Abstract
[Objective] The unfolded protein response (UPR) is an intracellular signal pathway for responding to endoplasmic reticulum (ER) stress. Whereas hypoxia, nutrient starvation and other conditions causes ER stress and activates UPR to up-regulate survival signal in tumor cells, excess ER stress stimulates the apoptotic signal pathway. Although UPR could be a promising target for a novel anticancer strategy, the precise mechanism underlying ER stress cascade remains elusive. In this study, we examined whether ER stress affects DNA repair machinery and enhances radiation-induced cell death in solid tumor cells.
[Methods] Human lung adenocarcinoma A549 cells were treated without or with ER stress inducers, tunicamycin (TM) or thapsigargin (TG). The expression of DNA-repair-related proteins was examined by Western blot analysis and semi-quantitative RT-PCR analysis. Reproductive cell death was assessed by clonogenic assay.
[Results] TM clearly suppressed the protein levels of Rad51, which is the key protein of the DNA homologous recombination repair pathway. However, mRNA of Rad51 was not affected by TM treatment. This down-regulation of Rad51 protein induced by TM or TG was inhibited by a proteasome inhibitor MG132. In clonogenic cell survival assays, the pretreatment of TM showed a significant radiosensitizing effect. These results suggested that ER stress induced Rad51 degradation through the ubiquitin-proteasome pathway and enhanced radiosensitivity by suppressing of DNA repair capacity.
