Hypoxia inducible factor-1 (HIF-1), a dimeric
transcription factor of the bHLH-PAS family, is comprised of HIF-1α, which is inducible by
hypoxia and ARNT or HIF-1β, which is constitutively expressed. HIF-1 is involved in cellular homeostasis under
hypoxia, in development and in several diseases affected by
oxygen availability, particularly
cancer. Since its expression is positively correlated with poor outcome prognosis for
cancer patients, HIF-1 is a target for
pharmaceutical therapy. We have previously shown that male germ cell Rac
GTPase activating protein (
MgcRacGAP), a regulator of Rho
proteins which are principally involved in cytoskeletal organization, binds to HIF-1α and inhibits its transcriptional activity. In this work, we have explored the mechanism of the
MgcRacGAP-mediated HIF-1 inactivation. We show that the Myo domain of
MgcRacGAP, which is both necessary and sufficient for HIF-1 repression, binds to the PAS-B domain of HIF-1α. Furthermore
MgcRacGAP competes with ARNT for binding to the HIF-1α PAS-B domain, as shown by in vitro binding pull down assays. In mammalian cells, ARNT overexpression can overcome the
MgcRacGAP-mediated inhibition and
MgcRacGAP binding to HIF-1α in vivo inhibits its dimerization with ARNT. We additionally present results indicating that
MgcRacGAP binding to HIF-1α is specific, since it does not affect the transcriptional activity of HIF-2, a close evolutionary relative of HIF-1 also involved in
hypoxia regulation and
cancer. Our results reveal a new mechanism for HIF-1 transcriptional activity regulation, suggest a novel
hypoxia-cytoskeleton link and provide new tools for selective HIF-1 inhibition.