Granzyme A Signaling

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are key immune effectors that eradicate infected cells and tumor cells. To destroy these targets, CTL and NK cells mostly use the granule exocytosis pathway which releases perforin and granzymes from cytolytic granules into the immunological synapse formed with the target. Granzyme A and granzyme B, the most abundant granzymes, are delivered to the target cell cytosol through perforin and independently induce cell death. The tryptase granzyme A activates cell death through a caspase-independent mechanism. Granzyme A causes characteristic features of apoptosis, including membrane blebbing, loss of mitochondrial transmembrane potential, nuclear fragmentation and chromatin condensation. However, instead of the usual apoptotic double-stranded oligonucleosomal DNA fragmentation, granzyme A causes a distinctive form of DNA damage known as single-stranded DNA nicking.During the induction of cell death, granzyme A targets a 270-420 kDa ER associated complex known as the SET complex, which contains two tumor suppressor proteins (pp32 and GAAD/NM23H1) and three granzyme A substrates (nucleosome assembly protein SET, DNA-binding protein HMG2 and the rate limiting base excision repair enzyme APE1). Granzyme A destroys the known functions of these substrates. Although the normal function of the SET complex is unknown, on the basis of the functions of its components, it has been proposed that it facilitates transcriptional activation and DNA repair in response to oxidative stress. Indeed, the proteins in this complex translocate rapidly to the nucleus in response to an increase in the level of ROS and after granzyme A loading with perforin. Granzyme A destroys three members of the SET complex, including the nucleosome-assembly protein SET, APE1 and HMG2. When these proteins are destroyed, the DNA nicking protein NM23H1 is free to nick DNA while the breaks are not repaired. Granzyme A targets other important nuclear proteins such as the linker histone H1, which is completely degraded and the tails are cleaved from the core histones. This opens up chromatin and enhances DNA fragmentation.