Role of CBP in regulating HIF-1-mediated activation of transcription.

The hypoxia-inducible factor-1 (HIF-1) is a key regulator of oxygen homeostasis in the cell. We have previously shown that HIF-1alpha and the transcriptional coactivator CBP colocalize in accumulation foci within the nucleus of hypoxic cells. In our further exploration of the hypoxia-dependent regulation of HIF-1alpha function by transcriptional coactivators we observed that coexpression of SRC-1 (another important coactivator of the hypoxia response) and HIF-1alpha did not change the individual characteristic nuclear distribution patterns. Colocalization of both these proteins proved to be mediated by CBP. Biochemical assays showed that depletion of CBP from cell extracts abrogated interaction between SRC-1 and HIF-1alpha. Thus, in contrast to the current model for the assembly of complexes between nuclear hormone receptors and coactivators, the present data suggest that it is CBP that recruits SRC-1 to HIF-1alpha in hypoxic cells. We also observed that CBP, HIF-1alpha/Arnt and HIF-1alpha/CBP accumulation foci partially overlap with the hyperphosphorylated form of RNA polymerase II, and that CBP had a stabilizing effect on the formation of the complex between HIF-1alpha and its DNA-binding partner, Arnt. In conclusion, CBP plays an important role as a mediator of HIF-1alpha/Arnt/CBP/SRC-1 complex formation, coordinating the temporally and hierarchically regulated intranuclear traffic of HIF-1alpha and associated cofactors in signal transduction in hypoxic cells.