DNA Hypermethylation at the Invasive Front of Oral Squamous Cell Carcinoma Confers Poorly Differentiated Characteristics and Promotes Migration of Cancer Cells

<b>Background/Objectives:</b> Oral squamous cell carcinoma (OSCC) is a common and aggressive oral cancer with high recurrence and mortality rates, largely due to late diagnosis and metastasis. Epigenetic regulation, particularly aberrant DNA methylation, plays a critical role in cancer progression. Altered methylation patterns disrupt cancer-related gene regulation. Our previous study found that oral cancer patients exhibit increased synthesis of S-adenosyl-L-methionine, a key methyl donor for cytosine methylation. Therefore, the aim of this study was to explore the relationship between global DNA methylation and OSCC progression and to evaluate the impact of DNA methylation heterogeneity on oral cancer cells. <b>Methods:</b> Immunohistochemistry (IHC) and immunofluorescence (IF) staining were used to examine 5-methylcytosine (5-mC) expression in OSCC clinical specimens and oral cancer cells. The DNA methyltransferase inhibitor 5-Aza-dC was used to assess the effects of DNA methylation on cell function and gene expression. RNA sequencing was used to identify key differentially expressed genes affected by 5-Aza-dC treatment. Cell migration was assessed using a wound closure assay. Protein and gene expression were analyzed using Western blotting and quantitative PCR. <b>Results:</b> An inverse relationship was found between 5-mC levels and cancer differentiation-poorly differentiated OSCC exhibited higher 5-mC levels. Additionally, higher 5-mC staining was observed at the invasion front of oral cancer tissues. In OSCC cells, 5-mC content correlated with migration ability. Furthermore, conditioned medium from cancer-associated fibroblasts enhanced both methylation levels and migration of OSCC cells. Treatment with 5-Aza-dC significantly increased epithelial differentiation, reduced epithelial-to-mesenchymal transition and cell adhesion-related genes, and inhibited OSCC cell migration. <b>Conclusions:</b> The findings highlight the critical role of DNA hypermethylation in OSCC progression, particularly in regulating differentiation, migration, and EMT. The interplay between the tumor microenvironment and epigenetic modifications underscores the complexity of OSCC biology and opens avenues for innovative therapeutic strategies.