PARt of the Response to DNA Damage

Double-stranded DNA breaks, such as those caused by radiation, activate DNA damage responses. One such response is gene transcription by nuclear factor κB (NF-κB); activation of this transcription factor requires several upstream signaling events, including sumoylation of IκB kinase γ [IKKγ; also known as NF-κB essential modulator (NEMO)] by protein inhibitor of activated STAT y (PIASy) and phosphorylation of IKKγ by the kinase ataxia telangiectasia mutated (ATM) (see also McCool and Miyamoto). To investigate how PIASy and ATM are recruited to IKKγ, Stilmann et al. searched for binding partners of IKKγ and identified poly(ADP-ribose)-polymerase 1 (PARP-1). In contrast to cells from wild-type mice, cells from Parp1 –/– mice did not show NF-κB activation in response to radiation, as assessed by nuclear accumulation of the p65 subunit of NF-κB in mouse embryonic fibroblasts (MEFs) and electrophoretic mobility shift assays (EMSAs) in macrophages and small intestinal epithelial cells. NF-κB activation in Parp1 –/– MEFs was rescued by expressing wild-type PARP-1, but not a catalytically inactive mutant or a mutant unable to bind DNA. The association of endogenous PARP-1 with IKKγ was induced by irradiation, transient in nature, and required poly(ADP-ribosyl)ation. Furthermore, PARP-1 enhanced the interaction between IKKγ and PIASy [an effect that required poly(ADP-ribose) binding motifs in PIASy], was required for sumoylation of IKKγ by PIASy, and promoted the association of ATM with IKKγ and PIASy. Parp1 –/– MEFs were more sensitive to irradiation-induced apoptosis compared with wild-type MEFs; in addition, induction of NF-κB target genes that counter apoptosis was impaired in Parp1 –/– MEFs. The authors propose that DNA damage stimulates PARP-1, which, through poly(ADP-ribosyl)ation, mediates the formation of a complex that enables the phosphorylation of IKKγ by ATM, the sumoylation of IKKγ by PIASy, and ultimately the activation of NF-κB. M. Stilmann, M. Hinz, S. C. Arslan, A. Zimmer, V. Schreiber, C. Scheidereit, A nuclear poly(ADP-ribose)-dependent signalosome confers DNA damage-induced IκB kinase activation. Mol. Cell 36 , 365–378 (2009). [PubMed] K. McCool, S. Miyamoto, A PAR-SUMOnious mechanism of NEMO activation. Mol. Cell 36 , 349–350 (2009). [PubMed]