Abstract 3942: Novel mechanism for inactivation of the DNA damage response during apoptosis

Abstract

Abstract Apoptosis, programmed cell death, is required in multicellular organisms for various physiological processes. Disruption of the equilibrium between cell death and proliferation is one of the mechanisms of tumor formation. Therefore, one of the challenges and one of the ongoing approaches for cancer therapy is to develop pro-apoptotic agents. One such agent is TRAIL (Tumor necrosis factor-Related Apoptosis-Inducing Ligand), a promising therapy in development because it induces apoptosis preferentially in cancer cells compared to normal cells. We recently identified the “apoptotic ring”, a previously unnoticed process of cellular apoptosis by the nuclear peripheral staining of phosphorylated DNA damage response (DDR) proteins including γ-H2AX, ATM, DNA-PK and Chk2. The γ-H2AX apoptotic ring forms in the early phase of the apoptotic response to a broad range of stimuli including TRAIL in different cell types. It differs from the focal pattern response (DDR nuclear foci) to DNA damaging agents. Because γ-H2AX is a fundamental chromatin modification in response to DNA double-strand breaks and serves as a molecular platform for the docking of DDR protein complexes and for amplifying the DDR, we were particularly interested to determine whether the apoptotic γ-H2AX response led to the full recruitment of the DDR factors. Here we looked at MDC1 (mediator of DNA damage checkpoint 1) which normally binds to γ-H2AX in response to DNA damage and amplifies the DDR. We will report that during TRAIL-induced apoptosis, MDC1 does not exhibit an apoptotic ring. Lately a new phosphorylation of histone H2AX on tyrosine 142 (P-Y142-H2AX) has been described, thereby preventing the binding of MDC1 to γ-H2AX. However, we find that Y142 phosphorylation is reduced in response to TRAIL and uncovered a novel mechanism to explain the inactivation of MDC1-mediated transduction of the DDR during apoptosis. Indeed, we demonstrate for the first time that MDC1 is cleaved by caspase-3 during apoptosis, separating the FHA (forkhead-associated) and the BRCT (breast cancer C-terminal) domains, and consequently disrupting the linkage to γ-H2AX and other DNA damage responsive proteins. 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 3942.