Cell cycle synchronization of FRTL5 cells. A physiological model system

We describe a "physiological" cell cycle synchronization model system. FRTL 5 cells, TSH-dependent for proliferation, were starved from TSH. The cell c ycle phases and the expression of markers associated to different cycle pha ses were evaluated. TSH starvation blocks proliferation without provoking d eath and induces virtually all the cells to accumulate in G0/G1 phase. TSH readdition allows 30% of these cells to enter the S phase. DNA topoisomeras e II 170-kDa isoform is not expressed in G0/G1 synchronized cells while it is expressed in logarithmic growing cells. The 80-kDa isoform is not expres sed in G0/G1 synchronized cells while it is expressed in 20% of logarithmic growing cells regardless of the cycle phase. c-myc mRNA is not expressed i n G0/G1 synchronized cells while it is detectable upon TSH readdition. This system provides a tool for the analysis of events associated with the G0/G 1 phase and the transition from G0/G1 to S phase.