Kurt W. Kohn, Joseph Riss, John N Weinstein, Yves Pommier and Carl J. Barrett

Implemented by : Margot Sunshine, Hong Cao and David Kane

Cells exposed to low oxygen levels (hypoxia) respond to this stressful condition by triggering the expression of a series of genes activated through the HIF family of transcription factors. This network of genes is conserved in metazoans from nematodes to mammals. In the absence of hypoxia, there are at least five oxygen-dependent pathways that inhibit transcription of HIF-target genes. All of these pathways are dependent on the protein VHL.

1. Degradation of HIF: HIF is subject to oxygen-dependent proline hydroxylation. Hydroxylated HIF binds VHL with its associated E3-ubiquitin transferase, causing ubiquitination and proteasomal degradation of HIF.
2. Repression of HIF-target genes: VHL, bound to proline-hydroxylated HIF, recruits the general gene repressor, KAP-1, to the HIF-target gene promoters.
3. Degradation of RNA polymerase II at HIF-target genes: After transcript initiation by phosphorylation of the C-terminal domain of Rpb1 (the large subunit of RNA polymerase II), Rpb1 is subject to proline hydroxylation. Hydroxylated Rpb1 binds VHL, leading to degradation of Rpb1 by the same mechanism described for (1) above. (One may suspect that proline hydroxylases (PHD2,3) associate with HIF, thereby facilitating action, not only on HIF itself, but also on Rpb1 at HIF-dependent genes. Transcripts begun in response to temporary hypoxia might then be interdicted by destruction of the RNA polymerase complex during transcript elongation.)
4. Deacetylation of histones at HIF-target genes: Promoter-bound HIF binds FIH, which in turn binds HDAC and recruits it to the HIF-dependent genes.
5. Inhibition of histone acetylation of HIF-target genes: FIH, that is associated with HIF-target promoters as in (4) above, hydroxylates an asparagine of HIF in an oxygen-dependent manner. Asparagine hydroxylation inhibits the binding of p300/CBP to HIF, thereby inhibiting the association of this histone acetylase with the promoter.

This molecular interaction map depicts the regulatory network that inhibits transcription in the presence of normal oxygen levels and switches HIF-mediated transcription on when oxygen levels are reduced. An analysis of the hypoxia-induced regulatory switch can be accessed here