MiR-3121-3p promotes tumor invasion and metastasis by suppressing Rap1GAP in papillary thyroid cancer in vitro

BACKGROUND: Rap1GAP is a tumor suppressor and is downregulated in human malignancies including papillary thyroid cancer (PTC). The mechanism of its suppression in PTC remains unclear. METHODS: Bioinformatic analyses were carried out to evaluate clinical significance and to predict upstream miRNA bindings of Rap1GAP. Three PTC cell lines, TPC-1, B-CPAP, and K1, were employed for functional verification and further experiments. We used dual-luciferase reporter gene assay to confirm the miRNA binding prediction, Western blotting and quantitative polymerase chain reaction (qPCR) to explore miRNA and Rap1GAP regulation, Transwell and wound healing assays to compare cell migration and invasion after protein knockout or overexpression, and Cell Counting Kit-8 (CCK-8) assay to evaluate cell proliferation. RESULTS: Rap1GAP expression was suppressed in thyroid cancer compared to adjacent normal tissues and was a potential diagnostic marker of PTC. Rap1GAP suppression was correlated to younger age, advanced T stage, N stage, extrathyroidal extension, BRAF-like tumors, and higher risk of recurrence. Combined analysis of bioinformatic prediction and dual-luciferase assay revealed binding between miR-3121-3p with 3'UTR of Rap1GAP promoter. MiR-3121-3p promoted cell migration, invasion, and proliferation via inhibiting Rap1GAP and thus upregulating MAPK pathway. Overexpression and knockdown of Rap1GAP could counteract the influence on cell migration and invasion carried out by miR-3121-3p mimic and inhibitor, respectively. Rap1GAP partially impaired the effect of miR-3121-3p in cell growth in the CCK-8 assay. CONCLUSIONS: Rap1GAP expression is suppressed in PTC and is a potential diagnostic marker. Its upstream regulator, miR-3121-3p, affects tumor metastasis and proliferation via regulating Rap1GAP expression. MAPK signaling pathway may be involved in this effect.