Bypass of the ts Block of tsJT60, a G0-Specific ts Mutant from Rat Fibroblasts, by Fetal Bovine Serum and Epidermal Growth Factor

tsJT60, a temperature-sensitive (ts) G0-mutant cell line from a Fischer rat, grows normally in the exponential growth phase at 34°C and 39.5°C, but when stimulated with fetal bovine serum (FBS), from the G0 phase they reenter the S phase at 34°C but not at 39.5°C. The ts-block was bypassed when G0-arrested tsJT60 cells were stimulated at 39.5°C with FBS plus epidermal growth factor (EGF). The presence of EGF for the first 6 h after serum stimulation caused tsJT60 cells to enter the S phase in the presence of FBS at 39.5°C. When EGF was added 6h after serum stimulation, entrance into the S phase was delayed by about 6 h. The sequential presence of two growth factors, EGF without FBS for 6 h then FBS without EGF, or the reversed sequence, failed to initiate DNA synthesis at 39.5°C. The binding of EGF was not temperature sensitive. The amounts of RNA and protein present doubled after stimulation with both FBS and EGF at 39.5°C. These and other findings suggest that EGF bypasses only some specific event in the entire prereplicative process that operates operating in serum-stimulated cells at 39.5°C.