Two-color fluorescence detection of Poly (ADP-Ribose) Polymerase-1 (PARP-1) cleavage and DNA strand breaks in etoposide-induced apoptotic cells

During apoptosis, the nuclear enzyme Poly(ADP-Ribose) Polymerase-1 (PARP-1) catalyzes the rapid and transient synthesis of poly(ADP-ribose) from NAD+ and becomes inactive when cleaved by caspases. The regulation of these two opposite roles of PARP-1 is still unknown. We have recently investigated PARP-1 activation/degradation in Hep-2 cells driven to apoptosis by actinomycin D. In the present work, we have extended our analysis to the effect of the DNA damaging agent etoposide, and paid attention to the relationship between PARP-1 cleavage and DNA fragmentation. An original fluorescent procedure was developed to simultaneously identify in situ the p89 proteolytic fragment of PARP-1 (by immunolabeling) and DNA degradation (by the TUNEL assay). The presence of p89 was observed both in cells with advanced signs of apoptosis (where the PARP-1 fragment is extruded from the nucleus into the cytoplasm) and in TUNEL-negative cells, with only incipient signs of chromatin condensation; this evidence indicates that PARP-1 degradation in etoposide-treated apoptotic cells may precede DNA cleavage.