P53 pathway is a major determinant in the radiosensitizing effect of Palbociclib: Implication in cancer therapy

Highlights

• Palbociclib promotes radiosensitivity in lung and colon cancer derived cell lines.

• Palbociclib promotes radiosensitivity independently of CDk4/6 chemical inhibition.

• Palbociclib associated radiosensitivity requires a functional p53 signalling.

• P53 could be a novel biomarker for the combination of Palbociclib and radiotherapy.

Abstract

Targeting cell cycle has become one of the major challenges in cancer therapy, being Palbociclib, a CDK4/6 inhibitor, an excellent example. Recently, it has been reported that Palbociclib could be a novel radiosensitizer agent. In an attempt to clarify the molecular basis of this effect we have used cell lines from colorectal (HT29, HCT116) lung (A549, H1299) and breast cancer (MCF-7). Our results indicate that the presence of a p53 wild type is strictly required for Palbociclib to exert its radiosensitizing effect, independently of the inhibitory effect exerted on CDK4/6. In fact, abrogation of p53 in cells with functional p53 blocks the radiosensitizing effect of Palbociclib. Moreover, no radiosensitizing effect is observed in cells with non-functional p53, but restoration of p53 function promotes radiosensitivity associated to Palbociclib. Furthermore, the presence of Palbociclib blocks the transcriptional activity of p53 in an ATM-dependent-fashion after ionizing radiation exposure, as the blockage of p21/WAF1 expression demonstrates. These observations are a proof of concept for a more selective therapy, based on the combination of CDK4/6 inhibition and radiotherapy, which would only benefit to those patients with a functional p53 pathway.

Introduction

Cancer therapy has evolved from chemotherapy to novel approaches by targeting specific alterations present in tumours, being protein kinases inhibitors a novel weapon in this therapeutic approach [1]. In this sense, cyclin-dependent kinases (CDKs), prolin-targeted serine/threonine kinases, are key proteins in the control of cell cycle through their interaction with cyclins and the subsequent regulation of cell cycle progression, thus becoming an interesting field in cancer research [2]. Specifically, CDK4/6 control the G1 phase to the S phase transition of cell cycle trough the interaction with Cyclin D [3]. The main target of the CDK4/6-Cyclin D complex is the retinoblastoma gene product (pRB), in which phosphorylation blocks its tumour suppressor function by modulating the activity of the downstream transcription factor E2F [4,5], hence emerging as a very attractive target for cancer therapy [6]. Palbociclib is an specific inhibitor of CDK4/6 that has shown anti-tumour activity in preclinical models [7]. Indeed, its use has been investigated in several pathologies as myeloma [8], lymphoma [9], liposarcoma [10] lung cancer [11], and specially in breast cancer [12], were CDK4/6 inhibitors have become a novel approach to overcome resistance to other current therapies [13]. In addition, combination of Palbociclib with other therapeutic options is showing promising results, as in the case of other kinases or autophagy inhibitors [[14], [15], [16]].

In addition to surgery, one of the main therapeutic options in cancer is the use of ionizing radiation (IR) alone or in combination with chemotherapy. However, resistance to IR is still a main problem in cancer therapy and the search of compounds able to promote radiosensitivity is a major challenge [17]. In this regard, preliminary evidences have shown a potential role for Palbociclib in the modulation of the cellular response to IR by promoting radiosensitivity in different experimental models [[18], [19], [20]]. However, few clues about the molecular mechanisms involved have been reported, except for the implication of the inhibition of MEK in K-ras mutant models [18], and the recent evidences found about the role of ATM [21]. Therefore, it is necessary to carry out studies to find the molecular determinants that control the radiosensitizing benefits associated to Palbociclib.

In an attempt to clarify this question, we have evaluated in vitro the effect of Palbociclib in different human cancer cells lines. Our data show that the tumour suppressor p53 is a critical determinant in the radiosensitizing effect associated to Palbociclib. Interestingly, the effect exerted by Palbociclib onto the p53 signalling pathway is due to the inhibition on ATM activity. In summary, our data support the idea that combination of Palbociclib and radiotherapy could be a promising approach to treat tumours, but only restricted to those patients with a functional p53 signalling pathway.