CAFFEINE INDUCES S-PHASE APOPTOSIS IN CIS-DIAMMINEDICHLOROPLATINUM-TREATED CELLS, WHEREAS CIS-DIAMMINEDICHLOROPLATINUM INDUCES A BLOCK IN G(2) M/

Caffeine overrides checkpoints in the G(2) phase of the cell cycle by inhibiting DNA repair at this phase and increases the cytotoxicity of antitumor drugs, such as cis-diamminedichloroplatinum (CDDP), The enha nced cell death induced by caffeine is characterized by apoptosis. In this paper, we demonstrate that this apoptotic event occurs in S phase of the cell cycle, whereas CDDP induces a block in G(2)/M, DNA histog ram analysis revealed that caffeine reduced G(2) arrest in CDDP-treate d EL-4 cells, In a synchronous population, the ratio of cyclin B:p34(c dc2) was UPregulated just before the cells went into the apoptotic pat hway. A rapid increase in DNA fragmentation was detected at 12-24 h, w hen marked regression of G(2)/M phase was observed. Moreover, the degr ee of DNA fragmentation in CDDP + caffeine-treated cells was not reduc ed when the cell cycle was arrested at metaphase by exposure to the sp indle-inhibitor nocodazole, It is possible that execution of the apopt otic program after treatment with caffeine did not require the EL-4 ce lls to reenter G(1) phase, The apoptotic cell fraction in the group of CDDP + caffeine was recognized as an S population by bivariate analys is of apoptosis and DNA content, These results suggest that enhancemen t of the apoptotic activity of CDDP-treated cells by caffeine is not a G(1)-phase event but an S-phase-specific event, whereas cells were ar rested in G(2)/M phase, and that it is regulated by G(2) checkpoint-re lated proteins. Cytometry 27:365-373,1997. (C) 1997 Wiley-Liss, Inc.