Complex regulation of the transactivation function of hypoxia-inducible factor-1αby direct interaction with two distinct domains of the creb-binding protein/p300

Abstract

Activation of transcription in response to low oxygen tension is mediated by the hypoxia-inducible factor-1 (HIF-1). HIF-1 is a heterodimer of two proteins: aryl hydrocarbon receptor nuclear translocator and the oxygen-regulated HIF-1α. The C-terminal activation domain of HIF-1α has been shown to interact with cysteine/histidine-rich region 1 (CH1) of the coactivator CBP/p300 in a hypoxia-dependent manner. However, HIF forms lacking C-terminal activation domain (naturally occurring or genetically engineered) are still able to activate transcription of target genes in hypoxia. Here, we demonstrate that the N-terminal activation domain (N-TAD) of HIF-1α interacts with endogenous CBP and that this interaction facilitates its transactivation function. Our results show that interaction of HIF-1α N-TAD with CBP/p300 is mediated by the CH3 region of CBP known to interact with,among other factors, p53. Using fluorescence resonance energy transfer experiments, we demonstrate that N-TAD interacts with CH3 in vivo. Coimmunoprecipitation assays using endogenous proteins showed that immunoprecipitation of CBP in hypoxia results in the recovery of a larger fraction of HIF-1α than of p53. Chromatin immunoprecipitation demonstrated that at 1%O2 CBP is recruited to a HIF-1α but not to a p53 target gene. Upon activation of both pathways, lower levels of chromatin-associated CBP were detected at either target gene promoter. These results identify CBP as the coactivator directly interacting with HIF-1α N-TAD and mediating the transactivation function of this domain. Thus, we suggest that in hypoxia HIF-1α is a major CBP-interacting transcription factor that may compete with other CBP-dependent factors, including p53, for limiting amounts of this coactivator, underscoring the complexity in the regulation of gene expression by HIF-1α. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.