DPP4 and osteosarcoma: The expression of PDGF-AA, endostatin, endothelin-1 and CD26 has been previously shown to be positively correlated with tumor progression in osteosarcoma [27,28,29,30] and the upregulation of serpin E1 and thrombospondin-1 in OS cells has been correlated with an increased risk of lung metastasis [31,32].