SERUM FACTOR REVEALING 2 DISTANT PHASES OF NEGATIVE PROLIFERATION CONTROL IN MITOGEN-STIMULATED NORMAL FIBROBLASTS

When quiescent normal skin fibroblasts are stimulated by mitogens such as epidermal growth factor (EGF), the proportion of cells entering a division cycle decreases with increasing cell density. The presence of a synthetic double-stranded RNA (poly I:C) enhances this density-related restriction. Fetal calf serum (FCS) as well as human serum (HS) and human platelet-poor plasma (HPPP) completely abrogate the inhibiting effect of cell density on EGF mitogenicity, both in the presence and absence of poly I:C. HS and HPPP are up to ten times more potent than FCS in overcoming density-related restriction of EGF mitogenicity in human skin fibroblasts, whereas the mitogenic potencies of FCS, HS and HPPP in the absence of EGF are identical. Thus the mitogenic activity of FCS, HS and HPPP and their ability to overcome the density-related restriction of EGF-induced proliferation may be due to different molecules. Addition of FCS or HS at various times after EGF exposure reveals two distinct control points within the prereplicative phase: one within the first 2 hours and the other between 10 and 20 hours after the beginning of EGF exposure. Thus, the interactions of EGF, serum, and poly I:C reveal the kinetics of a cell density-related mechanism of negative proliferation control.