Both estrogen receptor (ER) and Pit-1 proteins are essential for the estrog
en-activated expression of the rat prolactin gene. Our results show that ER
Pit-1 protein complex formation is reduced by estrogen in GH3 and PR1 rat
pituitary tumor cells. In the latter, this decrease was blocked by cyclohex
imide, a protein synthesis inhibitor. On the other hand, the direct additio
n of estrogen to PR1 cell lysates had no effect on the formation of ER Pit-
1 complexes. Estrogen-activated prolactin gene expression was also inhibite
d by cycloheximide, suggesting that some form of protein synthesis is invol
ved in ER Pit-1 complex formation and subsequent prolactin gene activation.
In support of this notion, we showed that estrogen-induced regulation of E
R Pit-1 complex formation could be transferred from cell lysates prepared f
rom estrogen-treated PR1 cells to control cell lysates. This is not true fo
r GH3 cells; instead, direct administration of estrogen to GH3 cell lysates
readily abolished ER Pit-1 protein complex formation in a dose-dependent m
anner, and such estrogen-induced regulation was blocked by the antiestrogen
ICI 182,780, These findings thus indicate that 1) interaction between ER a
nd Pit-1 proteins is estrogen-regulated in ways specific to different cell
types, and 2) auxiliary protein factor synthesis may be involved in this pr
ocess.