Monitoring of autophagy is complicated--salinomycin as an example

Salinomycin as a potent anticancer stem cell agent: State of the art and future directions

Cancer stem cells (CSCs) are a small subpopulation of cells within a tumor that can both self‐renew and differentiate into other cell types forming the heterogeneous tumor bulk. Since CSCs are involved in all aspects of cancer development, including tumor initiation, cell proliferation, metastatic dissemination, therapy resistance, and recurrence, they have emerged as attractive targets for cancer treatment and management. Salinomycin, a widely used antibiotic in poultry farming, was identified by the Weinberg group as a potent anti‐CSC agent in 2009. As a polyether ionophore, salinomycin exerts broad‐spectrum activities, including the important anti‐CSC function. Studies on the mechanism of action of salinomycin against cancer have been continuously and rapidly published since then. Thus, it is imperative for us to update its literature of recent research findings in this area. We here summarize the notable work reported on salinomycin's anticancer activities, intracellular binding target(s), effects on tumor microenvironment, safety, derivatives, and tumor‐specific drug delivery; after that we also discuss the translational potential of salinomycin toward clinical application based on current multifaceted understandings.

Resveratrol induces proliferation in preosteoblast cell MC3T3-E1 via GATA-1 activating autophagy

Resveratrol (RSV) could promote osteogenic activity, but its clinical application has been hampered in view of its poor bioavailability. Therefore, it is desirable to identify with certainty the molecular target of its bone mass boosting function, which is crucial to the design of an effective therapeutic strategy for the optimal treatment of osteoporosis. Emerging evidence has indicated that GATA-1, an important transcription factor in megakaryocyte and erythrocyte differentiation, can directly activate autophagy in erythrocytes, alluding to its impact on bone metabolism. In light of this, we sought to determine whether GATA-1 would be a putative target by which RSV would act on osteoblast proliferation and, if so, to explore the underlying mechanism involved in the process. We examined the cell viability, colony formation, cell cyclin expression, autophagy level, and the expression levels of GATA-1 and adenosine 5'-monophosphate (AMP)-activated protein kinase α (AMPKα) in osteoblastic cell strain MC3T3-E1. The results showed that RSV promoted the proliferation process in MC3T3-E1 coupled with increased expression of GATA-1 and phosphorylated AMPKα and activated autophagy. When GATA-1 was interfered with siRNA, both autophagy and proliferation were decreased. Administration of the agonist of phosphorylated AMPKα1 (Thr172) promoted the translocation of GATA-1 into the nucleus. Based on the above results, we concluded that RSV induces the proliferation of MC3T3-E1 by increasing GATA-1 expression, which thence activates autophagy; and of note, AMPKα is one of the upstream regulators of GATA-1.

Sulfuretin protects hepatic cells through regulation of ROS levels and autophagic flux

Palmitate (PA) exposure induces stress conditions featuring ROS accumulation and upregulation of p62 expression, resulting in autophagic flux blockage and cell apoptosis. Sulfuretin (Sul) is a natural product isolated from Rhus verniciflua Stokes; the cytoprotective effect of Sul on human hepatic L02 cells and mouse primary hepatocytes under PA-induced stress conditions was investigated in this study. Sul induced mitophagy by activation of p-TBK1 and LC3 and produced a concomitant decline in p62 expression. Autophagosome formation and mitophagy were assessed by the sensitive dual fluorescence reporter mCherry-EGFP-LC3B, and mitochondrial fragmentation was analyzed using MitoTracker Deep Red FM. A preliminary structure-activity relationship (SAR) for Sul was also investigated, and the phenolic hydroxyl group was found to be pivotal for maintaining the cytoprotective bioactivity of Sul. Furthermore, experiments using flow cytometry and western blots revealed that Sul reversed the cytotoxic effect stimulated by the autophagy inhibitors 3-methyladenine (3-MA) and chloroquine (CQ), and its cytoprotective effect was almost eliminated when the autophagy-related 5 (Atg5) gene was knocked down. These studies suggest that, in addition to its antioxidative effects, Sul stimulates mitophagy and restores impaired autophagic flux, thus protecting hepatic cells from apoptosis, and that Sul has potential future medical applications for hepatoprotection.

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

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

Combination of anthracyclines and anti-CD47 therapy inhibit invasive breast cancer growth while preventing cardiac toxicity by regulation of autophagy

BACKGROUND

A perennial challenge in systemic cytotoxic cancer therapy is to eradicate primary tumors and metastatic disease while sparing normal tissue from off-target effects of chemotherapy. Anthracyclines such as doxorubicin are effective chemotherapeutic agents for which dosing is limited by development of cardiotoxicity. Our published evidence shows that targeting CD47 enhances radiation-induced growth delay of tumors while remarkably protecting soft tissues. The protection of cell viability observed with CD47 is mediated autonomously by activation of protective autophagy. However, whether CD47 protects cancer cells from cytotoxic chemotherapy is unknown.

METHODS

We tested the effect of CD47 blockade on cancer cell survival using a 2-dimensional high-throughput cell proliferation assay in 4T1 breast cancer cell lines. To evaluate blockade of CD47 in combination with chemotherapy in vivo, we employed the 4T1 breast cancer model and examined tumor and cardiac tissue viability as well as autophagic flux.

RESULTS

Our high-throughput screen revealed that blockade of CD47 does not interfere with the cytotoxic activity of anthracyclines against 4T1 breast cancer cells. Targeting CD47 enhanced the effect of doxorubicin chemotherapy in vivo by reducing tumor growth and metastatic spread by activation of an anti-tumor innate immune response. Moreover, systemic suppression of CD47 protected cardiac tissue viability and function in mice treated with doxorubicin.

CONCLUSIONS

Our experiments indicate that the protective effects observed with CD47 blockade are mediated through upregulation of autophagic flux. However, the absence of CD47 in did not elicit a protective effect in cancer cells, but it enhanced macrophage-mediated cancer cell cytolysis. Therefore, the differential responses observed with CD47 blockade are due to autonomous activation of protective autophagy in normal tissue and enhancement immune cytotoxicity against cancer cells.

Targeting protein homeostasis with nelfinavir/salinomycin dual therapy effectively induces death of mTORC1 hyperactive cells

Uncontrolled cell growth in Tuberous Sclerosis Complex occurs due to inappropriate activation of mechanistic (mammalian) target of rapamycin complex 1 (mTORC1). The current therapy, rapamycin, produced promising clinical trial results, but patient tumours regrow if treatment is discontinued, revealing rapamycin has cytostatic properties rather than a cytotoxic effect. Taking advantage of the enhanced levels of endoplasmic reticulum (ER) stress present in TSC2-null cells, we investigated drug combinations producing a cytotoxic response. We found a nelfinavir and salinomycin combination specifically killed TSC2-deficient, mTORC1 hyperactive cells. Cytotoxicity was rescued by reducing protein synthesis, either through mTORC1 inhibition or cycloheximide treatment. This indicates that the drug combination targets the cells by tipping the protein homeostasis balance of the already metabolically stressed TSC2-deficient cells in favour of cell death. Furthermore, this drug combination also inhibited tumour formation in TSC2-deficient cell models and caused tumour spheroid death in 3D culture. Importantly, the 3D assay could differentiate the cytostatic agent, rapamycin, from the cytotoxic nelfinavir/salinomycin combination. Sporadic cancer cell lines with hyperactive mTORC1 signalling were also susceptible to this nelfinavir/salinomycin drug combination. This work indicates that the protein homeostasis pathway is an attractive therapeutic target in both Tuberous Sclerosis Complex and mTORC1-driven sporadic cancers.

Low-dose salinomycin induces anti-leukemic responses in AML and MLL

Development of anti-cancer drugs towards clinical application is costly and inefficient. Large screens of drugs, efficacious for non-cancer disease, are currently being used to identify candidates for repurposing based on their anti-cancer properties. Here, we show that low-dose salinomycin, a coccidiostat ionophore previously identified in a breast cancer screen, has anti-leukemic efficacy. AML and MLLr cell lines, primary cells and patient samples were sensitive to submicromolar salinomycin. Most strikingly, colony formation of normal hematopoietic cells was unaffected by salinomycin, demonstrating a lack of hemotoxicity at the effective concentrations. Furthermore, salinomycin treatment of primary cells resulted in loss of leukemia repopulation ability following transplantation, as demonstrated by extended recipient survival compared to controls. Bioinformatic analysis of a 17-gene signature identified and validated in primary MLLr cells, uncovered immunomodulatory pathways, hubs and protein interactions as potential transducers of low dose salinomycin treatment. Additionally, increased protein expression of p62/Sqstm1, encoded for by one of the 17 signature genes, demonstrates a role for salinomycin in aggresome/vesicle formation indicative of an autophagic response.

Together, the data support the efficacy of salinomycin as an anti-leukemic at non-hemotoxic concentrations. Further investigation alone or in combination with other therapies is warranted for future clinical trial.

Salinomycin co-treatment enhances tamoxifen cytotoxicity in luminal A breast tumor cells by facilitating lysosomal degradation of receptor tyrosine kinases

Luminal A breast cancer is the most common breast cancer subtype which is usually treated with selective estrogen receptor modulators (SERMS) like tamoxifen. Nevertheless, one third of estrogen receptor positive breast cancer patients initially do not respond to endocrine therapy and about 40% of luminal A breast tumors recur in five years. In this study, we investigated an alternative treatment approach by combining tamoxifen and salinomycin in luminal A breast cancer cell lines. We have found that salinomycin induces an additional cytotoxic effect by inhibiting the ligand independent activation of ERα. Thereby salinomycin increases the intracellular calcium level. This leads to a premature fusion of endosomes with lysosomes and thus to the degradation of Egfr family members. Since this process is essential for luminal A breast cancer cells to circumvent tamoxifen treatment, the combination of both drugs induces cytotoxicity in tamoxifen sensitive as well as resistant luminal A breast cancer cell lines.

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

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

Salinomycin inhibits cholangiocarcinoma growth by inhibition of autophagic flux

Introduction

Cholangiocarcinoma is characterized by aggressive tumor growth, high recurrence rates, and resistance against common chemotherapeutical regimes. The polyether-antibiotic Salinomycin is a promising drug in cancer therapy because of its ability to overcome apoptosis resistance of cancer cells and its selectivity against cancer stem cells. Here, we investigated the effectiveness of Salinomycin against cholangiocarcinoma in vivo, and analyzed interference of Salinomycin with autophagic flux in human cholangiocarcinoma cells.

Results

Salinomycin reduces tumor cell viability, proliferation, migration, invasion, and induced apoptosis in vitro. Subcutaneous and intrahepatic cholangiocarcinoma growth in vivo was inhibited upon Salinomycin treatment. Analysis of autophagy reveals inhibition of autophagic activity. This was accompanied by accumulation of mitochondrial mass and increased generation of reactive oxygen species.

Conclusions

This study demonstrates the effectiveness of Salinomycin against cholangiocarcinoma in vivo. Inhibition of autophagic flux represents an underlying molecular mechanism of Salinomycin against cholangiocarcinoma.

Methods

The two murine cholangiocarcinoma cell lines p246 and p254 were used to analyze tumor cell proliferation, viability, migration, invasion, and apoptosis in vitro. For in vivo studies, murine cholangiocarcinoma cells were injected into syngeneic C57-BL/6-mice to initiate subcutaneous cholangiocarcinoma growth. Intrahepatic tumor growth was induced by electroporation of oncogenic transposon-plasmids into the left liver lobe. For mechanistic studies in human cells, TFK-1 and EGI-1 were used, and activation of autophagy was analyzed after exposure to Salinomycin.

Therapeutic implications of tumor interstitial acidification

Interstitial acidification is a hallmark of solid tumor tissues resulting from the combination of different factors, including cellular buffering systems, defective tissue perfusion and high rates of cellular metabolism. Besides contributing to tumor pathogenesis and promoting tumor progression, tumor acidosis constitutes an important intrinsic and extrinsic mechanism modulating therapy sensitivity and drug resistance. In fact, pharmacological properties of anticancer drugs can be affected not only by tissue structure and organization but also by the distribution of the interstitial tumor pH. The acidic tumor environment is believed to create a chemical barrier that limits the effects and activity of many anticancer drugs. In this review article we will discuss the general protumorigenic effects of acidosis, the role of tumor acidosis in the modulation of therapeutic efficacy and potential strategies to overcome pH-dependent therapy-resistance.

Autophagy and Tubular Cell Death in the Kidney

Many common renal insults such as ischemia and toxic injury primarily target the tubular epithelial cells, especially the highly metabolically active proximal tubular segment. Tubular epithelial cells are particularly dependent on autophagy to maintain homeostasis and respond to stressors. The pattern of autophagy in the kidney has a unique spatial and chronologic signature. Recent evidence has shown that there is complex cross-talk between autophagy and various cell death pathways. This review specifically discusses the interplay between autophagy and cell death in the renal tubular epithelia. It is imperative to review this topic because recent discoveries have improved our mechanistic understanding of the autophagic process and have highlighted its broad clinical applications, making autophagy a major target for drug development.

Glucose starvation-mediated inhibition of salinomycin induced autophagy amplifies cancer cell specific cell death

Salinomycin has been used as treatment for malignant tumors in a small number of humans, causing far less side effects than standard chemotherapy. Several studies show that Salinomycin targets cancer-initiating cells (cancer stem cells, or CSC) resistant to conventional therapies. Numerous studies show that Salinomycin not only reduces tumor volume, but also decreases tumor recurrence when used as an adjuvant to standard treatments. In this study we show that starvation triggered different stress responses in cancer cells and primary normal cells, which further improved the preferential targeting of cancer cells by Salinomycin. Our in vitro studies further demonstrate that the combined use of 2-Fluoro 2-deoxy D-glucose, or 2-deoxy D-glucose with Salinomycin is lethal in cancer cells while the use of Oxamate does not improve cell death-inducing properties of Salinomycin. Furthermore, we show that treatment of cancer cells with Salinomycin under starvation conditions not only increases the apoptotic caspase activity, but also diminishes the protective autophagy normally triggered by the treatment with Salinomycin alone. Thus, this study underlines the potential use of Salinomycin as a cancer treatment, possibly in combination with short-term starvation or starvation-mimicking pharmacologic intervention.