Objective: To explore the molecular elements responsible for the enhanced apoptotic susceptibility in U937 cells mediated by silencing atactic telangiectasis mutation (ATM) gene.
Methods: Two U937 cell mutants, U937-ASPI3K (ATM gene negative) and U937-pZEOSV2 (+) (ATM gene positive) were used as a cell model system. Apoptosis was examined by measuring free nucleosome concentrations in U937 cells. Western blotting was employed to measure nuclear protein abundances of cdc25A, cdc25B, cdc25C, total p34cdc2, p34cdc2 (Thr161) or p34cdc2 (Thr14, Tyr15). RT-PCR was used to estimate cdc25 transcript levels.
Results: U937-ASPI3K exhibited an enhanced apoptotic susceptibility to lower dosage of irradiation, which could not be blocked by protein synthesis inhibitor. Protein serine-theronine phosphatase inhibitor or cyclin-dependent kinase (CDK) inhibitors could abolish the enhancement. Upon irradiation, p34cdc2 in U937-pZEOSV2 (+) was in an inactive state owing to phosphorylation of Thr 14 and Tyr15, which was associated with a dramatic decrease of nuclear cdc25A, cdc25B and cdc25C poteins. In contrast, p34cdc2 in U937-ASPI3K was in an active state owing to the low phosphorylation of Thr14 and Tyr15, 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.
Conclusion: Silencing ATM gene blocks the irradiation induced responsive decrease of nuclear cdc25 proteins, resulting in an abnormal activation p34cdc2 is the critical molecular mechanism for the enhanced apoptotic responses.