CREB BINDING-PROTEIN ACTS SYNERGISTICALLY WITH STEROID-RECEPTOR COACTIVATOR-1 TO ENHANCE STEROID RECEPTOR-DEPENDENT TRANSCRIPTION

Steroid receptors are ligand-regulated transcription factors that requ ire coactivators for efficient activation of target gene expression. T he binding protein of cAMP response element binding protein (CBP) appe ars to be a promiscuous coactivator for an increasing number of transc ription factors and the ability of CBP to modulate estrogen receptor ( ER)- and progesterone receptor (PR)-dependent transcription was theref ore examined. Ectopic expression of CBP or the related coactivator, p3 00, Enhanced ER transcriptional activity bg up to 10-fold in a recepto r- and DNA-dependent manner. Consistent with this, the 12S E1A adenovi ral protein, which binds to and inactivates CBP, inhibited ER transcri ptional activity, and exogenous CBP was able to partially overcome thi s effect, Furthermore, CBP was able to partially reverse the ability o f active ER to squelch PR-dependent transcription, indicating that CBP is a common coactivator for both receptors and that CBP is limiting w ithin these cells. To date, the only other coactivator able to signifi cantly stimulate receptor-dependent transcription is steroid receptor coactivator-l (SRC-I). Coexpression of CBP and SRC-1 stimulated ER and PR transcriptional activity in a synergistic manner and indicated tha t these two coactivators are not Functional homologues. Taken together , these data suggest that both CBP and SRC-1 may function in a common pathway to efficiently activate target gene expression.