Abstract
Caffeine, a widely consumed neuroactive compound, induces DNA damage checkpoint signalling override, and enhances sensitivity to DNA damaging agents. However, the precise underlying mechanisms have remained elusive. In fission yeast S. pombe, the Ataxia Telangiectasia Mutated (ATM) and Ataxia Telangiectasia mutated Related (ATR) orthologue Rad3 has been proposed as the cellular target of caffeine. Nevertheless, recent studies suggest that the Target of Rapamycin Complex 1 (TORC1) might be the main target. Caffeine mimics the effects of activating the Sty1-regulated stress response and the AMP-Activated Protein Kinase (AMPK) homologue Ssp1-Ssp2 pathways on cell cycle progression. Direct inhibition of TORC1 with the ATP-competitive inhibitor torin1, is sufficient to override DNA damage checkpoint signalling. It is, therefore, plausible, that caffeine modulates cell cycle kinetics by indirectly suppressing TORC1 through Ssp2 activation. Ssp1 and ssp2 deletion suppresses the effects of caffeine on cell cycle progression. In contrast, direct inhibition of TORC1 advances cell division in these mutants. These observations suggest that caffeine overrides DNA damage signalling, in part, via the indirect inhibition of TORC1 through Ssp2 activation. Alternatively, Ssp1 and Ssp2 may potentiate the effect of caffeine on Cdc25 activity. The AMPK-mTORC1 signalling axis plays an important role in aging and disease and presents a potential target for chemo- and radio-sensitization. Our results provide further insights of the underlying mechanisms by which caffeine modulates cell cycle progression in the context of Ssp1-AMPKαSsp2-TORC1 signalling activities and can potentially aid in the development of novel dietary regimens, therapeutics, and chemo-sensitizing agents.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
This version contains additional experiments using a number of mutant fission yeast strains and combinations of pharmacological interventions. It also contains additional analyses of molecular markers, quantification of cell size data and extensive editing throughout the manuscript. These changes are done to fulfil suggestions and constructive criticism from four anonymous reviewers.
