Tet methylcytosine dioxygenase 1 promotes hypoxic gene induction and cell migration in colon cancer.

Ten-eleven translocation 1 (TET1), a widely reported DNA demethylation protein, has been associated with tumorigenesis and metastasis. However, whether TET1 is an oncogene or tumor suppressor gene has been controversial; the mechanism of how TET1 affects cancer progression remains unclear. The current study aims to investigate how TET1 is changed in the tumor microenvironment and to explore the mechanisms of how TET1 affects colon cancer progression. Because hypoxia prevails on solid tumors, we established an important connection between hypoxia and DNA demethylation in tumorigenesis. By qPCR and RNA interference (RNAi) technology, we found that hypoxia increased TET1 expression with a hypoxia-inducible factor-1-alpha (HIF-1α)-dependent manner. By CHIP-qPCR and pyrosequencing technology, we demonstrated that TET1 regulated the target gene expression of HIF-1α through HIF-1α binding to hypoxia-responsive elements (HREs), and HIF-1α binding to HREs depended on CpG methylation levels. By Cell Counting Kit-8 (CCK-8) and transwell assay, we showed that loss of TET1 did not affect cell proliferation but inhibited migration. We also identified two novel gene mutants of TET1 in 120 paired tumor/normal tissue specimens by DNA sequencing and found that TET1 E2082K mutant blocked the TET1-enhanced cell migration. Our results showed that the downregulation of TET1 rescued the abnormally high levels of gene expression resulting from hypoxia in tumors and reduced the migration activity of tumor cells, suggesting a therapeutic role by interference with TET1 in colon cancer treatment. By demonstrating that hypoxia upregulated TET1 and that TET1 drove HIF-1α-responsive genes, we showed that an epigenetic mechanism and tumor microenvironment-driven models coexisted and mutually affected colon cancer.