CASP3 and gingival cancer: To investigate whether ZnO-NP-induced apoptosis is associated with the caspase cascade, we next analyzed the expressions of initiator caspase (caspases-8 and -9) and effector caspase (caspase-3), and its downstream substrate of poly-(ADP-ribose) polymerase (PARP) in ZnO-NP-treated Ca9-22 gingival cancer cells.