A lncRNA coordinates with Ezh2 to inhibit HIF-1α transcription and suppress cancer cell adaption to hypoxia

Hypoxia is a salient feature of the tumor microenvironment. HIF-1α is a master regulator of hypoxic adaption. The polycomb repressor complex 2 (PRC2) molecule EZH2 is known to play roles in essential cellular processes of cell fate decisions. However, how PRC2-mediated epigenetic dynamic changes take part in hypoxic adaption is not completely understood. Recently, we identified a long non-coding RNA (lncRNA) named HITT (HIF-1α Inhibitor at translation levels) that plays roles in modulating hypoxia-mediated angiogenesis and tumor growth in vivo. In this study, we reveal an important activity of HITT in evading hypoxia-induced Apoptosis by coordinating with PRC2 activity to regulate HIF-1α transcription. Genetic or chemical inhibition of PRC2 significantly elevates HIF-1α mRNA levels. The occupancy of EZH2 and its substrate H3K27me3 on the HIF-1α promoter is detected under normoxia, and is reduced by hypoxia. Restoring hypoxia-inhibited HITT expression rescues the association between EZH2/H3K27me3 and the HIF-1α promoter, which also simultaneously abrogates hypoxia-induced HIF-1α mRNA transcription. Further mechanistic studies revealed that HITT inhibits HIF-1α transcription by guiding EZH2 through the formation of an RNA-DNA triplex with the HIF-1α promoter. Importantly, HITT/Ezh2-regulated HIF-1α transcription leads to alerted HIF-1α protein output and elicits a significant effect to evade hypoxia-induced Apoptosis. Importantly, a close association between HIF-1α mRNA and HITT was further verified in human colon Cancer tissues in vivo. Collectively, these findings suggest a model for the epigenetic regulation of hypoxia-induced HIF-1α transcription modulated by lncRNA HITT, which provides important insights into how tumor cells sense and adapt to hypoxic stress.