ReviewDNA damage-induced activation of ATM and ATM-dependent signaling pathways
Introduction
Ataxia-telangiectasia (A-T) is a rare human disease characterized by cerebellar degeneration, immune system defects and cancer predisposition [1], [2], [3]. The disease has been the subject of intense scientific scrutiny, particularly since the identification in 1995 of the gene mutated in A-T, ATM [4], [5]. The ataxia-telangiectasia mutated protein (ATM) has emerged as a central player in the cellular response to ionizing radiation (IR), in which it plays a critical role in the activation of cell cycle checkpoints that lead to DNA damage-induced arrest at G1/S, S, and G2/M. The many properties of ATM have been the subject of several recent reviews [1], [2], [6], [7]. Here, we will highlight some of the new developments in the field and address some of the prominent, unanswered questions related to ATM structure and function.
Section snippets
The ATM protein
ATM is a member of the phosphatidylinositol 3-kinase-like family of serine/threonine protein kinases (PIKKs). Other members of this protein family include ATM- and Rad3-related (ATR), DNA-dependent protein kinase catalytic subunit (DNA-PKcs), mammalian target of rapamycin (mTOR), and ATX/hSMG-1 [1], [7]. The PIKKs represent a subclass of “atypical” protein kinases within the overall eukaryotic protein kinase family [8]. ATM, like other members of this protein kinase family, phosphorylates its
Activation of ATM in response to DNA damage
Perhaps one of the most important recent developments in the field has been insight into the mechanism of ATM activation in response to DNA damage. ATM is a predominantly nuclear protein, the levels of which do not change when cells are exposed to IR [20], [21], [22], [23]. However, the protein kinase activity of ATM, as determined using immunoprecipitation (IP) kinase assays, increases two- to three-fold following cellular exposure to IR [22] or the radiomimetic neocarzinostatin (NCS) [20]. IR
Does ATM detect DNA damage directly or indirectly?
Although these elegant experiments place ATM autophosphorylation as an important upstream event in the activation pathway, they do not address whether ATM is the primary sensor of DNA damage. If it were a direct sensor of DNA damage, ATM would be expected to interact directly with the damaged DNA. Addition of dsDNA does not stimulate ATM protein kinase activity in IP kinase assays [20], although addition of DNA does stimulate the activity of the purified ATM protein under some conditions in
Monitoring DNA damage-induced activation of ATM—the problem with p53
Despite the abundance of physiological substrates of ATM, one of the most frequently used “read-outs” of ATM activity is the phosphorylation of p53 on serine 15. IR-induced phosphorylation of p53 is both attenuated and delayed in A-T cells [60]. However, serine 15 phosphorylation of p53 is clearly evident in ATM-deficient cell lines at later times after exposure to IR, indicating that ATM is required for serine 15 phosphorylation predominantly during the initial phase of the DNA damage
Do other types of DNA damage trigger the activation of ATM?
To date, most studies have examined the effects of IR on the activation of ATM and ATM-dependent downstream pathways. IR induces damage to many biomolecules in the cell, including lipids and DNA (through the generation of single-strand breaks (SSBs), DSBs, and nucleotide damage), and triggers the formation of reactive oxygen species (ROS) (from ionization of water molecules and through lipid peroxidation). NCS, which also activates ATM, is a radiomimetic antibiotic that induces DSBs and SSBs by
Can ATM be activated in the absence of DNA damage?
Some studies have raised the interesting question of whether ATM can be activated by other forms of DNA damage, by changes in chromatin structure, or even by non-DNA-damaging events [12]. For example, serine 1981 phosphorylation of ATM occurs following exposure of cells to hypotonic buffer or to chloroquine, a topo II catalytic inhibitor, neither of which would be expected to induce DSBs [12]. Other studies have inferred activation of ATM and/or ATM-dependent downstream pathways from the
Concluding remarks
If 1995 was a breakthrough year for identification of the ATM gene, then 2003 will stand out as the year in which seminal advances were made in understanding how ATM is activated in response to DNA damage, and perhaps other cellular stressors. However, certain key questions remain unanswered. Precisely how ATM and the MRN complex detect DNA damage and whether serine 1981 phosphorylation of ATM is required for the localization of ATM to sites of DNA damage remain to be determined. Precisely how
Acknowledgements
We thank Dr. Yossi Shiloh for his many helpful comments and suggestions and Dr. N.S.Y. Ting and members of the Lees-Miller laboratory for helpful discussions. EUK is supported by a post-doctoral fellowship from the United States Department of Defense Breast Cancer Research Program (DAMD17-02-1-0318). SPLM is a Scientist of the Alberta Heritage Foundation for Medical Research, an Investigator of the Canadian Institutes for Health Research and holds the Engineered Air Chair in Cancer Research.
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