PI3K/Akt/mTOR pathway inhibitors enhance radiosensitivity in radioresistant prostate cancer cells through inducing apoptosis, reducing autophagy, suppressing NHEJ and HR repair pathways.

L Chang,J Bucci,J Ni,Y Li,P H Graham,P J Cozzi,J H Kearsley,J Hao

doi:10.1038/cddis.2014.415

Abstract

The PI3K/Akt/mTOR pathway has a central role in cancer metastasis and radiotherapy. To develop effective therapeutics to improve radiosensitivity, understanding the possible pathways of radioresistance involved and the effects of a combination of the PI3K/Akt/mTOR inhibitors with radiotherapy on prostate cancer (CaP) radioresistant cells is needed. We found that compared with parent CaP cells, CaP-radioresistant cells demonstrated G0/G1 and S phase arrest, activation of cell cycle check point, autophagy and DNA repair pathway proteins, and inactivation of apoptotic proteins. We also demonstrated that compared with combination of single PI3K or mTOR inhibitors (BKM120 or Rapamycin) and radiation, low-dose of dual PI3K/mTOR inhibitors (BEZ235 or PI103) combined with radiation greatly improved treatment efficacy by repressing colony formation, inducing more apoptosis, leading to the arrest of the G2/M phase, increased double-strand break levels and less inactivation of cell cycle check point, autophagy and non-homologous end joining (NHEJ)/homologous recombination (HR) repair pathway proteins in CaP-radioresistant cells. This study describes the possible pathways associated with CaP radioresistance and demonstrates the putative mechanisms of the radiosensitization effect in CaP-resistant cells in the combination treatment. The findings from this study suggest that the combination of dual PI3K/Akt/mTOR inhibitors (BEZ235 or PI103) with radiotherapy is a promising modality for the treatment of CaP to overcome radioresistance.

Highlights

The PI3K/Akt/mTOR pathway is an important intracellular signaling pathway in regulating cell growth, survival, adhesion and migration, during cancer progression, metastasis and radioresistance,[5,6,7,8] and is frequently activated in cancer cells
We found that the PI3K/Akt/mTOR pathway is closely linked with epithelial–mesenchymal transition (EMT) and cancer stem cell (CSC).[7]
The IC50 values at 24 h for CaP-RR and CaP cell lines are summarized in Supplementary Table S3. We found that both CaP-RR and CaP cells are more sensitive to four PI3K and mTOR inhibitors than normal RWPE-1 prostate cells (Po0.05), and that CaP-RR cells are less sensitive to four inhibitors than CaP cells in all CaP cell lines (1.5- to 2.5-fold; Po0.05; Supplementary Table S3)

Summary

Introduction

The PI3K/Akt/mTOR pathway is an important intracellular signaling pathway in regulating cell growth, survival, adhesion and migration, during cancer progression, metastasis and radioresistance,[5,6,7,8] and is frequently activated in cancer cells. PI3K and mTOR kinase activity and has been used in preclinical studies in many cancers to demonstrate excellent anticancer effects.[11] In addition, this inhibitor was the first PI3K/mTOR dual inhibitor to enter clinical trials in 2006.12. Under a low-dose radiation treatment, we have recently developed three CaP-RR cell lines with increased colony formation, invasion ability, sphere formation capability and enhanced epithelial–mesenchymal transition (EMT) and cancer stem cell (CSC) phenotypes and the activation of the PI3K/Akt/mTOR signaling pathway.[7] In addition, we found that the PI3K/Akt/mTOR pathway is closely linked with EMT and CSCs.[7] these CaP-RR cells, representative of the source of CaP recurrence after RT, may provide a very good model to mimic a clinical radioresistance condition as well as to examine the efficacy of these single and dual PI3K/ Akt/mTOR inhibitors for their radiosensitization effects

Methods

Results

Conclusion