The study of oral-cancer associated miRNAs and their target genes

104

Oral cancer is a deadly disease, ranking the eighth among all cancers in mortality. MicroRNAs (miRNAs) are small non-coding RNAs that mediate gene expression at the post-transcriptional level by degrading or repressing the translation of target mRNAs. Although accumulating evidence suggests that miRNAs function as oncogenes or tumor suppressors in human malignancy, there are still few studies focused on the role of miRNA(s) in oral carcinogenesis. To clarify the miRNAs involved in oral cancer, human miRNA microarray was used to identify the down-regulation of 54 miRNAs in three oral cancer cell lines when compared to normal oral keratinocytes (NOK). miR-99a as the most down-regulated in oral cancer cells (~32 folds). MiR-99a down-regulation was also confirmed both in tested oral cancer cell lines and clinical specimens. To study the role of miR-99a in oral cancer, etopic miR-99a expression inhibited oral cancer cell migration and invasion. Anti-miR-99a, silencing miR-99a functions, had the opposite effect. Myotubularin- related protein 3 (MTMR3) with one evolutionarily conserved seed region in the 3’-untranslated region was a novel miR-99a target. Depleting MTMR3 expression significantly reduced cell proliferation, migration, or invasion. There was an inverse expression of miR-99a and MTMR3 protein in oral cancer lines and clinical specimens. Together, miR-99a repressed oral cancer cell migration and invasion partly through decreasing MTMR3 expression. In addition to miR-99a, we also identified the downregulation of miR-22 in oral cancer cell lines and clinical specimens. Hyaluronan synthases 3 (HAS3) was predicted as a novel target of miR-22 and ectopic miR-22 repressed the levels of both endogenous and ectopic HAS3. The expression of HAS3 mRNA, involved in pro-inflammatory low molecular mass HA (LMM-HA) accumulation, was also highly expressed in oral cancer. To assess the functionality, ectopic HAS3 expression significantly increased oral cancer cell migration, invasion and xenograft tumorigenesis accompanied with the increase of pro-inflammatory TNF-α and MCP-1 expression. Conversely, HAS3 depletion significantly abrogated HAS3-mediated stimulation. The oncogenic action of HAS3 was partly through the activation of EGFR-Src signaling axis. The release of HAS3-derived LMM-HA into extracellular milieu enhanced transendothelial monocyte migration and MCP-1 expression, which could be attenuated by the addition of HAS3 antibody or an inhibitor, 4-Methylumbelliferone (4-MU). Consistent with the stimulatory effects of cytokines and growth factors on HAS3 expression, we also found TNF-α dose-dependently increased HAS3 mRNA expression partly through the activation of NF-κB in oral cancer cells. The increase of HAS3 mRNA expression significantly reduced the overall survival of late-stage oral cancer patients and high TNF-α expression further negated the clinical outcome among these patients. Overexpressed HAS3, a target of miR-22, enhanced crucial biological activities necessary for tumorigenesis and thus offered an advantageous environment for oral cancer progression. Taken together, MTMR3 and HAS3, rather than miR-99a or miR-22, might serve as theranostic targets for oral cancer treatment although more studies are needed to validate this possibility.