IFN-α induces transcription of hypoxia-inducible factor-1α to inhibit proliferation of human endothelial cells

Expression of hypoxia-inducible factor (HIF)-1 alpha, a transcription factor subunit increased by protein stabilization in response to hypoxia, is increased in human endothelial cells (ECs) by IFN-alpha under normoxic conditions. IFN-alpha increases HIF-1 alpha transcript levels within 2 h by up to 50% and doubles HIF-1 alpha protein expression. Based on pharmacological inhibition studies, the increase in HIF-1 alpha mRNA involves new transcription, is independent of new protein synthesis, and requires JAK signaling. Protein knockdown by small interfering RNA confirms the involvement of JAK1 and TYK2, as well of IFN-stimulated gene factor 3 (ISGF3). IFN-gamma does not significantly induce HIF-1 alpha mRNA, but increases the magnitude and duration of the IFN-alpha effect. IFN-alpha-induced HIF-1 alpha protein translocates to the nucleus and can bind to hypoxia response elements in DNA. However, IFN-alpha treatment fails to induce transcription of several prototypic HIF-responsive genes (VEGF-A, PPAR gamma, and prostacyclin synthase) due to an insufficient increase in HIF-1 alpha protein levels. Although certain other HIF-responsive genes (PHD3 and VEGF-C) are induced following IFN-alpha and/or IFN-gamma treatment, these responses are not inhibited by siRNA knockdown of HIF-1 alpha. Additionally, IFN-alpha induction of ISGF3-dependent genes involved in innate immunity (viperin, OAS2, and CXCL10) are also unaffected by knockdown of HIF-1 alpha. Interestingly, knockdown of HIF-1 alpha significantly reduces the capacity of IFN-alpha to inhibit endothelial cell proliferation. We conclude that IFN-alpha induces the transcription of HIF-1 alpha in human endothelial cells though a JAK-ISGF3 pathway under normoxic conditions, and that this response contributes to the antiproliferative activity of this cytokine.