INDUCTION OF ENTRY INTO S PHASE OF CELL CYCLE BY PDGF AND EGF ADMINISTERED ALONE AND IN COMBINATION IN A HIGH-SERUM REQUIRING MUTANT OF RAT 3Y1 FIBROBLASTS

抄録

3Y1tsF121 cells, a temperature-sensitive mutant of rat 3Y1 fibroblasts, are arrested in the G1 phase of the cell cycle when the cells proliferating at permissive temperature are placed at restrictive temperature. The cells arrested in the G1 phase are induced to enter the S phase when simply placed at permissive temperature or when added with a large dose of serum into the culture medium at restrictive temperature. This G1 phase-arrest overcoming activity in serum was not dialyzable and was lost after exposure of serum to 90°C for 30 min. Among the growth factors examined, platelet-derived growth factor (PDGF), epidermal growth factor (EGF), and fibroblast growth factor (FGF) were found to replace the serum for the G1 arrest overcoming activity. In any of the three factors, the maximal efficiency of the overcome of the G1-phase arrest (in terms of percentage of cells which entered the S phase) was lower than the maximal efficiency in serum. When PDGF and EGF were added in combination, the efficiency increased to the same level as the maximal efficiency seen in serum. In this case, however, the efficiency was lower than the sum seen with single applications of PDGF and EGF. Administration of EGF during the first half of the incubation period (0-24 h), followed by that of PDGF during the second half (24-48 h), or vice versa, resulted in the entry into S phase with an efficiency much the same as that seen with the simultaneous administration of PDGF and EGF during the entire incubation period. These results suggest that 1) PDGF and EGF either independently or cooperatively act on the events controlling the entry into S phase and involving the functional lesion in 3Y1tsF121 mutant. 2) PDGF and EGF have both common and unique actions in the events involving the lesion in 3Y1stF121, and complement each other in their unique actions. 3) PDGF and EGF need not be simultaneously administered for their cooperative action, and the ordered sequential stimulation of cells is not required. It was also found that, between EGF and PDGF, and between FGF and EGF, there was no detectable cooperative action. Dexamethasone perfectly suppressed the overcome of the G1 arrest mediated by serum or byEGF. The expression of the G1-arrest overcoming activity of each of PDFG, EGF required the simultaneous presence of a small quantity of serum which alone did not have the overcoming activity, suggesting that a serum component is required for the successful action of each of the three growth factor. This growth-factor helping activity in serum was not dialyzable and was resistant to heat (100℃).