Mechanistic insights into the role of miR‐6741‐3p in oral squamous cell carcinoma pathogenesis
Many functional studies and clinical analysis have linked miRNA dysregulation as a causative factor for tumor initiation and progression. Overexpression of oncogenic miRNAs and downregulation of tumor suppressor miRNAs are widely reported to drive the process of carcinogenesis. Epigenetic silencing of miRNAs with tumor suppressor features by DNA hypermethylation is a common hallmark of many cancers including OSCC. We, therefore, used the drug 5-Azacytidine, a DNMT inhibitor, for reactivation of tumor suppressor miRNAs silenced due to DNA hypermethylation. Treatment of cells from an OSCC cell line SCC131 with control and 5-Azacytidine, followed by the miRNA microarray analysis revealed upregulation and downregulation of 50 and 28 miRNAs respectively. miR-6741-3p was one of the miRNAs found to be upregulated following the 5-Azacytidine treatment. We validated the upregulation of miR-6741-3p following the 5-Azacytidine treatment using qRT-PCR and demonstrated its anti-proliferative activity in OSCC cells. miRNAs are epigenetic regulators which exert their effect through binding to one or more cognate target mRNAs and regulating their expression. Using bioinformatics and molecular approaches, we showed that miR-6741-3p post-transcriptionally regulates the expression of the oncogene SRSF3 at the protein level in OSCC by interacting with its 3’UTR in a sequence-specific manner. We analyzed the expression of miR-6741-3p transcript and SRSF3 protein in different cell lines and OSCC patient samples, and the results showed that the interaction between miR-6741-3p and SRSF3 is of physiological and clinical relevance. We demonstrated that miR-6741-3p reduced cell proliferation and anchorage-independent growth of OSCC cells, in part, by targeting the 3’UTR of SRSF3 and promoted apoptosis of OSCC cells independent of SRSF3. OSCC xenografts in nude mice using a synthetic miR-6741-3p mimic and a synthetic miR-6741-3p inhibitor indicated that miR-6741-3p suppresses tumor formation in OSCC, in part, by targeting SRSF3. We also demonstrated that miR-6741-3p decreases signaling through both the PI3K-AKT-MTOR and the ERK/MAPK pathways, in part, via the miR-6741-3p-SRSF3-ERK1/2-S6K1 axis. We also demonstrated that the intronic miR-6741-3p is involved in the autoregulation of its host gene PYCR2 and inhibits its expression at both the transcript and protein levels in OSCC cells by interacting with a target site present in the 3’UTR in a sequence-specific manner. Expression analysis of miR-6741-3p and PYCR2 across different cell lines and OSCC patient samples suggested that the interaction between miR-6741-3p and PYCR2 is of physiological and clinical relevance. We observed that MIR6741 lacks an independent promoter and seems to be co-transcribed with the PYCR2 transcript, using its host gene PYCR2 promoter. We identified and validated the host gene promoter activity and identified PAX6 as a positive regulator of the PYCR2 promoter activity. Using a combination of in silico analysis, ChIP analysis, and dual-luciferase reporter assays, we propose that PAX6 interacts with the PYCR2 promoter via an indirect mechanism. We further demonstrated that promoter methylation of the tumor suppressor gene PAX6 is responsible for the downregulation of miR-6741-3p in OSCC cells and speculate the involvement of the same mechanism in other cancers.