HIF1 is the main transcriptional regulator of the hypoxic response in mammalian cells and plays an important role in the adaptation of tumor cells to hypoxic microenvironments. While normally degraded under normoxic conditions via oxygen-dependent, ubiquitin-based mechanisms, HIF1α protein, which, with the constitutively expressed HIF1β forms the HIF1 heterodimer, is stabilized in hypoxic tumor cells. Stabilization of HIF1α protein under hypoxia leads to transactivation of a variety of angiogenic and tumorigenic genes including VEGF and matrix metalloproteinase 2. Clinically, high levels of HIF1 in tumors translate to increased patient morbidity and mortality. Our studies show that HIF1α, in addition to being regulated at the protein level, is also directly regulated by microRNAs, small 20-22 base ribonucleotides that specifically bind to the 3’ untranslated regions (UTRs) of target mRNA to inhibit translation or induce degradation. Based on bioinformatics and microRNA-target databases, we determined that HIF1α contains a potential binding site in its 3’ UTR for mir-199a, which, based on our microarray and RT-qPCR data, is significantly downregulated in hypoxic ovarian cancer cells compared to normoxia. Using a luciferase reporter construct in which luciferase expression is under control of the HIF1α 3’ UTR, we showed that overexpression of mir-199a in A2780 or MA148 cells leads to suppression of the luciferase signal. Mutation of the mir-199a binding site in the HIF1α 3’ UTR resulted in no change in luciferase signal with mir-199a overexpression, indicating that mir-199a directly targets the HIF1α 3’ UTR. Furthermore, decreasing mir-199a levels, either by incubating cells in hypoxia or using a morpholino specifically targeting mir-199a, leads to increased levels of HIF1α protein as determined by western blot and increased transcripts of HIF1-activated genes such as VEGF as measured by RT-qPCR. Conversely, overexpression of mir-199a leads to decreased HIF1α protein and VEGF transcript levels. Together, our findings have significant implications in the study of HIF1α regulation in cancers. Mir-199a may have a role in future therapeutic strategies to modulate the tumor microenvironment to limit hypoxia-driven tumor progression and metastasis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2070.