AsSIRTing the DNA damage response

Abstract

In mammalian cells, changes in signaling networks and expressed proteins ensure the adequate detection and management of damaged macromolecules. Here, we review an emergent pathway of maintenance of homeostasis following genotoxic stress. The RNA-binding protein HuR associates with sirtuin (SIRT)1 mRNA and maintains constitutively elevated levels of SIRT1 protein, a deacetylase that elicits a prosurvival function. SIRT1 was recently shown to deacetylate the Nijmegen breakage syndrome (NBS1) protein, thereby rendering it phosphorylatable by ataxia telangiectasia mutated protein (ATM). A component of the MRN (MRE11-RAD50-NBS1) nuclease complex, NBS1 is crucial for sensing DNA damage and mounting a genotoxic response. This article covers the regulatory pathway of HuR-->SIRT1-->NBS1, through which post-transcriptional and post-translational effectors contribute to the maintenance of genomic integrity.

Figure 1.

Schematic of the pathway HuR→SIRT1→NBS1. HuR upregulates SIRT1 expression levels post-transcriptionally by stabilizing the SIRT1 mRNA [6]. In turn, SIRT1 deacetylates (Ac) NBS1, rendering it a phosphorylation (P) substrate for ATM. Following its recruitment to the site of DNA damage, ATM is activated via mechanisms that are not fully understood, and phosphorylates Chk2, which can then phosphorylate HuR and thus inhibit its binding to SIRT1 mRNA, possibly creating a negative feedback regulatory loop. The Chk2 phosphorylation substrate, SIRT1 deacetylation target and HuR translational target p53 is also shown here because it impacts on the three major components of the genotoxic response: DNA repair, cell survival and growth arrest. The significance of NBS1 phosphorylation at Ser343 (*) awaits further study; the prediction that HuR associates with the NBS1 mRNA and influences its expression (broken line) needs to be tested experimentally.

Figure 2.

Dual model of SIRT1 action during the genotoxic response. (a) Under conditions of low-level or no cellular damage, HuR increases SIRT1 levels, which in turn deacetylates NBS1; ATM is recruited by the active NBS1-containing complex (MRN) to the site of the damaged DNA. The ensuing genotoxic response program includes the repair of DNA, inhibition of growth and avoidance of cell death. In this scenario the eventual activation of Chk2 by ATM could serve as a negative feedback loop (broken arrow), ensuring that the arrest and repair machineries do not remain in place indefinitely. (b) Under conditions of extreme genotoxic damage, HuR is phosphorylated, releasing the SIRT1 mRNA and thereby reducing SIRT1 expression levels. In turn, NBS1 remains acetylated, cannot be phosphorylated and is unable to activate ATM. The phenotypic consequences of this impaired response are a lack of DNA repair, an inability to activate control checkpoints and a reduction of cell survival. The converse negative feedback loop (broken arrow) is depicted here, as Chk2 eventually becomes inactive in the absence of ATM function.

Figure I.

Convergence of stress signals on SIRT1, an upstream regulator of multiple effectors of the stress response.