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

Since it is important to lower the death threshold in tumor cells and to enhance response to genotoxic damage, the mechanisms of bromodeoxyuridine (BrdU)-mediated radiosensitization were now examined in human C8161 melanoma. This revealed that antiapoptotic Bcl-2, and cell cycle controlling hyperphosphorylated Rb and cyclin A were downregulated by BrdU, even without irradiation. BrdU pretreatment and subsequent UV irradiation (10 J/m(2)) 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/m(2). 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.