Digoxin enhances radiation response in radioresistant A549 cells by reducing protein phosphatase 2A

Abstract

Protein phosphatase 2A (PP2A) is a ubiquitous multifunctional enzyme usually known as a tumor suppressor. Recent studies have reported that although inhibition of PP2A leads to acceleration of cell growth, it also induces damaged cells to pass through the cell cycle and renders them sensitive to radiotherapy. Here, we investigated the radiosensitizing effects of digoxin as a PP2A inhibitor in two non-small-cell lung cancer (NSCLC) cell types (H460 and A549) with differential sensitivity to radiation. Digoxin inhibited the proliferation of H460 and A549 cells in a dose-dependent fashion and was especially effective on radioresistant A549 cells. Interestingly, the radiosensitizing effect of digoxin was only present in the radioresistant A549 cells and xenografts. The combination of digoxin and ionizing radiation (IR) significantly reduced clonogenic survival and xenograft tumor growth (P<0.001), compared with IR alone. Digoxin suppressed PP2A protein expression and prevented IR-induced PP2A expression in A549 cells. Digoxin treatment combined with IR allowed the damaged cell to progress through the cell cycle via suppression of cell cycle-related proteins (p53, cyclin D1, cyclin B1, CDK4, and p-cdc2). Moreover, digoxin enhanced IR-induced DNA damage through reduction in levels of repair proteins and elevation of p-ATM foci formation up to 24 h (P<0.001). In conclusion, digoxin has a novel function as a PP2A inhibitor, and combined with IR produces a synergistic effect on radiosensitizing cells, thereby indicating a potentially promising therapeutic approach to radioresistant lung cancer treatment.

Digoxin Enhances the Anticancer Effect on Non-Small Cell Lung Cancer While Reducing the Cardiotoxicity of Adriamycin

“…Recently, digoxin was reported to act as radio-sensitizer, by inhibiting DSB repair and therefore potentiating DNA damage caused by IR treatment. The report indicated that digoxin could elevate the expression of gH2AX, promote the formation of gH2AX foci, and reduce the expression of RAD51 (Lu et al, 2014;Surovtseva et al, 2016;Lee et al, 2017), consistent with our finding in this study. Besides, we found that digoxin reduced the expression of XRCC1 and increased the expression of RPA.…”

Digoxin sensitizes gemcitabine-resistant pancreatic cancer cells to gemcitabine via inhibiting Nrf2 signaling pathway

“…Moreover, studies have shown that HIF-1α was involved in the process of chemoresistance, and inhibition of HIF-1α will increase the sensitivity of cancer cell to chemotherapeutic drugs [45], [46], [47]. In addition, Lee et al found that digoxin had a novel function as a phosphatase 2A (PP2A) inhibitor, and enhanced the radiosensitivity of radioresistant NSCLC cells through inhibiting PP2A [48]. These previous studies indicated that digoxin-mediated reversing of gemcitabine resistance in gemcitabine-resistant pancreatic cancer cells might be associated with inhibition of HIF-1α and PP2A.…”

The mechanistic role of cardiac glycosides in DNA damage response and repair signaling

“…Lee et al showed that digoxin not only enhanced DNA damage following IR treatments, but also reduced the levels of DNA repair proteins [ 60 ]. The study reported reduced expression and levels of Rad51, ERCC1, BRCA2, Ku70, Ku86, and DNA-PKcs in A549 cells but not H460 cells.…”