SAG/ROC2/RBX2 is a HIF-1 target gene that promotes HIF-1 alpha ubiquitination and degradation

Oncogene. 2008 Feb 28;27(10):1404-11. doi: 10.1038/sj.onc.1210780. Epub 2007 Sep 10.


SAG (sensitive to apoptosis gene) or ROC2/RBX2 is the second family member of ROC1/RBX1, a component of SCF (Skp1, Cullin, F-box protein) and VCB (von Hippel-Lindau (VHL), Cullin and Elongin B/C) E3 ubiquitin ligases. SAG protected cells from hypoxia-induced apoptosis when overexpressed. We report here that SAG was subjected to hypoxia induction at the levels of mRNA and protein. Hypoxia induction of SAG was largely HIF-1alpha dependent. A consensus HIF-1-binding site, GCGTG was identified in the first intron of the SAG gene. In response to hypoxia, HIF-1 bound to this site and transactivated SAG expression. SAG transactivation required both the intact binding site in cis and HIF-1alpha in trans. On the other hand, like its family member, ROC1, SAG promoted VHL-mediated HIF-1alpha ubiquitination and degradation, which was significantly inhibited upon small interfering RNA silencing of SAG or ROC1. Furthermore, the endogenous HIF-1alpha at both basal and hypoxia-induced levels was significantly increased upon SAG silencing. Finally, SAG forms in vivo complex with Cul-5 and VHL under hypoxia condition. These results suggest an HIF-1-SAG feedback loop in response to hypoxia, as follows: hypoxia induces HIF-1 to transactivate SAG. Induced SAG then promotes HIF-1alpha ubiquitination and degradation. This feedback loop may serve as a cellular defensive mechanism to reduce potential cytotoxic effects of prolonged HIF-1 activation under hypoxia.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Base Sequence
  • Cell Line
  • Gene Targeting
  • HeLa Cells
  • Humans
  • Hypoxia / enzymology
  • Hypoxia-Inducible Factor 1, alpha Subunit / metabolism*
  • Mice
  • Mice, Knockout
  • Molecular Sequence Data
  • RNA, Messenger / metabolism
  • Ubiquitin-Protein Ligases / biosynthesis
  • Ubiquitin-Protein Ligases / genetics
  • Ubiquitin-Protein Ligases / metabolism*
  • Ubiquitin-Protein Ligases / physiology


  • Hypoxia-Inducible Factor 1, alpha Subunit
  • RNA, Messenger
  • RNF7 protein, human
  • Ubiquitin-Protein Ligases