Elongation of G1 phase by transient exposure of rat 3Y1 fibroblasts to caffeine during the previous and present generations.
When density-arrested rat 3Y1 fibroblasts were stimulated to enter S phase by seeding sparsely in fresh medium, caffeine inhibited this stimulation. When compared at the doses that gave the same levels of the inhibition of entry into S phase, caffeine inhibited protein synthesis to a far lesser extent than cycloheximide. This indicates that caffeine affects some event(s) specific to entry into S phase rather than general protein synthesis. When cells synchronized at early S phase were exposed to caffeine, progression of S and G2 phases was prolonged by only 1 h (from 6 h to 7 h). However, after removal of caffeine at mitosis, the G1 phase was prolonged for 5 h (from 11 h to 16 h). These results are consistent with our model that the initiation of S phase is regulated throughout the period between the adjacent S phases. When cells were incubated with normal medium containing serum during the S and G2 periods, a subsequent 6-h pulse exposure to caffeine caused prolongation of G1 phase for 7 h (from 11 h to 18 h). On the other hand, when cells were incubated in the absence of serum during these periods, the prolongation was only 2 h (from 16 h to 18 h). Similarly, entry into S phase was prolonged only 2 h, when a 6-h pulse exposure to caffeine was given immediately after release from density arrest or serum-deprivation arrest. These results indicate the involvement of the relaxation process, which is not affected by caffeine, when serum-deprived cells or density-arrested cells restore the process prerequisite for entry into S phase.
Okuda
A
A
Department of Virology, Medical Institute of Bioregulation, Kyushu University Fukuoka, Japan.
Kimura
G
G
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002478:Cells, Cultured; D003513:Cycloheximide / Q000494:pharmacology; D003864:Depression, Chemical; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D007399:Interphase / Q000187:drug effects*; D014176:Protein Biosynthesis; D051381:Rats
D002110:Caffeine; D003513:Cycloheximide