title: A protein phosphatase feedback mechanism regulates the basal phosphorylation of Chk2 kinase in the absence of DNA damage.
abstract: The checkpoint kinase Chk2 is an effector component of the ATM-dependent DNA damage response (DDR) pathway. The activation of Chk2 by genotoxic stress involves its phosphorylation on T68 by ATM and additional auto/transphosphorylations. Here we demonstrate that in unperturbed cells, chemical inhibition of Chk2 by VRX0466617 (VRX) enhances the phosphorylation of Chk2-T68 throughout the cell cycle phases. This event, dependent on the presence of ATM and catalytically functional Chk2, is not consequential to DNA damage, as neither gamma-H2AX nuclear foci nor increased ATM activation is detected in VRX-treated cells, suggesting the involvement of other regulatory proteins. As serine/threonine protein phosphatases (PPs) regulate the phosphorylation and deactivation of proteins of the DDR pathway, we analyzed their role in phospho-T68-Chk2 regulation. We found that intracellular inhibition of PP1 and PP2A-like activities by okadaic acid markedly raised the accumulation of Chk2-pT68 without DNA damage induction, and this phenomenon was also seen when PP1-C, PP2A-C, and Wip1/PPM1D were simultaneously knockdown by siRNA. Altogether, these data indicate a novel mechanism in undamaged cells where PPs function to maintain the balance between ATM and its direct substrate Chk2 through a regulatory circuit.
authors.affiliation: Dept of Experimental Oncology, Fondazione IRCCS Istituto Nazionale Tumori, Milano, Italy.
authors.forename: Luigi
authors.initials: L
authors.lastname: Carlessi
authors.forename: Giacomo
authors.initials: G
authors.lastname: Buscemi
authors.forename: Enrico
authors.initials: E
authors.lastname: Fontanella
authors.forename: Domenico
authors.initials: D
authors.lastname: Delia
mesh_terms: D064007:Ataxia Telangiectasia Mutated Proteins; D015153:Blotting, Western; D018797:Cell Cycle Proteins / Q000378:metabolism; D002460:Cell Line; D045744:Cell Line, Tumor; D064447:Checkpoint Kinase 2; D004249:DNA Damage; D004268:DNA-Binding Proteins / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D025461:Feedback, Physiological; D006801:Humans; D008954:Models, Biological; D019319:Okadaic Acid / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D054645:Protein Phosphatase 1 / Q000235:genetics / Q000378:metabolism; D054648:Protein Phosphatase 2 / Q000235:genetics / Q000378:metabolism; D000071636:Protein Phosphatase 2C; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D034622:RNA Interference; D013844:Thiazoles / Q000494:pharmacology; D013912:Threonine / Q000378:metabolism; D025521:Tumor Suppressor Proteins / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D013844:Thiazoles; D025521:Tumor Suppressor Proteins; C520468:VRX0466617; D019319:Okadaic Acid; D013912:Threonine; D064447:Checkpoint Kinase 2; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; C578634:CHEK2 protein, human; D017346:Protein Serine-Threonine Kinases; C000607458:PPM1D protein, human; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2; D000071636:Protein Phosphatase 2C