Caspase-independent apoptosis induced in rat liver cells by plancitoxin I, the major lethal factor from the crown-of-thorns starfish Acanthaster planci venom

Plancitoxin I, the major lethal factor from the crown-of-thorns starfish Acanthaster planci venom, is quite unique not only in exhibiting potent hepatotoxicity but also in sharing high sequence homology with mammalian deoxyribonulease II. In this study, morphological and biochemical changes in rat liver epithelial cells (TRL 1215 cells) treated with the toxin were examined to understand the mechanism by which plancitoxin I displays hepatotoxicity. AlamarBlue assay established that plancitoxin I is cytolethal to TRL 1215 cells. This cytolethalithy was ascribable to apoptotic cell death. Nuclear fragmentation evidenced by either Diff-Quick or Hoechst 33258 staining, DNA fragmentation by TUNEL assay and electrophoretic analysis on agarose gel and phosphatidylserine externalization by flow cytometric analysis of annexin V-FITC stained cells were all characteristics of apoptosis. The observed apoptosis was shown to be independent of the caspase 3 cascade that is generally accepted as the effector of the apoptotic process. Very interestingly, experiments using FITC-labeled plancitoxin I proved that the toxin can enter the nucleus of TRL 1215 cells. Our results suggested that plancitoxin I induces apoptosis of TRL 1215 cells through the following procedure: binding to a specific receptor in the cytoplasmic membrane, entering the cell, entering the nucleus and degrading DNA.