Abstract
BackgroundHypoxia induces the epithelial-mesenchymal transition, EMT, to promote cancer metastasis. In addition to transcriptional regulation mediated by hypoxia-inducible factors, HIFs, other epigenetic mechanisms of gene regulation, such as histone modifications and DNA methylation, are utilized under hypoxia. However, whether DNA demethylation mediated by TET1, a DNA dioxygenase converting 5-methylcytosine, 5mC, into 5-hydroxymethylcytosine, 5hmC, plays a role in hypoxia-induced EMT is largely unknown.ResultsWe show that TET1 regulates hypoxia-responsive gene expression. Hypoxia/HIF-2α regulates the expression of TET1. Knockdown of TET1 mitigates hypoxia-induced EMT. RNA sequencing and 5hmC sequencing identified the set of TET1-regulated genes. Cholesterol metabolic process genes are among the genes that showed high prevalence and statistical significance. We characterize one of the genes, INSIG1 (insulin induced gene 1), to confirm its expression and the 5hmC levels in its promoter. Knockdown of INSIG1 also mitigates hypoxia-induced EMT. Finally, TET1 is shown to be a transcriptional co-activator that interacts with HIF-1α and HIF-2α to enhance their transactivation activity independent of its enzymatic activity. TET1 acts as a co-activator to further enhance the expression of INSIG1 together with HIF-2α. We define the domain in HIF-1α that interacts with TET1 and map the domain in TET1 that confers transactivation to a 200 amino acid region that contains a CXXC domain. The TET1 catalytically inactive mutant is capable of rescuing hypoxia-induced EMT in TET1 knockdown cells.ConclusionsThese findings demonstrate that TET1 serves as a transcription co-activator to regulate hypoxia-responsive gene expression and EMT, in addition to its role in demethylating 5mC.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-014-0513-0) contains supplementary material, which is available to authorized users.
Highlights
Hypoxia induces the epithelial-mesenchymal transition, EMT, to promote cancer metastasis
We further identified the region in the proximal promoter of TET1 gene that responded to hypoxia and hypoxiainducible factors (HIFs)-2α
The results showed that the genes including glucose transporter 3 (GLUT3), hexokinase 1 (HK1), phosphoglycerate kinase 2 (PGK2), pyruvate kinase M (PKM), and lactate dehydrogenase A (LDHA) were induced by hypoxia and knockdown of TET1 or INSIG1 abolished or decreased their expression levels in two different cell lines (Figure 4a,b, and Additional file 11a,b)
Summary
Hypoxia induces the epithelial-mesenchymal transition, EMT, to promote cancer metastasis. In addition to transcriptional regulation mediated by hypoxia-inducible factors, HIFs, other epigenetic mechanisms of gene regulation, such as histone modifications and DNA methylation, are utilized under hypoxia. Hypoxia induces the epithelial-mesenchymal transition (EMT) to promote cancer metastasis [3,4,5,6]. In addition to transcriptional regulation mediated by hypoxiainducible factors (HIFs), other epigenetic mechanisms of gene regulation, such as histone modifications and DNA methylation, are utilized under hypoxia [7,8,9]. Certain chromatin changes have been observed during EMT. Other specific chromatin changes have been observed during hypoxia or TGF-β-induced EMT [11,12] For example in Snail-induced EMT, loss of H3K4Me3, H3K4Ac, and H3K27Ac, and gain of H3K27Me3 were observed for genes repressed, whereas gain of H3K4Me3, H3K4Me1, and loss of H3K27Me3 were observed for genes activated [10].
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