Cell-specific and hypoxia-dependent regulation of human HIF-3α: inhibition of the expression of HIF target genes in vascular cells

Hypoxia-inducible factors (HIF) are transcription factors responding to reduced oxygen levels and are of utmost importance for regulation of a widespread of cellular processes, e. g., angiogenesis. In contrast to HIF-1 alpha/HIF-2 alpha, the relevance of HIF-3 alpha for the regulation of the HIF pathway in human vascular cells is largely unknown. HIF-3 alpha mRNA increases under hypoxia in endothelial and vascular smooth muscle cells. Analysis of HIF-3 alpha isoforms revealed a cell type-specific pattern, but only one isoform, HIF-3 alpha 2, is hypoxia-inducible. Reporter gene assays of the appropriate promoter localized a 31-bp fragment, mediating this hypoxic regulation. The contribution of HIF-1/2 and NF kappa B to the HIF-3 alpha induction was verified. Functional studies focused on overexpression of HIF-3 alpha isoforms, which decrease the hypoxia-mediated expression of VEGFA and Enolase2. These data support the notion of a hypoxia-induced inhibitory function of HIF-3 alpha and demonstrate for the first time the existence of this negative regulation of HIF-signaling in vascular cells.