Osteosarcoma is an aggressive bone tumor primarily affecting children and adolescents. Its cause is not yet fully understood, and there is an urgent need for more effective treatment. In the present study we identified several miRNAs whose expression is altered in osteosarcoma compared to adjacent normal tissue. Moreover, expression levels of one of those miRNAs, miR-506-3p, correlated negatively with expression of RAB3D (a Ras-related protein). Suppression of miR-506-3p in osteosarcoma led to increased expression of RAB3D, which in turn led to increased CDK4 (cyclin-dependent kinase 4) and MMP9 (matrix metalloprotein 9) activities. Our results suggest that miR-506-3p acts as a tumor suppressor in osteosarcoma and that its downregulation leads to tumor cell proliferation and metastasis due to upregulation of RAB3D- and CDK4-mediated signaling. miR-506-3p thus appears be a potentially useful target for adjuvant therapy in osteosarcoma patients.

Accumulating evidence indicates that a ligand of programmed cell death receptor‐1 (PD‐L1) participates in the progression and recurrence of multiple malignancies, including osteosarcoma. Nevertheless, the role of PD‐L1 in chemoresistance development is not fully understood. In the current study, we aim to clarify the interaction of miR‐519d‐3p and PD‐L1 in the development of cisplatin resistance. Immunohistochemistry, quantitative reverse‐transcription polymerase reaction, and Western blot were used to evaluate PD‐L1 expression. MTT and transwell migration assays were used to measure cell growth and motility, respectively. ENCORI, miRCode, and miRDB databases were recruited to predict candidate miRNAs targeting PD‐L1. The binding sequences of miR‐519d‐3p and PD‐L1 3ʹ untranslated region were identified by dual‐luciferase reporter and RNA immunoprecipitation assays. Flow cytometric analysis was conducted to measure the cycle distribution and cell apoptosis. Metastatic mouse models were generated with cisplatin‐resistant sublines by intravenous injection. We found that PD‐L1 expression was positively correlated to cisplatin resistance and metastasis, whereas miR‐519d‐3p expression was reduced in cisplatin‐resistant specimens and was negatively correlated to cisplatin resistance and metastasis of osteosarcoma. We demonstrated that miR‐519d‐3p overexpression reversed cisplatin resistance, induced G1/S phase arrest and apoptosis. In addition, we proved that miR‐519d‐3p inhibited lung metastasis by establishing cisplatin‐resistant MG63 metastatic xenograft models. The present findings suggest that miR‐519d‐3p/PD‐L1 axis is a novel signaling pathway contributing to cisplatin resistance. Our study provides new clues for curing refractory osteosarcoma beyond immune checkpoint inhibitors.