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Molecular and Cellular Biology, March 2000, p. 2138-2146, Vol. 20, No. 6
Population Council, The Rockefeller
University, New York, New York 10021,1 and
Institut fur Zellbiologie, Universitatsklinikum, D-45122 Essen,
Germany2
Received 8 September 1999/Returned for modification 26 October
1999/Accepted 17 December 1999
The steroid hormone progesterone acts via high-affinity nuclear
receptors that interact with specific DNA sequences located near the
promoter of the hormone-responsive gene. Recent studies suggested that
the hormone-occupied progesterone receptor (PR) mediates gene
activation by recruiting a cellular coregulatory factor, termed
coactivator, to the target promoter. The identity and mechanism of
action of the coactivator(s) that regulates transcriptional activity of
PR are currently under investigation. Here we provide evidence that the
hormone-occupied PR forms a multisubunit receptor-coactivator complex
containing two previously described coactivators, CREB-binding protein
(CBP) and steroid receptor coactivator 1 (SRC-1, a member of the p160
family of coactivators), in nuclear extracts of human breast tumor T47D
cells. The association of CBP and SRC-1/p160 with the receptor complex
is entirely hormone dependent. Both CBP and SRC-1/p160 possess
intrinsic histone acetyltransferase (HAT) activity, and it has been
recently proposed that these coactivators function by modulating
chromatin structure at the promoter of the target gene. Interestingly,
addition of purified CBP to the nuclear extracts of T47D cells markedly
stimulated progesterone- and PR-dependent transcription from a
nucleosome-free, progesterone response element (PRE)-linked reporter
DNA template. Furthermore, depletion of SRC-1/p160 by
immunoprecipitation from these transcriptional extracts also
significantly impaired PR-mediated RNA synthesis from a naked
PRE-linked DNA template. These results strongly implied that CBP and
SRC-1/p160 facilitate receptor-mediated transcription in these cell
extracts through mechanisms other than chromatin remodeling. We also
observed that the adenoviral oncoprotein E1A, which interacts directly
with CBP, repressed PR-mediated transactivation when added to the
nuclear extracts of T47D cells. Supplementation with purified CBP
overcame this inhibition, indicating that the inhibitory effect of E1A
is indeed due to a blockade of CBP function. Most importantly, we noted
that binding of E1A to CBP prevented the assembly of a coactivation
complex containing PR, CBP, and SRC-1/p160, presumably by disrupting
the interaction between CBP and SRC-1/p160. These results strongly
suggested that E1A repressed receptor-mediated transcription by
blocking the formation or recruitment of coactivation complexes.
Collectively, our results support the hypothesis that the assembly of a
multisubunit coactivation complex containing PR, CBP, and SRC-1/p160 is
a critical regulatory step during hormone-dependent gene activation by
PR and that the fully assembled complex has the ability to control
transcription through mechanisms that are independent of the
histone-modifying activities of its component coactivators.
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
E1A-Mediated Repression of Progesterone Receptor-Dependent
Transactivation Involves Inhibition of the Assembly of a
Multisubunit Coactivation Complex
*
Corresponding author. Mailing address: Center for
Biomedical Research, Population Council, The Rockefeller University,
1230 York Ave., New York, NY 10021. Phone: (212) 327-8761. Fax: (212) 327-7678. E-mail: milan{at}popcbr.rockefeller.edu.
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