Evidence that the coactivator CBP/p300 is important for phenobarbital-induced but not basal expression of the CYP2H1 gene.

We have previously identified an upstream 556-bp enhancer domain for the chicken CYP2H1 gene that responds to phenobarbital and binds several transcription factors, including the orphan chicken xenobiotic receptor (CXR). By contrast, the promoter lacks a CXR site and is not inducible by phenobarbital. Although it has been established that CXR can interact with the coactivator SRC-1, there are no reports as to whether other coactivators may be important for phenobarbital-mediated inducibility. Our studies using the adenovirus E1A wild-type protein, which inhibits the coactivators cAMP response element binding protein (CBP) and CBP associated factor (p/CAF), provide evidence for the involvement of one or both of these coactivators at the enhancer but not at the promoter of the CYP2H1 gene. The observations that mutant E1A proteins did not affect the enhancer activity and that inhibition by wild-type E1A was reversed by CBP and p/CAF confirmed the involvement of these coactivators in the induction process. We propose that the intrinsic histone acetyl transferase activity of one or both of these coactivators participates in chromatin remodeling thereby stimulating drug induction of the promoter. This proposal was supported by experiments with the histone deacetylase inhibitor, trichostatin A, which resulted in the superinduction of the drug response but had little effect on basal expression of the CYP2H1 gene. The work provides evidence for the first time for the involvement of the coactivators CBP and p/CAF in the phenobarbital-mediated induction of the CYP2H1 gene.