Salinomycin, a p-glycoprotein inhibitor, sensitizes radiation-treated cancer cells by increasing DNA damage and inducing G2 arrest

Mechanism of action of salinomycin on growth and migration in pancreatic cancer cell lines

OBJECTIVES

Pancreatic cancer is one of the most aggressive and lethal cancers worldwide and there are few effective treatments. Recently, salinomycin (Sal) was reported to alter proliferation and apoptosis in various tumors. This prompted us to investigate the effect of Sal on pancreatic cancer cells and to explore the possible molecular mechanism in vitro.

METHODS

After treatment with Sal, pancreatic cancer cell viability and apoptosis were analyzed by Hoechst 33342 staining and flow cytometry, respectively. Invasion and metastasis of pancreatic cancer cells were assayed by a Transwell migration assay. Flow cytometry was also used to assessed the fraction of CD133(+) cell subpopulations. The expression of proliferating cell nuclear antigen (PCNA), Bcl-2, E-cadherin, and Wnt/β-catenin signaling-related proteins were detected by RT-PCR and western blot.

RESULTS

Sal inhibited the growth and migration of pancreatic cancer cells in vitro in a dose- and time-dependent manner. We found that the proportion of CD133(+) cell subpopulations decreased after treatment with Sal in pancreatic cancer cell lines at the same time. The percentage of apoptotic cells was increased after Sal treatment. Compared with control groups, Bax and E-cadherin were significantly upregulated, and Bcl-2 and PCNA were significantly downregulated in Sal-treated cells. The expression of Wnt/β-catenin signaling-related proteins (β-catenin and p-GSK-3β) was inhibited.

CONCLUSIONS

These results indicate that Sal could influence the cell growth and migration in pancreatic cancer cells in vitro, which may occur by inhibition of Wnt/β-catenin signaling.

Salinomycin as a drug for targeting human cancer stem cells

Cancer stem cells (CSCs) represent a subpopulation of tumor cells that possess self-renewal and tumor initiation capacity and the ability to give rise to the heterogenous lineages of malignant cells that comprise a tumor. CSCs possess multiple intrinsic mechanisms of resistance to chemotherapeutic drugs, novel tumor-targeted drugs, and radiation therapy, allowing them to survive standard cancer therapies and to initiate tumor recurrence and metastasis. Various molecular complexes and pathways that confer resistance and survival of CSCs, including expression of ATP-binding cassette (ABC) drug transporters, activation of the Wnt/β-catenin, Hedgehog, Notch and PI3K/Akt/mTOR signaling pathways, and acquisition of epithelial-mesenchymal transition (EMT), have been identified recently. Salinomycin, a polyether ionophore antibiotic isolated from Streptomyces albus, has been shown to kill CSCs in different types of human cancers, most likely by interfering with ABC drug transporters, the Wnt/β-catenin signaling pathway, and other CSC pathways. Promising results from preclinical trials in human xenograft mice and a few clinical pilote studies reveal that salinomycin is able to effectively eliminate CSCs and to induce partial clinical regression of heavily pretreated and therapy-resistant cancers. The ability of salinomycin to kill both CSCs and therapy-resistant cancer cells may define the compound as a novel and an effective anticancer drug.

A novel carbazole derivative, MHY407, sensitizes cancer cells to doxorubicin-, etoposide-, and radiation treatment via DNA damage

In this study, we synthesized a novel carbazole derivative, MHY407, as a sensitizer of cancer cells to increase DNA damage. We then evaluated the anticancer effects of MHY407 and identified the molecular mechanism for the sensitization of breast cancer cell lines. MHY407 significantly increased DNA damage as determined by DNA breakage, levels of damage-responsive proteins, and DNA foci. In addition, MHY407 increased p21 and decreased cyclin D1 protein levels. MHY407 also involved increased cell cycle arrest at the S phase. Furthermore, in doxorubicin and etoposide-treated breast cancer cells, co-treatment with MHY407 reduced cell viability and increased apoptosis. Co-treatment of MHY407 with doxorubicin or etoposide increased DNA damage-related proteins and foci formation, suggesting that increased DNA damage by MHY407 plays an important role in the sensitization. In addition, MHY407 also sensitized the cancer cells to DNA damaging radiation treatment. These results may contribute to the development of MHY407-based treatments for cancer patients receiving DNA-damage therapy.

Lower salinomycin concentration increases apoptotic detachment in high-density cancer cells

The present study identified a novel salinomycin (Sal) sensitization mechanism in cancer. We tested whether Sal reduced proliferation in a high-density population by counting attached cell numbers after Sal treatment. Sal reduced proliferation in high-density cell populations. Longer exposure to Sal further reduced proliferation. Sal concentrations of 0.1 and 5 μM had similar sensitization effects, suggesting that Sal toxicity was minimal with longer exposure to a high-density cell population. The results suggest that Sal can be applied at a relatively low concentration for a longer time to overcome drug-resistant solid tumors. The 0.5 μM Sal treatment resulted in fewer attached cells than that of the 5 μM Sal treatment with a longer exposure. The lower Sal concentration mainly increased the number of easily detachable cells on the surface. In particular, 0.5 μM Sal increased cellular detachment of newly produced daughter cells. The easily-detachable cells were undergoing apoptosis. It seems that the 0.5 μM Sal treatment also increased cellular toxicity. These novel findings may contribute to the development of Sal-based therapy for patients with drug-resistant cancer or a high-density solid tumor.

Impact of Salinomycin on human cholangiocarcinoma: induction of apoptosis and impairment of tumor cell proliferation in vitro

Background

Cholangiocarcinoma (CC) is a primary liver cancer with increasing incidence worldwide. Despite all efforts made in past years, prognosis remains to be poor. At least in part, this might be explained by a pronounced resistance of CC cells to undergo apoptosis. Thus, new therapeutic strategies are imperatively required. In this study we investigated the effect of Salinomycin, a polyether ionophore antibiotic, on CC cells as an appropriate agent to treat CC. Salinomycin was quite recently identified to induce apoptosis in cancer stem cells and to overcome apoptosis-resistance in several leukemia-cells and other cancer cell lines of different origin.

Methods

To delineate the effects of Salinomycin on CC, we established an in vitro cell culture model using three different human CC cell lines. After treatment apoptosis as well as migration and proliferation behavior was assessed and additional cell cycle analyses were performed by flowcytometry.

Results

By demonstrating Annexin V and TUNEL positivity of human CC cells, we provide evidence that Salinomycin reveals the capacity to break apoptosis-resistance in CC cells. Furthermore, we are able to demonstrate that the non-apoptotic cell fraction is characterized by sustainable impaired migration and proliferation. Cell cycle analyses revealed G2-phase accumulation of human CC cells after treatment with Salinomycin. Even though apoptosis is induced in two of three cell lines of CC cells, one cell line remained unaffected in regard of apoptosis but revealed as the other CC cells decreased proliferation and migration.

Conclusion

In this study, we are able to demonstrate that Salinomycin is an effective agent against previously resistant CC cells and might be a potential candidate for the treatment of CC in the future.

Salinomycin sensitizes antimitotic drugs-treated cancer cells by increasing apoptosis via the prevention of G2 arrest

Here, we investigated whether Sal could sensitize cancer cells to antimitotic drugs. We demonstrated that Sal sensitized paclitaxcel (PAC)-, docetaxcel (DOC)-, vinblastin (VIN)-, or colchicine (COL)-treated cancer cell lines, suggesting that Sal has the ability to sensitize the cells to any form of microtubule-targeting drugs. Sensitization to the antimitotic drugs could be achieved with very low concentrations of Sal, suggesting that there is a possibility to minimize Sal toxicity associated with human cancer patient treatments. Sensitization by Sal increased apoptosis, which was observed by C-PARP production. Sal sensitized the cancer cells to antimitotic drugs by preventing G2 arrest, suggesting that Sal contributes to the induction of mitotic catastrophe. Sal generally reduced cyclin D1 levels in PAC-, DOC-, and VIN-treated cells. In addition, Sal treatment increased pH2AX levels and reduced p21 levels in antimitotic drugs-treated cells. These observations suggest that the mechanisms underlying Sal sensitization to DNA-damaging compounds, radiation, and microtubule-targeting drugs are similar. Our data demonstrated that Sal sensitizes cancer cells to antimitotic drugs by increasing apoptosis through the prevention of G2 arrest via conserved Sal-sensitization mechanisms. These results may contribute to the development of Sal-based chemotherapy for cancer patients treated with antimitotic drugs.