The importin α/β-specific inhibitor Ivermectin affects HIF-dependent hypoxia response pathways

Hypoxia-inducible transcription factors (HIFs) regulate hundreds of genes involved in cellular adaptation to reduced oxygen availability. HIFs consist of an O-2-labile alpha-subunit (primarily HIF-1 alpha and HIF-2 alpha) and a constitutive HIF-1 beta subunit. In normoxia the HIF-alpha subunit is hydroxylated by members of a family of prolyl-4-hydroxylase domain (PHD) proteins, PHD1-3, resulting in recognition by von Hippel-Lindau protein, ubiquitination and proteasomal degradation. In contrast, reduced oxygen availability inhibits PHD activity resulting in HIF-1 alpha stabilisation and nuclear accumulation. Nuclear import of HIF-1 alpha mainly depends on classical nuclear localisation signals (NLS) and involves importin alpha/beta heterodimers. Recently, a specific inhibitor of nuclear import has been identified that inhibits importin alpha/beta-dependent import with no effects on a range of other nuclear transport pathways involving members of the importin protein family. In this study we evaluated the physiological activity of this importin alpha/beta-inhibitor (Ivermectin) in the hypoxia response pathway. Treatment with Ivermectin decreases binding activity of HIF-1 alpha to the importin alpha/beta-heterodimer. Moreover, HIF-1 alpha nuclear localisation, nuclear HIF-1 alpha protein levels, HIF-target gene expression, as well as HIF-transcriptional activity are reduced upon Ivermectin treatment. For the first time, we demonstrate the effect of specific importin alpha/beta-inhibition on the hypoxic response on the molecular level.