Original ArticleThe BET bromodomain inhibitor JQ1 radiosensitizes non-small cell lung cancer cells by upregulating p21
Introduction
Non-small cell lung cancer (NSCLC), one of the most commonly diagnosed cancers, is the leading cause of cancer-related death worldwide [28]. Although radiotherapy servers as an important and effective treatment option for NSCLC, a subgroup of patients present rapid progression and resistance to radiotherapy, and eventually relapse shortly after treatment ends [33]. The maximum doses are limited due to normal tissue tolerance, and increasing radiation dose is often unfeasible since it also might increase adverse effects of normal tissues. Other efforts to optimize radiotherapy treatment, such as precision in dose delivery and optimization of treatment plans, are not beneficial for some patients and large inter-individual differences are observed in terms of response outcomes [33]. Consequently, the development of novel radiosensitizing strategies in the treatment of NSCLC is urgently required.
Epigenetics is becoming more extensively studied in cancer biology as a mechanism regulating cancer cell biology. It mainly deals with regulation of gene expression through DNA methylation, miRNA, or histone modifications such as acetylation, methylation, phosphorylation, or ubiquitination. Radiotherapy effects on genetic alterations (or vice versa) have been studied widely over the past few years. However, the influence of radiotherapy on epigenetic alterations has been relatively little studied, even though these alterations potentially lead to changes in transcriptional activity and thereby to altered cellular resistance to radiation [15], [40], [41]. As epigenetic alterations can be reversed by drug treatment, they are interesting candidate targets for anticancer therapy or radiotherapy sensitizers.
The BET family of bromodomains, which includes BRD2, BRD3, BRD4, and bromodomain testis-specific protein (BRDT), are epigenetic reader proteins that bind acetylated lysine residues on histones. Directly or through recruitment of other transcriptional regulators, these proteins play critical roles in the control of diverse biological processes, including inflammation, cell cycle, maintenance of higher-order chromatin structure and DNA damage signaling [16], [17], [27]. Small-molecule inhibitors that bind BRD4, preventing its interaction, were found to be able to lead to repression of BRD4 transcriptional targets, and have been shown to induce cytotoxicity in a variety of malignancies [13], [16], [17], [21], [31]. Considering its ability to enhance the effects of anticancer therapy and the potential low side effect profile, BET inhibitors are a promising new class of antineoplastic pharmaceuticals. However, whether it plays an important role in improving efficacy of radiotherapy has never been investigated.
In this study, we evaluated the effect of a small-molecule inhibitor of BET bromodomains JQ1 on the radiosensitivity of NSCLC cell lines. Our results suggested that JQ1 sensitized NSCLC cells to radiation both in vitro and in vivo, and that this effect of JQ1 was likely attributable, at least in part, to the inhibition of DNA repair and induction of G2/M cell cycle arrest and apoptosis that resulted from the upregulation of p21 expression. To our knowledge, this is the first evidence showing that the radiosensitizing effects of JQ1 in NSCLC. These proofs of concept studies indicate that BET bromodomain inhibition may offer a potential strategy for enhancing the effect of radiotherapy and reducing radioresistance.
Section snippets
Cell lines and reagents
A549 (p53 wt), H460 (p53 wt) and H1299 (p53 mutation) were maintained in RPMI1640 culture medium (Gibco) supplemented with 10% heat-inactivated fetal bovine serum (FBS; Gibco), at 37 °C in a humidified incubator containing 5% CO2. JQ1 was purchased from Selleckchem, resuspended in dimethylsulfoxide (DMSO), and stored in aliquots at −80 °C (1 mM, stock solution). The drug was prepared at a final concentration of 500 nM (working solution) in DMSO, and added to the culture medium and homogenized
JQ1 significantly enhanced the radiosensitivity of NSCLC cells
To determine the effect of JQ1 on the radiosensitivity of NSCLC, A549 and H460 cells were pre-incubated with JQ1 (500 nM) or DMSO as a vehicle control for 48 h prior to exposure to different doses of ionizing radiation (IR) treatment. As shown in Fig. 1A–C, using clone formation assay, we found that JQ1 significantly radiosensitized A549 and H460 cells. The surviving fractions (SF) at 2 Gy (SF2) for A549 and H460, were reduced from 0.82 ± 0.09 to 0.49 ± 0.01 (P < 0.001) and 0.66 ± 0.04 to
Discussion
JQ1, a highly selective BET bromodomain inhibition, has displayed broad antineoplastic effects in a range of tumors carrying different genetic lesions. Although phase II studies of JQ1 as a single agent for certain types of cancer are currently under way, the effects of JQ1 in combination with radiation have never been reported before. In the current study, we have demonstrated for the first time that the BET bromodomain inhibitor JQ1 sensitized NSCLC cells to radiation both in vitro and
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grants: 81402576, No. 81402408).
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Both authors contribute equally.