Inactivation of enzymes involved in DNA repair.
The enzyme poly (ADP-ribose) polymerase, or PARP, was the first protein identified as a substrate for caspases. PARP is involved in repair of DNA damage and functions by catalyzing the synthesis of poly (ADP-ribose) and by binding to DNA strand breaks and modifying nuclear proteins. The ability of PARP to repair DNA damage is prevented following cleavage of PARP by caspase-3.
Inactivation of enzymes involved in cell replication.
DNA topoisomerase II is a nuclear enzyme essential for DNA replication and repair. Caspases can inactivate this enzyme leading to DNA damage.
Breakdown of structural nuclear proteins.
Lamins are intra-nuclear proteins that maintain the shape of the nucleus and mediate interactions between chromatin and the nuclear membrane. Degradation of lamins by caspase 6 results in the chromatin condensation and nuclear fragmentation commonly observed in apoptotic cells.
Fragmentation of DNA.
The fragmentation of DNA into nucleosomal units - as seen in DNA laddering assays - is caused by an enzyme known as CAD, or caspase activated DNase. Normally CAD exists as an inactive complex with ICAD (inhibitor of CAD, also known as DNA fragmentation factor45). During apoptosis, ICAD is cleaved by caspases, including caspase 3,to release CAD. Since CAD is a DNase with a high specific activity (comparable to or higher than DNase I and DNase II) rapid fragmentation of the nuclear DNA follows.