Are Caspases (and Caspase-activated Executors) Actively Imported into the Nucleus?
The nuclear localisation of either precursor or mature caspases is still a matter of debate. In early immunohistochemical investigations, caspase-3 and caspase-1 immunoreactivity was detected in the nuclei of a subpopulation of cells in neuroblastic tumors with favourable prognosis73 and nuclear caspase-3 staining was observed in healthy tissue of different origin, as for example testis and colon.74 However, these studies did not differentiate between the zymogens and the processed active caspases. Subsequently, subcellular fractionation and affinity labelling experiments identified multiple mature caspases in the nuclear fraction of cells induced to undergo apoptosis.75, 76, 77 These were: caspase-3,75, 76, 77 caspase-6,76 caspase-2,75 and caspase-8.75, 78 Mature caspase-9 and -7 were reported to translocate to the nucleus in some studies, but not in others.75 Among the caspase zymogens, only procaspase-2 has been definitely identified as a bona fide nuclear protein.65 Nuclear localis
