Epigenetic Silencing of TET1 Mediated Hydroxymethylation of Base Excision Repair Pathway During Lung Carcinogenesis
Abstract Backgroud: The methylcytosine dioxygenase Ten-eleven translocation 1 (TET1) is an important regulator for the balance of DNA methylation and hydroxymethylation through various pathways. Increasing evidence has suggested that TET1 probably involved in DNA methylation and demethylation dysregulation during chemical carcinogenesis. However, the role and mechanism of TET1 during lung cancer remains unclear. Methods: The gene and protein expression were detected by qRT-PCR, Western blot and immunohistochemistry during lung carcinogenesis. Methylation and hydroxymethylation status were evaluated by MeDIP-qPCR and hMeDIP-qPCR. The effect and mechanism of TET1 on lung cancer were identified both in vitro and in vivo models. Results: In this study, we found that TET1 expression was significantly down-regulated and the methylation level was significantly up-regulated in 3-MCA-induced cell malignant transformation model, rat chemical carcinogenesis model, and human lung cancer tissues. Demethylation experiment further confirmed that DNA methylation negatively regulated TET1 gene expression. TET1 overexpression inhibited cell proliferation, migration and invasion in vitro and in vivo, while knockdown of TET1 resulted in an opposite phenotype. DNA hydroxymethylation level in the promoter region of base excision repair (BER) pathway key genes XRCC1, OGG1, APEX1 significantly decreased and the degree of methylation gradually increased in malignant transformed cells. After differential expression of TET1, the level of hydroxymethylation, methylation and expression of these genes also changed significantly. Furthermore, TET1 binds to the promoter of XRCC1, OGG1, and APEX1 to maintain them hydroxymethylated. Blockade of BER pathway key gene alone or in combination significantly diminished the effect of TET1. Conclusions: Our study demonstrated for the first time that TET1 gene expression is regulated by DNA methylation and TET1-mediated hydroxymethylation regulates BER pathway to inhibit the proliferation, migration and invasion during the 3-MCA-induced lung carcinogenesis. These results may suggest that TET1 gene can be used as a potential biomarker and therapy target for lung cancer.