Abstract 3877: An inhibitor-2 complex regulates cell cycle checkpoints in response to DNA damage

Optimal DNA damage responses such as cell cycle checkpoints are essential for cell survival and prevention of genetic instability. Comprehensive knowledge on the regulation of cell cycle checkpoints will provide molecular insights into tumorigenesis as well as tumor responses to radiation and chemotherapy. The signal transduction network orchestrated by the ATM protein kinase through phosphorylation of downstream targets is critical to activate cell cycle checkpoints in response to ionizing radiation (IR). Among them, Inhibitor 2 (I-2), a regulatory subunit of Protein Phosphatase 1 (PP1), is phosphorylated by ATM on Serine 43 after IR and required for PP1 activation. However, the detailed mechanism of the signaling network involving ATM, PP1 and I-2 has not been elucidated. In the present study, we show that I-2 phosphorylation is required for activation of the S-phase and G2/M checkpoints. We also identified a complex associated with the phosphorylated form of I-2. Several DNA damage responsive proteins such as ATM, Brca1, Cdc7 and Chk2 are presented in the complex. Further, we demonstrate that Brca1 is required for ATM-mediated I-2 phosphorylation and PP1 activation after DNA damage. We find Brca1 is essential for recruiting I-2 to DNA damage sites, and Brca1 interaction with I-2 increases after DNA damage. This DNA damage-induced Brca1/I-2 interaction further lead to I-2/PP1 dissociation, the process required for PP1 activation after DNA damage. Taken together, our results highlighted that a comprehensive pathway mediated by I-2 phosphorylation in response to DNA damage. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3877.