title: Activation of ATM and Chk2 kinases in relation to the amount of DNA strand breaks.
abstract: The diverse checkpoint responses to DNA damage may reflect differential sensitivities by molecular components of the damage-signalling network to the type and amount of lesions. Here, we determined the kinetics of activation of the checkpoint kinases ATM and Chk2 (the latter substrate of ATM) in relation to the initial yield of genomic DNA single-strand (SSBs) and double-strand breaks (DSBs). We show that doses of gamma-radiation (IR) as low as 0.25 Gy, which generate vast numbers of SSBs but only a few DSBs per cell (<8), promptly activate ATM kinase and induce the phosphorylation of the ATM substrates p53-Ser15, Nbs1-Ser343 and Chk2-Thr68. The full activation of Chk2 kinase, however, is triggered by treatments inflicting >19 DSBs per cell (e.g. 1 Gy), which cause Chk2 autophosphorylation on Thr387, Chk2-dependent accumulation of p21waf1 and checkpoint arrest in the S phase. Our results indicate that, in contrast to ATM, Chk2 activity is triggered by a greater number of DSBs, implying that, below a certain threshold level of lesions (<19 DSBs), DNA repair can occur through ATM, without enforcing Chk2-dependent checkpoints.
authors.affiliation: Department of Experimental Oncology, Istituto Nazionale Tumori, Via G Venezian 1, 20133 Milan, Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Buscemi
authors.forename: Paola
authors.initials: P
authors.lastname: Perego
authors.forename: Nives
authors.initials: N
authors.lastname: Carenini
authors.forename: Makoto
authors.initials: M
authors.lastname: Nakanishi
authors.forename: Luciana
authors.initials: L
authors.lastname: Chessa
authors.forename: Junjie
authors.initials: J
authors.lastname: Chen
authors.forename: Kumkum
authors.initials: K
authors.lastname: Khanna
authors.forename: Domenico
authors.initials: D
authors.lastname: Delia
mesh_terms: D064007:Ataxia Telangiectasia Mutated Proteins; D018797:Cell Cycle Proteins; D002460:Cell Line; D002467:Cell Nucleus / Q000235:genetics; D064447:Checkpoint Kinase 2; D004247:DNA / Q000235:genetics; D004249:DNA Damage / Q000235:genetics*; D004277:DNA, Single-Stranded / Q000235:genetics; D004268:DNA-Binding Proteins; D006801:Humans; D008214:Lymphocytes; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D013379:Substrate Specificity; D025521:Tumor Suppressor Proteins
chemical_list: D018797:Cell Cycle Proteins; D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins; D025521:Tumor Suppressor Proteins; D004247:DNA; D064447:Checkpoint Kinase 2; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; C578634:CHEK2 protein, human; D017346:Protein Serine-Threonine Kinases