miR-31 is upregulated in oral premalignant epithelium and contributes to the immortalization of normal oral keratinocytes

Oral squamous cell carcinoma (OSCC) is a prevalent malignancy worldwide. MicroRNAs are short non-coding RNAs that regulate gene expression and are crucial for tumorigenesis. Previously, we have identified that miR-31 is frequently upregulated in OSCC and that this miR-31 increase, together with downstream effector modulation, enhances oral carcinogenesis. We have identified higher levels of miR-31 expression in oral potential malignant disorder (OPMD) tissues compared with normal oral mucosa. Exogenous miR-31 and human telomerase reverse transcriptase (hTERT) expression were introduced into cultured normal oral keratinocytes (NOKs), which led to the immortalization; these lines were designated M31OK1 and M31OK3. These immortalized lines maintained the capability to undergo squamous differentiation. In addition, migration by both cell lines was attenuated by hTERT and miR-31 knockdown. M31OK1 carries a p53 gene mutation at codon 273. A serum-tolerant subline, M31OK1-D, exhibits potent anchorage- independent growth that is attenuated by knockdown of both hTERT and miR-31. miR-31-targeted factors inhibiting HIF (FIH), which upregulated vascular endothelial growth factor (VEGF), was found crucial for oral tumorigenesis. The proliferation, migration and epithelial-mesenchymal transition of M31OK1-D are associated with downregulation of FIH and upregulation of VEGF, which require miR-31 expression. High miR-31 expression is correlated with higher VEGF expression and lower E-cadherin expression in OPMD tissue. It can be concluded that miR-31 collaborates with hTERT to immortalize NOKs and that this may contribute to early stage oral carcinogenesis. The targeting of downstream factors by miR-31 may further advance the neoplastic progression of immortalized NOKs, allowing them to become malignant.