Inhibitory PAS domain protein is a negative regulator of hypoxia-induciblegene expression

Alteration of gene expression is a crucial component of adaptive responses to hypoxia. These responses are mediated by hypoxia-inducible transcription factors (HIFs)(1,2). Here we describe an inhibitory PAS (Per/Arnt/Sim) dom ain protein, IPAS, which is a basic helix-loop-helix (bHLH)/PAS protein str ucturally related to HIFs. IPAS contains no endogenous transactivation func tion but demonstrates dominant negative regulation of HIF-mediated control of gene expression. Ectopic expression of IPAS in hepatoma cells selectivel y impairs induction of genes involved in adaptation to a hypoxic environmen t, notably the vascular endothelial growth factor (VEGF) gene, and results in retarded tumour growth and tumour vascular density in vivo. In mice, IPA S was predominantly expressed in Purkinje cells of the cerebellum and in co rneal epithelium of the eye. Expression of IPAS in the cornea correlates wi th low levels of expression of the VEGF gene under hypoxic conditions. Appl ication of an IPAS antisense oligonucleotide to the mouse cornea induced an giogenesis under normal oxygen conditions, and demonstrated hypoxia-depende nt induction of VEGF gene expression in hypoxic corneal cells. These result s indicate a previously unknown mechanism for negative regulation of angiog enesis and maintenance of an avascular phenotype.