Abstract
Cdc25 is required for Cdc2 dephosphorylation and is thus essential for cell cycle progression. Checkpoint activation requires dual inhibition of Cdc25 and Cdc2 in a Rad3-dependent manner. Caffeine is believed to override activation of the replication and DNA damage checkpoints by inhibiting Rad3-related proteins in both S chizosaccharomyces pombe and mammalian cells. In this study, we have investigated the impact of caffeine on Cdc25 stability, cell cycle progression and checkpoint override. Caffeine induced Cdc25 accumulation in S . pombe independently of Rad3. Caffeine delayed cell cycle progression under normal conditions but advanced mitosis in cells treated with replication inhibitors and DNA-damaging agents. In the absence of Cdc25, caffeine inhibited cell cycle progression even in the presence of hydroxyurea or phleomycin. Caffeine induces Cdc25 accumulation in S . pombe by suppressing its degradation independently of Rad3. The induction of Cdc25 accumulation was not associated with accelerated progression through mitosis, but rather with delayed progression through cytokinesis. Caffeine-induced Cdc25 accumulation appears to underlie its ability to override cell cycle checkpoints. The impact of Cdc25 accumulation on cell cycle progression is attenuated by Srk1 and Mad2. Together our findings suggest that caffeine overrides checkpoint enforcement by inducing the inappropriate nuclear localization of Cdc25.
Highlights
The ability to rapidly delay cell cycle progression in response to environmental and genotoxic insults, is essential for the maintenance of genomic integrity and/or cell viability
We have investigated the effect of caffeine on Cdc25 stability, cell cycle progression and DNA damage/replication checkpoint activation in S. pombe
Exposure to 10 mM caffeine resulted in rapid accumulation of HA-tagged Cdc25 under control of the endogenous promoter in log phase S. pombe cells (Fig. 1A)
Summary
The ability to rapidly delay cell cycle progression in response to environmental and genotoxic insults, is essential for the maintenance of genomic integrity and/or cell viability. Cells have evolved molecular signalling pathways that sense DNA damage or environmental stress and activate cell cycle checkpoints. Cell cycle progression in Schizosaccharomyces pombe is regulated by the activity of the cyclin-dependent kinase (CDK) Cdc and its regulatory cyclin Cdc (Lu et al, 2012). Negative regulation of Cdc, and cell cycle progression, is enforced by the Mik and Wee kinases which phosphorylate Tyr to inhibit its activity. The Cdc phosphatase positively regulates Cdc activity by dephosphorylating Tyr and is essential for G2/M cell cycle progression in S. pombe (Lu et al, 2012). The effective inhibition of Cdc and Cdc activity is essential for full activation of the DNA damage and stress activated cell cycle checkpoints (Alao and Sunnerhagen, 2008)
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