La. Selvey et al., HUMAN PAPILLOMAVIRUS (HPV) TYPE-18 E7 PROTEIN IS A SHORT-LIVED STEROID-INDUCIBLE PHOSPHOPROTEIN IN HPV-TRANSFORMED CELL-LINES, Journal of General Virology, 75, 1994, pp. 1647-1653
We used a capture ELISA to quantify the E7 protein of human papillomav
irus type 18 (HPV-18). In HeLa cells, which express low levels of immu
noreactive E7 protein (iE7), iE7 had a mean half-life of 13.5 min. In
HPV-18 E7 recombinant baculovirus (E7rec BV)-infected Sf21 cells, whic
h express higher levels of E7, the half-life of iE7 was much longer (9
0 min and > 24 h, with two different E7rec BVs). For two transformed h
uman cervical cell lines expressing HPV-18 E7, exposure of the cells t
o hydrocortisone resulted in a twofold increase in steady-state levels
of the E7 protein: no similar effect was observed with progesterone,
oestrogen or testosterone. The half-life of iE7 was unaltered by hydro
cortisone or progesterone exposure. An immunoassay which distinguished
Ser(33)-phosphorylated E7 from E7 not phosphorylated at this residue
(Ser(33)dephospho-E7), showed that in HeLa and Sf21 cells the majority
of E7 was phosphorylated: the half-life of both species of E7 was sim
ilar in HeLa cells, but the half-life of Ser(33)dephospho-E7 was much
shorter (90 min) in Sf21 cells than that of Ser(33)phospho-E7 (> 24 h)
. A HeLa-fibroblast fusion cell line with tumorigenic potential (CGL-1
) had a similar ratio of dephospho-E7 to total E7 (0.06), as a similar
fusion cell line (CGL-4) with no tumorigenic potential (0.03). We con
clude that E7 is a labile phosphoprotein, and that the expression and
steady-state level of the E7 protein in eukaryotic cells may be influe
nced by the hormonal environment of the cells.