Wl. Kraus et al., DETERMINANTS FOR THE REPRESSION OF ESTROGEN-RECEPTOR TRANSCRIPTIONAL ACTIVITY BY LIGAND-OCCUPIED PROGESTIN RECEPTORS, Journal of steroid biochemistry and molecular biology, 63(4-6), 1997, pp. 175-188
There is considerable evidence for cross-talk between the estrogen and
progestin signaling pathways, including examples of repression or att
enuation of estrogen-stimulated endpoints by progestin receptor (PR) a
gonists and antagonists. We have previously described an experimental
system for examining aspects of this cross-talk, namely the repression
of estrogen receptor (ER) transcriptional activity by liganded PR (Kr
aus, W. L., Weis, K. E., Katzenellenbogen, B. S., Mol. Cell. Biol. 15
(1995) 1847-1857). Under promoter and cell type conditions where ligan
ded PR was not a good activator of transcription, PR isoforms were sho
wn to act as potent ligand-dependent repressors of ER transcriptional
activity. In the current study, we have identified multiple determinan
ts of this repression by systematically manipulating potentially impor
tant variables in this system (e.g. PR A:PR B ratio, sequence of the r
esponse elements, receptor structure, and ligand type). Alterations in
several of these parameters had profound effects on the ability of PR
to repress the activity of ER. Decreases in the PR A:PR B ratio and c
hanges in the sequence of the progestin response element in the report
er gene construct abolished the repressive action of agonist-occupied
PR A on ER transcriptional activity. In addition, point or deletion mu
tations in the amino-terminal A/B region of ER, including a triple poi
nt mutation which eliminates phosphorylation sites previously shown to
be important in the activity of the receptor, made the ER more sensit
ive to the repressive actions of liganded PR. The PR ligands that prom
oted the most potent repression of ER activity were those with 11 beta
phenyl substitutions, suggesting that the phenyl moiety in the 11 bet
a position is the important structural feature leading to strong repre
ssion. Interestingly, changes in the structure of the ER ligand and th
e sequence of the estrogen response element did not influence the magn
itude of repression by PR. The fact that alterations in these check po
ints along the estrogen signaling pathway had little or no effect on t
he magnitude of repression suggests that liganded PR interferes with t
he ability of ER to interact productively with the transcriptional mac
hinery; in other words, PR-mediated repression occurs downstream of th
e events leading to the ligand-dependent conversion of ER to a transcr
iptionally active form. Our results indicate that a number of paramete
rs which are naturally varied in vivo, such as the sequence of PR DNA
binding sites and the PR A:PR B ratio, can dramatically alter the repr
ession of ER activity by liganded PR, and may explain the differential
affects of progestin-occupied PR on the expression of different estro
gen regulated genes. (C) 1997 Elsevier Science Ltd. All rights reserve
d.