A stable bovine papillomavirus E1 expressing cell line (C2E1) was used
to investigate the effects of E1 protein on the requirement for growt
h factors during serum-induced reentry from quiescence to proliferatio
n, Flow cytometric bivariate DNA/PCNA analysis was utilized to study t
he expression of proliferating cell nuclear antigen (PCNA) concomitant
with this transition. C2E1 cells, unlike the control cells (CNEO), we
re able to reenter the cell cycle when stimulated with low serum (1%).
Stimulation with 10% serum revealed that C2E1 cells entered the first
cell cycle faster than CNEO, indicating that E1 protein decreased the
time of progression from CO stage upon serum activation. It was also
shown that PCNA expression started earlier in C2E1 cells than in CNEO
cells after quiescent cells were stimulated with 10% serum, Addition o
f 1% serum was able to induce PCNA expression in C2E1 but not in CNEO
cells in the first 24 h after stimulation. Using Triton X-100 treatmen
t, it was found that the distribution between bound and unbound forms
of PCNA was altered in E1-expressing cells compared to CNEO cells, Bas
ed on these results, it is suggested that E1 might possess mitogen-lik
e properties. (C) 1995 Wiley-Liss, Inc.