Serum-induced G0/G1 transition in chemically transformed 3T3 cells. Independence of protein synthesis, stable "memory", and enhancement by cycloheximide pretreatment

Abstract

Quiescent, chemically transformed (benzo-alpha-pyrene) BALB/c 3T3 cells (BP A31) enter the cell division cycle when exposed to complete medium containing 10% fetal calf serum (FCS); the number of cells recruited is a function of the duration of serum exposure. The recruitment of cells by short (less than 4 h) serum pulses is not inhibited by simultaneous exposure to cycloheximide (CH), and therefore the initial commitment does not require protein synthesis. The cells enter S phase with a constant delay following the removal of CH, even if CH exposure has been continued for as long as 20 h after the end of the serum pulse. The cell recruitment by serum pulses was inhibited by 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole (DRB), an inhibitor of cytoplasmic mRNA accumulation. These data suggest that serum exposure produces a stable 'memory' that is necessary and sufficient for the eventual progression through G1 to S phase that occurs when protein synthesis is resumed after the removal of CH; this 'memory' probably consists of mRNA species that are induced by serum and that are stable in the absence of protein synthesis. Unexpectedly, pretreatment of quiescent BP A31 cells with CH (8-24 h) dramatically increased the fraction of the total cell population that is recruited by a serum pulse of fixed duration.