Recent evidence indicates that arrest of mammalian cells at the G(2)/M checkpoint involves inactivation and translocation of Cdc25C, which is mediated by phosphorylation of Cdc25C on
serine 216. Data obtained with a phospho-specific antibody against
serine 216 suggest that activation of the DNA damage checkpoint is accompanied by an increase in
serine 216 phosphorylated Cdc25C in the nucleus after exposure of cells to gamma-radiation. Prior treatment of cells with 2 mM
caffeine inhibits such a change and markedly reduces radiation-induced
ataxia-telangiectasia-mutated (ATM)-dependent Chk2/Cds1 activation and phosphorylation. Chk2/Cds1 is known to localize in the nucleus and to phosphorylate Cdc25C at
serine 216 in vitro.
Caffeine does not inhibit Chk2/Cds1 activity directly, but rather, blocks the activation of Chk2/Cds1 by inhibiting ATM
kinase activity. In vitro, ATM phosphorylates Chk2/Cds1 at
threonine 68 close to the N terminus, and
caffeine inhibits this phosphorylation with an IC(50) of approximately 200 microM. Using a phospho-specific antibody against
threonine 68, we demonstrate that radiation-induced, ATM-dependent phosphorylation of Chk2/Cds1 at this site is
caffeine-sensitive. From these results, we propose a model wherein
caffeine abrogates the G(2)/M checkpoint by targeting the ATM-Chk2/Cds1 pathway; by inhibiting ATM, it prevents the
serine 216 phosphorylation of Cdc25C in the nucleus. Inhibition of ATM provides a molecular explanation for the increased radiosensitivity of
caffeine-treated cells.