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Abstract

E1A-mediated repression of progesterone receptor-dependent transactivation involves inhibition of the assembly of a multisubunit coactivation complex (PDF) (HTML) 
Xu,Yue; Klein-Hitpass,Ludger; Bagchi,Milan K.
Molecular and Cellular Biology 20(6): 2138-2146
Publication date: 2000

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The steroid hormone progesterone acts via high-affinity nuclear receptors that interact with specific DNA sequences locatednear the promoter of the hormone-responsive gene. Recent studiessuggested that the hormone-occupied progesterone receptor (PR)mediates gene activation by recruiting a cellular coregulatoryfactor, termed coactivator, to the target promoter. The identityand mechanism of action of the coactivator(s) that regulates transcriptionalactivity of PR are currently under investigation. Here we provideevidence that the hormone-occupied PR forms a multisubunit receptor-coactivatorcomplex containing two previously described coactivators, CREB-bindingprotein (CBP) and steroid receptor coactivator 1 (SRC-1, a memberof the p160 family of coactivators), in nuclear extracts of humanbreast tumor T47D cells. The association of CBP and SRC-1/p160with the receptor complex is entirely hormone dependent. BothCBP and SRC-1/p160 possess intrinsic histone acetyltransferase(HAT) activity, and it has been recently proposed that these coactivatorsfunction by modulating chromatin structure at the promoter ofthe target gene. Interestingly, addition of purified CBP to thenuclear extracts of T47D cells markedly stimulated progesterone-and PR-dependent transcription from a nucleosome-free, progesteroneresponse element (PRE)-linked reporter DNA template. Furthermore,depletion of SRC-1/p160 by immunoprecipitation from these transcriptionalextracts also significantly impaired PR-mediated RNA synthesisfrom a naked PRE-linked DNA template. These results strongly impliedthat CBP and SRC-1/p160 facilitate receptor-mediated transcriptionin these cell extracts through mechanisms other than chromatinremodeling. We also observed that the adenoviral oncoprotein E1A,which interacts directly with CBP, repressed PR-mediated transactivationwhen added to the nuclear extracts of T47D cells. Supplementationwith purified CBP overcame this inhibition, indicating that theinhibitory effect of E1A is indeed due to a blockade of CBP function.Most importantly, we noted that binding of E1A to CBP preventedthe assembly of a coactivation complex containing PR, CBP, andSRC-1/p160, presumably by disrupting the interaction between CBPand SRC-1/p160. These results strongly suggested that E1A repressedreceptor-mediated transcription by blocking the formation or recruitmentof coactivation complexes. Collectively, our results support thehypothesis that the assembly of a multisubunit coactivation complexcontaining PR, CBP, and SRC-1/p160 is a critical regulatory stepduring hormone-dependent gene activation by PR and that the fullyassembled complex has the ability to control transcription throughmechanisms that are independent of the histone-modifying activitiesof its componentcoactivators.

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