Getting SET for Cell Death

Science's STKE  18 Mar 2003:
Vol. 2003, Issue 174, pp. tw112-TW112
DOI: 10.1126/stke.2003.174.tw112

Cytotoxic T cells kill virus-infected cells and malignant cells by releasing granzyme (Gzm) proteases A and B together with perforin (which enables Gzm access to the cell interior). Whereas GzmB initiates apoptosis through caspase-dependent and independent pathways, GzmA kills cells through a caspase-independent mechanism. Thus, cells resistant to caspase-mediated apoptosis can still be killed by GzmA. An endoplasmic reticulum (ER)-associated protein complex (SET complex) that contains the tumor suppressor pp32, GzmA-activated DNase activity, and three GzmA substrates, including the nucleosome assembly protein SET, has been implicated in this process, but the identity of the DNase--and how it is regulated by GzmA--has remained unclear. Fan et al. purified the SET complex by affinity chromatography using inactive GzmA and separated it to identify a fraction with DNase activity and one that inhibited DNase activity. The authors used SDS-DNA-polyacrylamide gel electrophoresis to define a 17-kD protein as the DNase. They used mass spectroscopy together with Western analysis to identify it as NM23-H1, a nucleoside diphosphate kinase that has been implicated in suppression of tumor metastasis. SET bound to NM23-H1 and inhibited its DNase activity. GzmA, which cleaved SET but not NM23-H1, released SET inhibition. In HeLa cells stained for SET and NM23-H1, the two proteins colocalized with an ER marker; after lymphokine-activated killer (LAK) cell attack, they translocated to the nucleus. Western analysis of LAK-treated cells demonstrated a SET fragment in the nucleus. Silencing NM23-H1 with RNA interference inhibited GzmA-dependent DNA nicking, whereas silencing SET or overexpressing NM23-H1 enhanced it. NM23-H1 was previously reported to have DNase activity; the authors speculate that Ape1, another substrate for GzmA in the SET complex, normally repairs these nicks, with Ape1 and NM23-H1 working together in DNA repair. Chakravarti and Hong discuss this research and research implicating the SET protein pp32 in caspase-dependent apoptosis.

Z. Fan, P. J. Beresford, D. Y. Oh, D. Zhang, J. Lieberman, Tumor suppressor NM23-H1 is a granzyme A-activated DNase during CTL-mediated apoptosis, and the nucleosome assembly protein SET is its inhibitor. Cell 112, 659-672 (2003). [Online Journal]

D. Chakravarti, R. Hong, SET-ting the stage for life and death. Cell 112, 589-593 (2003). [Online Journal]