Mammalian gene expression program resiliency: the roles of multiple coactivator mechanisms in hypoxia-responsive transcription.

CBP and its paralog p300 (CBP/p300 collectively) are transcriptional coactivators that are among the most interconnected proteins in the mammalian protein-protein "interactome" with over 315 described interaction partners. CBP/p300 are protein/histone acetyltransferases, but most of the protein-binding domains of CBP/p300 are unique to these two coactivators, indicating that CBP/p300 should be highly limiting. The CH1 domain of CBP/p300 was considered essential for most, if not all, hypoxia-inducible transcription by binding to hypoxia-inducible-factor-1alpha (HIF-1alpha). Mutating CH1 had little effect, however, on the hypoxia-induced transcription of the HIF-target genes Higd1a, Egln1 (prolyl-hydroxylase), Bnip3 (Bcl2-interacting-protein-3), and Pfkl (phosphofructokinase). In contrast, HIF-targets Pgf (placental growth factor) and Egln3 were strongly affected by the CH1 mutation, while Stc1 (stanniocalcin-1) and Slc2a1 (glucose-transporter-1) were moderately affected. HIF targets were also dependent on coactivation mechanisms that are sensitive to trichostatin A (TSA(S)). Paradoxically, TSA inhibits histone deacetylases (HDACs) that are usually associated with transcriptional repression, implying that HDACs can also function as coactivators. Thus, activator-specific transcription in mammals requires seemingly unrelated coactivator mechanisms, and individual target genes vary in their requirements for each mechanism. Gene expression program resiliency is therefore coupled with gene specific regulation by avoiding uniform reliance on a "keystone" coactivator interaction.