Sensitization to radiation-induced DNA damage accelerates loss of bcl-2 and increases apoptosis and autophagy

Abstract

Since it is important to lower the death threshold in tumor cells and to enhance response to genotoxic damage, the mechanism of bromodeoxyuridine (BrdU)-mediated radiosensitization were now examined in human C8161 melanoma . This revealed that anti-apoptotic Bcl-2, and cell cycle controlling hyperphosphorylated Rb and cyclin A were down-regulated by BrdU ,even without irradiation . BrdU pretreatment and subsequent UV irradiation (10 J/m2) accelerated an early increase in the ratio of pro-apoptotic Bax to that of Bcl-2 , increased apoptosis-associated PARP cleavage and potentiated DNA damage compared to irradiated unsensitized cells. BrdU also synergized with radiation to increase autophagic features, such as perinuclear vacuole formation . More specifically, conversion of LC3B-I into LC3B-II by immune blotting and an increased pattern of cytoplasmic and nuclear LC3B by fluorescent immunostaining ,supported induction of autophagy in BrdU-pretreated cells irradiated with 25 J/m2. This report shows for the first time that radiation sensitizers like BrdU enhance and modify radiation-induced cell death by accelerating an increased bax/bcl-2 ratio in unirradiated cells ,and subsequently increasing radiation-induced apoptosis and / or autophagy depending on radiation dosage.