeletal muscle and heart cell damage, resulting in skeletal muscle degeneration, heart failure, and even death in animals and humans, if improperly used. However, the mechanism of its toxic action in myoblasts is not well understood. Using mouse myoblasts (C2C12) and human rhabdomyosarcoma (RD and Rh30) cells as an experimental model for myoblasts, here we found that maduramicin inhibited cell proliferation and induced cell death in a concentration-dependent manner. Further studies revealed that maduramicin induced accumulation of the cells at G₀/G₁ phase of the cell cycle, and induced apoptosis in the cells. Concurrently, maduramicin downregulated protein expression of cyclin D1, cyclin-dependent kinases (CDK4 and CDK6), and CDC25A, and upregulated expression of the CDK inhibitors (p21^Cip1 and p27^Kip1), resulting in decreased phosphorylation of Rb. Maduramicin also induced expression of BAK, BAD, DR4, TRADD and TRAIL, leading to activation of caspases 8, 9 and 3 as well as cleavage of poly ADP ribose polymerase (PARP). Taken together, our results suggest that maduramicin executes its toxicity in myoblasts at least by inhibiting cell proliferation and inducing apoptotic cell death.

Cytotoxicity of monensin, narasin and salinomycin and their interaction with silybin in HepG2, LMH and L6 cell cultures

The cytotoxic effect of monensin, narasin and salinomycin followed by their co-action with silybin in the cell line cultures of human hepatoma (HepG2), chicken hepatoma (LMH) or rat myoblasts (L6) have been investigated. The effective concentration of the studied ionophoric polyethers has been assessed within two biochemical endpoints: mitochondrial activity (MTT assay) and membrane integrity (LDH assay) after 24h incubation of each compound and farther, the cytotoxicity influenced in course of their interaction with silybin was determined. The most affected endpoints were found for inhibition of mitochondrial activity of the hepatoma cell lines and their viability depended on concentration of the ionophoric polyether, as well as on the cell line tested. The rat myoblasts were more sensitive target for cellular membrane damage when compared to inhibition of mitochondrial activity. An interaction between the ionophoric polyethers and silybin resulted a considerable cytotoxicity decrease within all studied cell lines; the combination index (CI) showed differences of interaction mode and dependence on cell culture, concentration of silybin, as well as the assay used. The obtained results are of interest in respect to recent findings on applicability of salinomycin and monensin for human therapy.

Monensin inhibits epidermal growth factor receptor trafficking and activation: synergistic cytotoxicity in combination with EGFR inhibitors

Targeting the EGFR, with inhibitors such as erlotinib, represents a promising therapeutic option in advanced head and neck squamous cell carcinomas (HNSCC). However, they lack significant efficacy as single agents. Recently, we identified the ability of statins to induce synergistic cytotoxicity in HNSCC cells through targeting the activation and trafficking of the EGFR. However, in a phase I trial of rosuvastatin and erlotinib, statin-induced muscle pathology limited the usefulness of this approach. To overcome these toxicity limitations, we sought to uncover other potential combinations using a 1,200 compound screen of FDA-approved drugs. We identified monensin, a coccidial antibiotic, as synergistically enhancing the cytotoxicity of erlotinib in two cell line models of HNSCC, SCC9 and SCC25. Monensin treatment mimicked the inhibitory effects of statins on EGFR activation and downstream signaling. RNA-seq analysis of monensin-treated SCC25 cells demonstrated a wide array of cholesterol and lipid synthesis genes upregulated by this treatment similar to statin treatment. However, this pattern was not recapitulated in SCC9 cells as monensin specifically induced the expression of activation of transcription factor (ATF) 3, a key regulator of statin-induced apoptosis. This differential response was also demonstrated in monensin-treated ex vivo surgical tissues in which HMG-CoA reductase expression and ATF3 were either not induced, induced singly, or both induced together in a cohort of 10 patient samples, including four HNSCC. These results suggest the potential clinical utility of combining monensin with erlotinib in patients with HNSCC.

Combination of SAHA and bortezomib up-regulates CDKN2A and CDKN1A and induces apoptosis of Epstein-Barr virus-positive Wp-restricted Burkitt lymphoma and lymphoblastoid cell lines

Epstein-Barr virus (EBV) latent proteins exert anti-apoptotic effects on EBV-transformed lymphoid cells by down-regulating BCL2L11 (BIM), CDKN2A (p16(INK4A) ) and CDKN1A (p21(WAF1) ). However, the potential therapeutic effects of targeting these anti-apoptotic mechanisms remain unexplored. Here, we tested both in vitro and in vivo effects of the combination of histone deacetylase (HDAC) and proteasome inhibitors on the apoptosis of six endemic Burkitt lymphoma (BL) lines of different latency patterns (types I and III and Wp-restricted) and three lymphoblastoid cell lines (LCLs). We found that the combination of HDAC and proteasome inhibitors (e.g. SAHA/bortezomib) synergistically induced the killing of Wp-restricted and latency III BL and LCLs but not latency I BL cells. The synergistic killing was due to apoptosis, as evidenced by the high percentage of annexin V positivity and strong cleavage of PARP1 (PARP) and CASP3 (caspase-3). Concomitantly, SAHA/bortezomib up-regulated the expression of CDKN2A and CDKN1A but did not affect the level of BCL2L11 or BHRF1 (viral homologue of BCL2). The apoptotic effects were dependent on reactive oxygen species generation. Furthermore, SAHA/bortezomib suppressed the growth of Wp-restricted BL xenografts in nude mice. This study provides the rationale to test the novel application of SAHA/bortezomib on the treatment of EBV-associated Wp-restricted BL and post-transplant lymphoproliferative disorder.

Monensin inhibits canonical Wnt signaling in human colorectal cancer cells and suppresses tumor growth in multiple intestinal neoplasia mice

The Wnt signaling pathway is required during embryonic development and for the maintenance of homeostasis in adult tissues. However, aberrant activation of the pathway is implicated in a number of human disorders, including cancer of the gastrointestinal tract, breast, liver, melanoma, and hematologic malignancies. In this study, we identified monensin, a polyether ionophore antibiotic, as a potent inhibitor of Wnt signaling. The inhibitory effect of monensin on the Wnt/β-catenin signaling cascade was observed in mammalian cells stimulated with Wnt ligands, glycogen synthase kinase-3 inhibitors, and in cells transfected with β-catenin expression constructs. Furthermore, monensin suppressed the Wnt-dependent tail fin regeneration in zebrafish and Wnt- or β-catenin-induced formation of secondary body axis in Xenopus embryos. In Wnt3a-activated HEK293 cells, monensin blocked the phoshorylation of Wnt coreceptor low-density lipoprotein receptor related protein 6 and promoted its degradation. In human colorectal carcinoma cells displaying deregulated Wnt signaling, monensin reduced the intracellular levels of β-catenin. The reduction attenuated the expression of Wnt signaling target genes such as cyclin D1 and SP5 and decreased the cell proliferation rate. In multiple intestinal neoplasia (Min) mice, daily administration of monensin suppressed progression of the intestinal tumors without any sign of toxicity on normal mucosa. Our data suggest monensin as a prospective anticancer drug for therapy of neoplasia with deregulated Wnt signaling.

Gramicidin A induces metabolic dysfunction and energy depletion leading to cell death in renal cell carcinoma cells

Ionophores are lipid-soluble organic molecules that disrupt cellular transmembrane potential by rendering biologic membranes permeable to specific ions. They include mobile-carriers that complex with metal cations and channel-formers that insert into the membrane to form hydrophilic pores. Although mobile-carriers possess anticancer properties, investigations on channel-formers are limited. Here, we used the channel-forming ionophore gramicidin A to study its effects on the growth and survival of renal cell carcinoma (RCC) cells. RCC is a histologically heterogeneous malignancy that is highly resistant to conventional treatments. We found that gramicidin A reduced the in vitro viability of several RCC cell lines at submicromolar concentrations (all IC50 < 1.0 μmol/L). Gramicidin A exhibited similar toxicity in RCC cells regardless of histologic subtype or the expression of either the von Hippel-Lindau tumor suppressor gene or its downstream target, hypoxia-inducible factor-1α. Gramicidin A decreased cell viability equal to or greater than the mobile-carrier monensin depending on the cell line. Mechanistic examination revealed that gramicidin A blocks ATP generation by inhibiting oxidative phosphorylation and glycolysis, leading to cellular energy depletion and nonapoptotic cell death. Finally, gramicidin A effectively reduced the growth of RCC tumor xenografts in vivo. These results show a novel application of gramicidin A as a potential therapeutic agent for RCC therapy.

Cyclooxygenase-2 is induced by p38 MAPK and promotes cell survival

The Na+ ionophore monensin affects cellular pH and, depending on its concentration, causes the survival or death of tumor cells. In the present study, we elucidated the survival pathway activated in U937 cells, a human lymphoma-derived cell line. These cells treated with monensin at a concentration of 5 µM were growth-arrested in G1, activated p38 mitogen-activated protein kinase (MAPK) and showed an increased expression of cyclooxygenase-2 (COX-2). The latter two molecular events were linked, as pharmacological inhibition of the MAPK did not allow COX-2 increased expression. Furthermore, we showed that p38 and COX-2 keep monensin-stressed U937 cells alive, as pharmacological inhibition of each enzyme caused cell death.

Cytostatic and cytotoxic properties of monensic acid and its biometal(II) complexes against human tumor / non-tumor cell lines

The anticancer activity of monensic acid (MonH) and its biometal(II) complexes [M(Mon)2(H2O)2](M = Mg, Ca, Mn, Co, Ni, Zn) was evaluated against cultured human permanent cell lines established from glioblastoma multiforme (8MGBA) and cancers of the lung (A549), breast (MCF-7), uterine cervix (HeLa) and liver (HepG2). The viability and proliferation of the non-tumor human embryonic cell line Lep3 was also tested. The investigations were carried out using a thiazolyl blue tetrazolium bromide test, neutral red uptake cytotoxicity assay, crystal violet staining, colony forming method and double staining with acridin orange and propidium iodide. The results obtained reveal that the compounds applied at concentrations of 0.5–25 µg mL−1 for 24–72 h decrease the viability and proliferation of the treated cells in a time- and concentration-dependent manner. The metal(II) complexes studied (especially those of Co(II), Ni(II) and Zn(II)) have been found to express stronger cytotoxic and cytostatic activities than the non-coordinated monensic acid. The non-tumor human cell line showed strong chemosensitivity towards compounds tested comparable to that of cultured human tumor cell lines.

Expression profile of apoptosis related genes and radio-sensitivity of prostate cancer cells

Radio-resistant or recurrent prostate cancer represents a serious health risk for approximately 20%-30% of patients treated with primary radiation therapy for clinically localized prostate cancer. In the present study, we investigated the expression profiles of 84 genes involved in various apoptosis pathways in two prostate cancer cell lines LNCaP (P53+ and AR+) and PC3 (P53- and AR-). We also studied the effect of monensin, an apoptosis inducing reagent, in X-ray-induced cell killing. Comparison of gene expressions between unirradiated LNCaP and PC3 cells revealed distinguished gene expression patterns. The data showed a significantly higher expression level of genes involved in the caspase/card family and the TNF ligand/receptor family in PC3 cells, whereas, LNCaP cells exhibited higher expressions in the p53 related genes. At 2 and 4 hrs post a 10 Gy X-ray exposure, changes of gene expressions were detected in a significant fraction of the genes in LNCaP cells, but no significant changes were found in PC3 cells. There was no significant apoptosis-inducing effect of X-rays (up to 10 Gy) in both cell lines; however, monensin was shown to be effective in inducing apoptosis in LNCaP, but not in PC3 cells. In addition, the effect of combined treatment of monensin and X-rays in LNCaP cells appeared to be synergistic. Our results suggest that monensin may be effective for both cancer cell killing and radiosensitizing, and the different expression profiles in apoptosis related genes in cancer cells may be correlated with their sensitivity to apoptosis inducing reagents.

Palytoxin causes nonoxidative necrotic damage to PC12 cells in culture

Palytoxin (PTX) is a potent marine toxin that causies serious damage to various tissues and organs. It has been reported to affect the transport of cations across the plasma membranes, which is commonly recognized as being the principal mechanism of its highly toxic action on mammals, including humans. However, although some marine toxins have been shown to cause toxic effects on the nervous system by interfering with the transmission of nerve impulses, the effect of PTX on neuronal cells has not yet been fully elucidated. Therefore, the toxic action of PTX on PC12 cells was examined as an in vitro model experiment to elucidate the neurotoxic properties of this toxin, and PTX was shown to reduce the viability of PC12 cells in a concentration-dependent manner. The cytotoxic action of PTX was not significantly altered by the presence of the antioxidant N-acetylcysteine and reduced-form glutathione in the cultures. Fluorescence staining of the cells and the electrophoretic analysis of genomic DNA showed that PTX failed to cause chromatin condensation and DNA fragmentation within the cells. On the other hand, the exposure to PTX caused positive staining of the cytoplasmic space of the cells with propidium iodide and the release of lactate dehydrogenase into the culture medium. Based on these observations, PTX is considered to cause cell death as a consequence of disrupting the plasma membranes, thus causing nonoxidative necrotic damage to PC12 cells.

Monensin is a potent inducer of oxidative stress and inhibitor of androgen signaling leading to apoptosis in prostate cancer cells

Current treatment options for advanced and hormone refractory prostate cancer are limited and responses to commonly used androgen pathway inhibitors are often unsatisfactory. Our recent results indicated that sodium ionophore monensin is one of the most potent and cancer-specific inhibitors in a systematic sensitivity testing of most known drugs and drug-like molecules in a panel of prostate cancer cell models. Because monensin has been extensively used in veterinary applications to build muscle mass in cattle, the link to prostate cancer and androgen signaling was particularly interesting. Here, we showed that monensin effects at nanomolar concentrations are linked to induction of apoptosis and potent reduction of androgen receptor mRNA and protein in prostate cancer cells. Monensin also elevated intracellular oxidative stress in prostate cancer cells as evidenced by increased generation of intracellular reactive oxygen species and by induction of a transcriptional profile characteristic of an oxidative stress response. Importantly, the antiproliferative effects of monensin were potentiated by combinatorial treatment with the antiandrogens and antagonized by antioxidant vitamin C. Taken together, our results suggest monensin as a potential well-tolerated, in vivo compatible drug with strong proapoptotic effects in prostate cancer cells, and synergistic effects with antiandrogens. Moreover, our data suggest a general strategy by which the effects of antiandrogens could be enhanced by combinatorial administration with agents that increase oxidative stress in prostate cancer cells.

Salinomycin induces apoptosis and overcomes apoptosis resistance in human cancer cells

Salinomycin is a polyether antibiotic isolated from Streptomyces albus that acts in different biological membranes as a ionophore with a preference for potassium. It is widely used as an anticoccidial drug in poultry and is fed to ruminants to improve nutrient absorption and feed efficiency. Salinomycin has recently been shown to selectively deplete human breast cancer stem cells from tumorspheres and to inhibit breast cancer growth and metastasis in mice. We show here that salinomycin induces massive apoptosis in human cancer cells of different origin, but not in normal cells such as human T lymphocytes. Moreover, salinomycin is able to induce apoptosis in cancer cells that exhibit resistance to apoptosis and anticancer agents by overexpression of Bcl-2, P-glycoprotein or 26S proteasomes with enhanced proteolytic activity. Salinomycin activates a distinct apoptotic pathway that is not accompanied by cell cycle arrest and that is independent of tumor suppressor protein p53, caspase activation, the CD95/CD95L system and the proteasome. Thus, salinomycin should be considered as a novel and effective anticancer agent that overcomes multiple mechanisms of apoptosis resistance in human cancer cells.

High-throughput cell-based screening of 4910 known drugs and drug-like small molecules identifies disulfiram as an inhibitor of prostate cancer cell growth

PURPOSE

To identify novel therapeutic opportunities for patients with prostate cancer, we applied high-throughput screening to systematically explore most currently marketed drugs and drug-like molecules for their efficacy against a panel of prostate cancer cells.

EXPERIMENTAL DESIGN

We carried out a high-throughput cell-based screening with proliferation as a primary end-point using a library of 4,910 drug-like small molecule compounds in four prostate cancer (VCaP, LNCaP, DU 145, and PC-3) and two nonmalignant prostate epithelial cell lines (RWPE-1 and EP156T). The EC(50) values were determined for each cell type to identify cancer selective compounds. The in vivo effect of disulfiram (DSF) was studied in VCaP cell xenografts, and gene microarray and combinatorial studies with copper or zinc were done in vitro for mechanistic exploration.

RESULTS

Most of the effective compounds, including antineoplastic agents, were nonselective and found to inhibit both cancer and control cells in equal amounts. In contrast, histone deacetylase inhibitor trichostatin A, thiram, DSF, and monensin were identified as selective antineoplastic agents that inhibited VCaP and LNCaP cell proliferation at nanomolar concentrations. DSF reduced tumor growth in vivo, induced metallothionein expression, and reduced DNA replication by downregulating MCM mRNA expression. The effect of DSF was potentiated by copper in vitro.

CONCLUSIONS

We identified three novel cancer-selective growth inhibitory compounds for human prostate cancer cells among marketed drugs. We then validated DSF as a potential prostate cancer therapeutic agent. These kinds of pharmacologically well-known molecules can be readily translated to in vivo preclinical studies and clinical trials.

Antitumor compounds from actinomycetes: from gene clusters to new derivatives by combinatorial biosynthesis

Covering: up to October 2008. Antitumor compounds produced by actinomycetes and novel derivatives generated by combinatorial biosynthesis are reviewed (with 318 references cited.) The different structural groups for which the relevant gene clusters have been isolated and characterized are reviewed, with a description of the strategies used for the generation of the novel derivatives and the activities of these compounds against tumor cell lines.

Mitochondrial damage as an early event of monensin-induced cell injury in cultured fibroblasts L929

The present study was designed to identify, submicroscopically, the primary organelle or target structure for monensin in cultured murine fibroblasts L929. In addition, the effect of the drug on cell size and surface membranes of the cells were analysed; cellular proliferation, collagen secretion, and necrosis and apoptosis were re-evaluated. At the lowest concentration of monensin the foremost ultrastructural alteration occurred in the mitochondria, characterized by increased matrix density with disorganized and less distinct crystae. Incubation with monensin at higher concentrations resulted in severe mitochondrial damage and marked dilatation of the Golgi apparatus and rough endoplasmic reticulum cisternae. Fibroblasts exposed to higher concentrations of monensin were enlarged with decreased number of filopodia and hollows in the surface membrane. Moreover, monensin inhibited the cell proliferation, increased immunohistochemical positiveness for collagen type I in a dose-dependent manner, and, at high concentrations, caused cell necrosis whereas apoptosis was not induced. Taken together, these results show that monensin induces early mitochondrial damage, possibly causing an energy deficit that led to inhibition of fibroblasts proliferation and accumulation of collagen causing dilatation of Golgi apparatus and rough endoplasmic reticulum. Moreover, the mitochondrial damage would also explain the monensin-induced necrosis.