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PurposeThe incidence of thyroid cancer is increasing and the proliferation of thyroid cancer cells is incompletely understood. microRNAs may play key roles in thyroid cancer progression.MethodsWe analyzed miR-340-5p in thyroid cancer tissue and normal tissue, and using informatics to predict its target. Cell lines and a mouse model were used to study the role of miR-340-5p in cancer proliferation.ResultsOverexpression of miR-340-5p was found in thyroid cancer specimens. Tumors with higher pathological grade had higher levels of miR-340-5p. Overexpression of miR-340-5p significantly enhanced cell viability and colony formation. Treatment of anti-miR-340-5p, however, showed opposite alterations. We predicted that bone morphogenetic protein 4 (BMP4) is a possible target, and found a negative correlation between miR-340-5p and BMP4 levels in thyroid cancer tissue. miR-340-5p reduced BMP4 expression. BMP4 overexpression attenuated the effects of miR-340-5p in cell viability and colony formation. In addition, using a xenograft mouse model we proved that anti-miR-340-5p was able to inhibit tumor growth.ConclusionsmiR-340-5p promotes thyroid cancer proliferation by inhibiting BMP4. Anti-miR-340-5p can be a promising strategy to control thyroid cancer.