title: Failure to inactivate CDK activity is responsible for the enhanced apoptotic response in U937 cells mediated by silencing ATM gene.
abstract: Silencing ATM gene gave rise to enhanced apoptotic response to irradiation and irradiation-like chemotherapy agents, this paper explored the crucial identities of the molecular elements responsible for the enhanced apoptotic response in U937 cells mediated by silencing ATM gene. Two U937 cell mutants named U937-ASPI3K (ATM, negative) and U937-pZeosv2(+) (ATM, wild-type) were used as a cell model system to identify the critical molecule(s) responsible for the varied apoptotic response in the absence or presence of ATM gene. Apoptosis was examined by measuring concentrations of free nucleosome in U937 cells. Western blot was employed to measure nuclear protein abundance of CDC25A, CDC25B, CDC25C, total p34cdc2, p34cdc2, (Thr 161) or p34cdc2 (Thr 14, Tyr 15). RT-PCR was used to estimate CDC25 transcript levels. U937-ASPI3K exhibited an enhanced apoptotic response to lower dosage of irradiation, which could not be blocked by protein synthesis inhibitor. Protein serine-threonine phosphatase inhibitor or cyclin-dependent kinase (CDK) inhibitors, on the other hand, abolished the enhancement indicated that protein phosphorylation/dephosphorylation modification and CDK activity are required for the enhanced apoptotic response in the absence of ATM gene. Upon irradiation, p34cdc2 in U937-pZeosv2(+) was maintained in an inactive state by phosphorylation on threonine 14 (Thr 14) and tyrosine 15 (Tyr 15), which was associated with a dramatic decrease of nuclear CDC25A, CDC25B and CDC25C proteins. In contrast, p34cdc2 in U937-ASPI3K maintained in an active state by dephosphorylation on threonine 14 (Thr 14) and tyrosine 15 (Tyr 15), which was associated with constant nuclear CDC25A, CDC25B and CDC25C protein abundance before and after irradiation. The responsive decrease of nuclear CDC25 proteins occurred at the post-transcription level. Silencing ATM gene blocks the responsive decrease of nuclear CDC25 proteins, which is responsible for failure to inactivate p34cdc2 after irradiation. Active p34cdc2 and CDK2, in turn, acts as the death executors to trigger apoptosis. In summary, aberrantly activated CDK activity is the critical molecular mechanism central to enhanced apoptotic responses in the absence of ATM gene.
authors.affiliation: Department of Hematology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030.
authors.forename: Jinniu
authors.initials: J
authors.lastname: Deng
authors.forename: Jianfeng
authors.initials: J
authors.lastname: Zhou
authors.forename: Fankai
authors.initials: F
authors.lastname: Meng
authors.forename: Dengju
authors.initials: D
authors.lastname: Li
authors.forename: Hanying
authors.initials: H
authors.lastname: Sun
mesh_terms: D017209:Apoptosis / Q000187:drug effects / Q000235:genetics* / Q000528:radiation effects; D064007:Ataxia Telangiectasia Mutated Proteins; D016203:CDC2 Protein Kinase / Q000378:metabolism; D018797:Cell Cycle Proteins; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D004268:DNA-Binding Proteins; D020868:Gene Silencing / Q000187:drug effects / Q000528:radiation effects; D006801:Humans; D017346:Protein Serine-Threonine Kinases / Q000235:genetics*; D025521:Tumor Suppressor Proteins; D020298:U937 Cells; D020687:cdc25 Phosphatases / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D025521:Tumor Suppressor Proteins; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase; D018844:Cyclin-Dependent Kinases; C517004:CDC25A protein, human; D020687:cdc25 Phosphatases