Modulation of Caspase-3 activity using a redox active vitamin K3 analogue, plumbagin, as a novel strategy for radioprotection

Radiation induced damage to normal cells is a major shortcoming of conventional radiotherapy, which necessitates the development of novel radio-protective drugs. An ideal radio-modulator would protect normal cells while having cytotoxic effects on cancer cells. Plumbagin is a potent anti-tumour agent and has been shown to sensitize tumour cells to radiation-induced damage. In the present study, we have evaluated the radio-protective potential of plumbagin and found that it protected normal lymphocytes against radiation-induced apoptosis, but did not protect cancer cells against radiation. Plumbagin offered radioprotection even when it was added to cells after irradiation. The ability of only thiol based antioxidants to abrogate the radio-protective effects of plumbagin suggested a pivotal role of thiol groups in the radio-protective activity of plumbagin. Further, protein interaction network (PIN) analysis was used to predict the molecular targets of plumbagin. Based on the inputs from plumbagin's PIN and in light of its well-documented ability to modulate thiol groups, we proposed that plumbagin may act via modulation of caspase enzyme which harbours a critical catalytic cysteine. Indeed, plumbagin suppressed radiation-induced increase in homogenous caspase and caspase-3 activity in lymphocytes. Plumbagin also inhibited the activity of recombinant caspase-3 and mass spectrometric analysis revealed that plumbagin covalently interacts with caspase-3. Further, the in vivo radioprotective efficacy of plumbagin (single dose of 2mg/kg body weight) was demonstrated by its ability to rescue mice against radiation (7.5 Gy; Whole Body Irradiation) induced mortality. These results indicate that plumbagin prevents radiation induced apoptosis specifically in normal cells by inhibition of caspase-3 activity.